Research Progress on Tunable Absorbers for Various Wavelengths Based on Metasurfaces

Abstract

In complex electromagnetic environments, traditional static absorbers struggle to meet dynamic control requirements. Tunable absorbers based on metasurfaces have emerged as a research hotspot due to their ability to flexibly control electromagnetic wave properties. This paper provides a systematic review of research progress in tunable absorbers across the microwave, terahertz, and infrared bands, with a focus on analyzing the physical mechanisms, material systems, and performance characteristics of five dynamic control methods: electrical control, magnetic control, optical control, temperature control, and mechanical control. Electrical control achieves rapid response through materials such as graphene and varactor diodes; magnetic control utilizes ferrites and other materials for stable tuning; optical control relies on photosensitive materials for ultrafast switching; temperature control employs phase-change materials for large-range reversible regulation; and mechanical control expands tuning freedom through structural deformation. Research indicates that multi-band compatibility faces challenges due to differences in structural scale and physical mechanisms, necessitating the integration of emerging materials and synergistic control strategies. This paper summarizes the core performance metrics and typical applications of absorbers across various bands and outlines future development directions such as multi-field synergistic control and low-power design, providing theoretical references and technical pathways for the development of intelligent tunable absorber devices.

1. Introduction

In the context of increasingly complex electromagnetic environments, the demand for precise control of electromagnetic waves has driven the emergence of metamaterials. Metamaterials are a class of artificially engineered materials based on subwavelength structures, exhibiting unique properties that are not available in naturally occurring materials. They have demonstrated significant application value in areas such as military stealth [1,2,3,4,5], communications [6,7,8,9,10], imaging [11,12,13,14], and energy utilization [15,16,17,18,19]. In particular, metamaterials have introduced new strategies for the design of absorbing materials. As early as 2008, Landy et al. [20] proposed the first metamaterial perfect absorber. By employing split-ring resonators, dielectric layers, and metallic wires as unit structures, they achieved near-perfect absorption in the microwave band. This pioneering work inspired extensive research across multiple domains. However, traditional absorbers based on static metamaterials are limited by their inherent electromagnetic responses and are unable to meet the growing demand for dynamic applications. Specifically, such absorbers typically exhibit fixed resonance frequencies and bandwidths that cannot be adjusted once fabricated, as their electromagnetic properties remain insensitive to external environmental changes. This rigid response mechanism severely restricts their potential in dynamic electromagnetic scenarios, such as frequency agility, environment adaptivity, and real-time stealth. To overcome these limitations and achieve dynamic tunability of absorption performance without altering the intrinsic structural parameters, researchers have turned to the development of tunable absorbers. The emergence of metasurfaces has significantly alleviated these constraints [21,22]. As the two-dimensional counterparts of metamaterials, metasurfaces offer advantages such as low loss, ultra-thin profiles, low cost, ease of fabrication, and seamless integration. By engineering subwavelength artificial unit cells, metasurfaces enable flexible control over the amplitude, phase, and polarization of electromagnetic waves [23,24]. As illustrated in Figure 1, recent studies have demonstrated that metasurface-based tunable absorbers exhibit excellent dynamic response characteristics across a wide range of spectral regimes [25,26,27,28,29,30,31,32,33,34,35], including microwave, terahertz, and infrared bands, thereby opening up new avenues for the development of next-generation intelligent absorbers.

It is worth emphasizing that the electromagnetic characteristics vary considerably across different spectral regions. For instance, the penetration ability of microwaves contrasts sharply with the photon energy-dominated behavior of infrared radiation. These differences naturally give rise to distinct tuning mechanisms and material systems for each band [36,37,38]. Building on this premise, the present work provides a systematic review of recent advances in tunable absorbers operating in the microwave, terahertz, and infrared regimes. Particular attention is devoted to their underlying physical mechanisms, material platforms, and tuning strategies. The goal of this study is to establish a solid theoretical foundation while offering practical references for the design and development of next-generation cross-band tunable absorber devices.

The remainder of this paper is organized as follows. Section 2 introduces the principle and requirements of adjustable absorbers. Section 3 introduces the principal approaches used to achieve tunability. Section 4, Section 5 and Section 6 examine metasurface absorbers in the microwave, terahertz, and infrared bands, respectively, with a focus on their working principles and representative application scenarios. Finally, Section 7 provides an integrated summary and discusses future research directions for tunable metasurfaces.

2. Principle and Requirements of Adjustable Absorber

As indispensable devices for future applications in complex electromagnetic environments, tunable absorbers must simultaneously satisfy two interrelated performance dimensions: dynamic response capability and spectral compatibility. The former describes the ability of the device to respond in real time to variations in the electromagnetic environment, while the latter reflects its adaptability across different frequency regimes. The synergistic optimization of these two aspects defines the central technical bottleneck in absorber development and offers a systematic pathway for advancing performance through material innovation, structural engineering, and tuning mechanisms [39,40,41,42,43,44,45].

Dynamic response capability represents the defining feature that distinguishes tunable absorbers from conventional static counterparts. It relies on the precise and rapid modulation of electromagnetic parameters in both temporal and spatial domains, which can be characterized by three technical metrics: response speed, tuning range, and tuning precision. These factors together determine the practicality of absorbers in real-world scenarios [46,47,48,49].

First, response speed dictates whether a device can adapt to rapid changes in the electromagnetic environment, with markedly different timescale requirements across applications. In military stealth and electronic countermeasure scenarios, absorbers require millisecond-level responsiveness to track radar scans or moving targets in real time. In addition, stealth absorbers are typically required to achieve more than 90% average absorption across broad frequency ranges, with radar cross-section (RCS) reduction exceeding 15 dB. Angle and polarization insensitivity are also crucial to ensure effectiveness under diverse detection conditions. In high-speed communication systems, the response time must be further reduced to the microsecond–nanosecond scale to match high-speed modulation and beamforming demands. Beyond response speed, THz communication absorbers and modulators must support gigahertz-to-terahertz bandwidths, maintain low insertion loss (typically <3 dB), and ensure compatibility with chip-scale integration to enable high-speed and low-latency data transmission. In optically controlled metasurfaces, the requirement can be as stringent as picosecond-level or faster, enabling ultrafast modulation from the terahertz to optical regimes [50,51,52,53]. These cross-timescale performance demands directly influence material system selection and tuning mechanism design.

Second, the dynamic tuning range reflects the extent of achievable modulation and is a decisive factor for both absorption efficiency and functional versatility. For instance, infrared thermal management often demands an absorption variation of ΔA > 80% to yield effective thermal radiation control. Radar stealth applications typically require radar cross-section (RCS) reduction exceeding 15 dB across the operational band, while radio-frequency filtering and channel selection may need depth variations over 30 dB to suppress interference. Achieving such performance requires material systems with broad dielectric, conductive, or plasmonic tunability, coupled with mechanisms capable of sustaining large dynamic ranges [54,55,56,57,58]. Third, tuning precision is indispensable for high-performance operation. Phase-change absorbers based on VO₂, for example, often require temperature control within ±0.1 K to minimize hysteresis. Graphene-based electrically tunable devices demand millivolt-level voltage resolution to achieve fine Fermi-level control. In narrowband, high-Q absorbers, frequency tuning steps must remain within 1% of the central resonance to preserve selectivity, imposing strict requirements on driving circuits and micro/nanostructural design [59,60,61,62,63,64].

It is important to emphasize that response speed, range, and precision are inherently coupled. Expanding the tuning range often involves nonlinear processes such as phase transitions or thermal diffusion, which reduce speed. Improving precision typically requires closed-loop control, which may increase latency and power consumption. This “speed–range–precision” trade-off forms the core optimization challenge in absorber design. Practical devices must therefore adopt multi-objective strategies, balancing performance according to application-specific requirements to achieve coordinated matching among materials, structures, and tuning mechanisms.

From the perspective of spectral compatibility, the growing convergence of stealth, thermal management, and communication technologies has created demand for absorbers capable of multi-band operation. The ultimate objective is to realize broadband, continuously tunable absorption spanning from the microwave (0.3–300 GHz) to the terahertz (0.1–10 THz) and infrared (0.3–400 THz) regimes. However, several obstacles hinder this goal. Conventional metasurfaces, with resonance behavior tied to fixed geometric scales, struggle to cover such disparate wavelength regimes. Moreover, electromagnetic interactions differ across bands: microwaves are dominated by cavity currents and capacitive effects, while terahertz and infrared regimes involve plasmonic excitations, interfacial polarizations, and phonon resonances. Achieving cooperative multi-physics responses in a single structure remains a fundamental challenge [65,66,67,68,69,70].

Beyond performance metrics, the fundamental physical mechanisms of absorbers can be interpreted through impedance matching and critical coupling. According to transmission line theory, perfect absorption occurs when the effective impedance of the absorber matches that of free space (Z ≈ Z₀ = 377 Ω), eliminating reflection and transmission so that all energy is dissipated inside the structure. This matching is typically realized by tailoring subwavelength resonators, dielectric layers, and metallic backplanes so that effective permittivity and permeability satisfy μ/ε ≈ μ₀/ε₀. However, impedance matching alone does not guarantee perfect absorption if material loss and radiative leakage are unbalanced. Critical coupling addresses this by requiring the internal loss rate to equal the external radiation loss rate, ensuring that all incident energy is coupled into and dissipated within the absorber. In practice, impedance matching enables efficient wave coupling, while critical coupling governs energy dissipation, and their synergy forms the theoretical foundation of absorber design.

The realization of multi-band and broadband absorption relies on engineering resonant pathways and impedance continuity. Multi-band absorbers typically integrate resonant elements of different sizes or symmetries within a unit cell, each contributing a discrete absorption peak. Mechanisms such as coupled resonances, mode splitting, and hybridization of localized surface plasmons enable multiple frequencies to be absorbed simultaneously. Broadband absorbers, in contrast, aim to merge or extend absorption bands. Strategies include stacking resonators with staggered resonant frequencies, designing graded or multilayer impedance-matching structures, or exploiting critical coupling across multiple modes. Additionally, loss engineering—by carefully adjusting dielectric or conductive damping—can broaden resonances while preserving impedance matching. Together, these approaches provide complementary design pathways: multi-band absorbers enhance spectral selectivity, whereas broadband absorbers ensure continuous coverage, and hybrid designs often combine both to meet the stringent requirements of stealth, sensing, and communication systems.

Finally, to provide a comparative perspective, this paper classifies current research efforts by spectral regime (

Table 1. Comparative Analysis of Multiband Tunable Metasurface Absorbers.

Property	Microwave Band	Terahertz Band	Infrared Band
Primary Tuning Mechanism	Electrical/Magnetic/ Mechanical Control	Electrical/ Thermal Control	Thermal Control
Typical Tuning Materials	Ferrites/Diodes, etc.	Graphene/ Liquid Crystals, etc.	VO2/GST Phase-Change Materials, etc.
Response Time	ns-μs Level	fs-ns Level	μs-ms Level
Typical Tuning Range	Δf > 5 GHz	Δf > 0.5 THz	Δλ > 2 μm
Dominant Loss Mechanism	Ohmic Loss	Carrier Scattering	Dielectric Loss

), summarizing representative materials, mechanisms, response speeds, tuning ranges, and applications across microwave, terahertz, and infrared bands.

3. Methods for Achieving Tunability in Metasurfaces

With the continuous improvement in artificially engineered structures in controlling electromagnetic waves, metasurface technology has emerged as a core platform for enabling wavefront manipulation, polarization conversion, frequency selection, and absorption-based stealth functionalities. However, conventional static metasurfaces have fixed structural parameters, meaning that their resonant frequency and absorption performance are locked after fabrication, making them unsuitable for dynamic tasks in varying environments. To enable active adjustment of electromagnetic properties, researchers have proposed a variety of tunable metasurface design strategies. By incorporating materials or structures with field-responsive characteristics, metasurfaces can be endowed with the capability to dynamically alter their equivalent electromagnetic parameters in real time. As shown in Figure 2, current approaches to achieving tunable metasurfaces can be broadly classified into five categories: electrical, magnetic, optical, thermal, and mechanical tuning. Each method relies on the influence of external variables—such as electric fields, magnetic fields, light, temperature, or mechanical force—to dynamically modulate electromagnetic parameters including resonant frequency, absorption strength, polarization state, and phase response. These different tuning mechanisms exhibit distinct advantages in terms of tuning speed, integrability, adjustment range, and application scenarios, forming a diverse and multi-dimensional research landscape. The following sections provide a systematic review of the five mainstream tuning mechanisms, focusing on their working principles, representative materials, key structural designs, performance metrics, and developmental challenges.

3.1. Electrical Tuning

Electrical tuning is currently the most mature and widely applied dynamic control technique for tunable metasurface absorbers. Its basic principle is to apply an external electric field to modify the internal carrier distribution, charge migration, or polarization state of the functional material, thereby altering its intrinsic electromagnetic parameters such as permittivity, conductivity, or permeability. This, in turn, enables dynamic adjustment of the absorber’s operating state. In practice, electrical tuning mechanisms can be divided into three main types: Material response tuning based on carrier modulation in novel materials; Circuit parameter tuning using active electronic devices; and Structural response tuning in dielectric anisotropic materials. In terms of novel material systems, graphene stands out as the most representative electrically tunable material due to its unique band structure and electronic properties. Graphene and other emerging two-dimensional materials offer tunable Fermi levels and highly linear conductivity control, making them particularly effective in the microwave and terahertz frequency ranges. By applying a gate voltage, their effective conductivity can be continuously adjusted, enabling fine control over absorption frequency, bandwidth, and absorption intensity, suitable for the design of ultrafast modulators and broadband-tunable devices [71,72,73,74,75,76].

Another prominent approach is circuit-based tuning using active components, particularly embedded varactor diodes. The junction capacitance of a varactor diode changes monotonically with applied reverse bias voltage, making it an effective means to adjust the equivalent capacitance of a resonant unit and thus control its resonant frequency. Such devices are compact, require simple drive circuitry, and offer rapid response times, making them especially suitable for tunable absorbers operating in the microwave-to-millimeter-wave bands. In typical designs, varactor diodes are embedded in the gap regions of metallic metasurface resonators as electrically controlled loads. By finely adjusting their capacitance, one can modulate reflection phase, enhance absorption intensity, or dynamically switch between multi-frequency absorption states. While their tuning range may be somewhat narrower than that of materials like graphene, their strong engineering feasibility, clear tuning pathway, and low cost make them widely used in practical applications such as communication systems and radar camouflage [77,78,79,80,81].

Liquid crystal materials, as electrically responsive anisotropic dielectrics, also show considerable potential for electrically tunable absorbers [82,83,84,85]. The molecular orientation of liquid crystals is highly sensitive to external electric fields; when a field is applied, their dielectric tensor changes significantly, thereby adjusting the macroscopic effective permittivity. This property can be exploited to tune resonant frequency, polarization response, or absorption strength. Liquid-crystal-based tuning is often applied in low- or mid-frequency ranges and is particularly suited for large-area flexible absorbers or multi-polarization control devices. However, their relatively slow response speed and susceptibility to ambient temperature variations limit their applicability in high-frequency scenarios.

In summary, electrical tuning relies on the direct influence of external electric fields on the electromagnetic response of materials, offering excellent programmability, high tuning precision, and rapid response. It is especially well-suited for applications requiring high-frequency dynamic adjustment and system integration. Current development trends are moving toward low-power operation, high stability, and multi-dimensional cooperative tuning in conjunction with other mechanisms (such as thermal or optical control), positioning electrical tuning as a foundational technology for intelligent tunable metasurface systems.

3.2. Magnetic Control

Similarly to electrical control, magnetic control is an external-field-driven mechanism. Its basic principle is to apply an external magnetic field to modulate the permeability or magnetization state of magnetic materials, thereby enabling dynamic control over the electromagnetic response of metasurface absorbers. This approach relies on the sensitivity of magnetic materials to magnetic fields, particularly when ferromagnetic or ferrimagnetic media are incorporated into resonant structures. Through phenomena such as magnetically induced anisotropy or ferromagnetic resonance, the propagation and absorption characteristics of electromagnetic waves can be effectively tuned. Research on magnetic control can be traced back to the early exploration of microwave ferrite devices. However, studies on its integration with metasurfaces emerged much later, primarily constrained by material size, power consumption, and packaging limitations. In recent years, advances in nanoscale magnetic materials such as yttrium iron garnet (YIG), CoFe₂O₄, and thin-film magnetic control devices [86,87,88,89] have brought new breakthroughs. The introduction of magnetic fluids and programmable magnetic structures has further expanded the degrees of freedom available for magnetic tuning.

Magnetic control exhibits high tuning depth and excellent frequency stability in the low-frequency microwave band, making it especially suitable for broadband absorbers and devices with nonreciprocal requirements, such as isolators and circulators. Compared with electrical control, magnetic control typically does not require complex bias circuits or active device drivers, allowing for simpler system designs and inherent immunity to electromagnetic interference. This makes it well suited for operation in high-power or complex electromagnetic environments. Nevertheless, the implementation of magnetic control depends on the application of an external magnetic field, which imposes strict requirements on system size, magnetic source strength, and precise field distribution. Further progress in novel magnetic materials and compact magnetic control structures is urgently needed.

It is noteworthy that magnetic tuning has proven to be most effective in the microwave regime, whereas its application in the terahertz (THz) and infrared (IR) domains remains far less prevalent. This disparity primarily arises from several fundamental limitations. First, the operational frequency of conventional magnetic materials such as ferrites is intrinsically limited by their saturation magnetization, restricting efficient magnetic response to lower-frequency bands. Second, integrating magnetic components into deep-subwavelength metasurface architectures at THz/IR frequencies poses substantial nanofabrication challenges, particularly in maintaining material quality and achieving uniform magnetic biasing at micro- and nanoscales. Additionally, external low-frequency magnetic fields exhibit inefficient coupling into THz/IR metasurfaces due to strong electromagnetic shielding and impedance mismatch at these frequencies, significantly limiting practical field penetration and active modulation efficacy.

Thus, although magnetic tuning offers robust performance in microwave metamaterials, its extension into higher-frequency regimes requires advances in high-anisotropy magnetic compounds, innovative field delivery mechanisms, and hybrid metadevice designs.

3.3. Optical Control

Optical control is an important method for dynamically tuning the electromagnetic properties of metasurfaces using an external optical field. Its core advantages include ultrafast response, high tuning precision, and non-contact modulation. Among various tuning mechanisms, optical control stands out for its ability to induce dramatic changes in material parameters within extremely short time scales, making it ideal for applications with stringent bandwidth and speed requirements, such as terahertz communications, dynamic imaging, and infrared detection. From a physical perspective, optical control operates through several main pathways. First, laser illumination of semiconductor materials can effectively generate photocarriers, thereby altering the material’s conductivity [90]. This mechanism is particularly effective in the terahertz and infrared bands. Specifically, optical excitation promotes electrons from the valence band to the conduction band, creating a large number of free carriers that change the material’s electromagnetic response. Since carrier generation and recombination occur on picosecond to nanosecond timescales, this method achieves extremely fast modulation speeds.

Second, optical control can adjust resonant responses by modifying the local surface plasmon excitation conditions in metallic nanostructures [91,92]. Under intense light, changes in electron density distribution or temperature gradients can shift the plasmon resonance, enabling dynamic control over reflection, transmission, and absorption peaks. This mechanism is applicable not only to conventional metallic metasurfaces but also to plasmonic materials such as doped oxides and graphene, offering strong tuning capabilities in the infrared-to-visible spectrum. In addition, certain materials exhibit pronounced nonlinear optical effects under intense illumination, including third-order nonlinearity, electro-optic effects, and thermally induced refractive index changes [93,94,95,96]. These nonlinear responses can be exploited to design metasurfaces that dynamically respond to complex light-field variations, thereby extending the scope of optical control. For example, nonlinear-optics-based metasurface structures have been successfully applied to all-optical modulators, nonlinear beam shapers, and tunable infrared absorbers.

Despite its many advantages, optical control still faces several technical challenges [97,98]. High-efficiency optical tuning often requires high-intensity or precisely modulated laser sources, which increases system complexity and power consumption. Furthermore, achieving precise control typically demands complex optical setups, including collimators, modulators, and focusing elements, limiting the practicality of optical control in portable and low-power systems. Additionally, some photosensitive materials suffer from poor thermal stability, limited modulation cycles, or material degradation [99,100], posing long-term reliability concerns. To address these issues, future research should focus on developing low-threshold, high-stability optically active materials and structures, as well as reducing system complexity and power consumption through improved material design, structural optimization, and system integration. These advances will be essential for promoting the practical deployment of optical control technologies in high-performance dynamic metasurfaces.

3.4. Temperature Regulation

Temperature regulation is a tunable mechanism based on the thermal sensitivity of a material’s electromagnetic parameters. The fundamental principle is that variations in external temperature induce reversible changes in the electrical conductivity, dielectric constant, or plasma frequency of the absorber’s constituent materials, thereby enabling dynamic adjustment of its electromagnetic response. This approach offers notable advantages such as structural simplicity, large tuning range, and good repeatability. It is particularly well-suited for applications requiring moderate response speed but high adaptability to thermal environments, including thermal camouflage, infrared regulation, and environmental sensing systems. From the perspective of material physics, temperature regulation primarily relies on functional materials that are highly sensitive to thermal stimuli [100,101,102]. A representative example is vanadium dioxide (VO₂), which undergoes a phase transition from an insulating state to a metallic state near 68 °C. During this process, its electrical conductivity increases by several orders of magnitude, while the dielectric constant and infrared reflectivity exhibit abrupt changes. The phase transition is both reversible and stable, making VO₂ an ideal candidate for constructing temperature-responsive metasurfaces. In the terahertz or infrared regimes, adjusting the temperature of VO₂ can significantly modulate the absorption peak position, intensity, and bandwidth, enabling adaptive absorption performance under complex operating conditions.

In addition to VO₂, certain thermally sensitive semiconductors also exhibit excellent temperature response characteristics. For instance, indium antimonide (InSb) has a plasma frequency in the terahertz regime that is highly sensitive to temperature. Its carrier concentration and mobility change significantly with temperature, leading to a reconstruction of the dielectric function and, consequently, dynamic adjustment of the absorber’s reflection/transmission properties. Similarly, strontium titanate (STO) possesses a tunable dielectric constant in the low-to-intermediate temperature range and has been widely employed in temperature-regulated metamaterial devices. These materials hold great promise for designing high-frequency thermally tunable metasurfaces [103,104,105,106]. Compared with electric or magnetic field regulation, temperature regulation does not require continuous excitation; once the target temperature is stabilized, the material’s electromagnetic response remains constant. This makes it suitable for steady-state thermal management, infrared stealth, and environment-adaptive systems. However, the method also has notable drawbacks, including slow response speed, complex temperature control systems, and limited tuning precision. To address these limitations, recent research has focused on developing novel thermally sensitive materials with low thermal inertia and high response speed, or on coupling temperature regulation with electrical, optical, or magnetic tuning mechanisms to enhance overall system performance.

3.5. Mechanical Regulation

Mechanical regulation modifies the geometry of a metasurface through external mechanical forces, thereby adjusting its electromagnetic response. The core principle lies in physically deforming the metasurface’s structural elements—such as altering unit-cell spacing, size, or arrangement—to achieve tunability. This method is intuitive, straightforward to implement, and particularly suited for low-frequency applications and designs requiring a large tuning range. Among existing approaches, microelectromechanical systems (MEMS) technology is one of the most mature and widely adopted strategies for mechanical regulation [107,108,109,110]. MEMS devices can precisely control microscale displacement or rotation of the metasurface structure, enabling dynamic adjustment of its properties. For example, in reconfigurable resonators, micro-actuators can modify the aperture angle or inductive path of an electromagnetic structure, effectively changing its resonance frequency and absorption strength. MEMS devices not only offer high integration and controllability but can also be seamlessly integrated with existing microelectronic platforms, providing a solid foundation for miniaturized, mass-producible, tunable metasurfaces.

Flexible substrates and deformable structural designs further expand the versatility of mechanical regulation [111,112,113,114,115,116]. By incorporating elastic polymers such as polydimethylsiloxane (PDMS) or stretchable two-dimensional materials, metasurface patterns can be transferred onto flexible backplanes, enabling stress-induced modulation at the macroscale. Deformation is not limited to planar stretching; three-dimensional transformations can also be achieved through origami-inspired folding or deployable mechanisms, substantially extending the tuning range and complexity. For instance, stretching a flexible substrate can alter the periodic spacing of the structure to dynamically control reflectance or transmittance, while folding/wrinkling structures can function as on–off switches for absorption performance. Such designs have demonstrated excellent potential in broadband operation, wide-angle incidence adaptation, and wearable electromagnetic devices.

Despite the clear advantages of mechanical regulation in terms of design freedom and deformation amplitude, its limitations are equally apparent. The response speed is generally much slower than that of electrical or optical tuning, as the process involves macroscopic mechanical motion, making it unsuitable for high-frequency or ultrafast real-time control [117]. In addition, mechanical systems are susceptible to environmental factors such as vibration, temperature fluctuations, and material fatigue, which may degrade repeatability and stability over long-term operation [118]. Furthermore, achieving mechanical deformation control at the micro/nanoscale remains technically challenging, restricting its applicability in high-frequency, subwavelength structures. Nonetheless, mechanical regulation offers a direct tuning strategy based on structural deformation, making it particularly advantageous for low-frequency bands, large frequency shifts, and reconfigurable device designs [119]. When combined with electrical, magnetic, optical, or thermal mechanisms, mechanical regulation has the potential to play a key role in multi-field synergistic tunable absorbers, paving the way for high-performance, adaptive, and intelligent electromagnetic response systems.

3.6. Comparative Discussion

The five principal mechanisms outlined above—electrical, magnetic, optical, thermal, and mechanical control—collectively illustrate the diverse strategies available for achieving tunability in metasurface absorbers. Each method exhibits distinct advantages and limitations when evaluated in terms of response speed, modulation depth, integration compatibility, and energy consumption. For instance, electrical tuning based on graphene, varactors, or liquid crystals offers fast response and fine precision but often requires continuous biasing. Magnetic tuning provides stable frequency adjustment in the microwave regime but remains less effective in higher-frequency bands due to material and fabrication constraints. Optical tuning achieves ultrafast modulation on the picosecond scale, making it suitable for THz and infrared regimes, yet its reliance on external laser excitation increases system complexity. Temperature regulation offers large tuning ranges through the use of thermally sensitive materials, though it typically suffers from slower response speeds. Mechanical tuning, meanwhile, enables intuitive structural reconfiguration and broadband adaptability, but its macroscopic deformation mechanisms are inherently limited in terms of speed and long-term stability.

Within this broader landscape, phase-change materials (PCMs) such as VO₂ and GST deserve particular emphasis as they introduce unique functionalities that transcend individual categories. On the one hand, PCMs can be viewed as thermal tuning media, since their phase transition is often triggered by temperature; on the other hand, their bistable and nonvolatile characteristics provide advantages unattainable by conventional thermally responsive materials. Specifically, once a PCM undergoes a phase transition, the new state can be maintained without continuous external excitation, enabling energy-efficient memory-like behavior. This property allows PCM-integrated absorbers to combine the wide tuning ranges of thermal control with the retention and programmability features crucial for intelligent photonic devices. At the same time, PCMs face challenges such as relatively high transition thresholds, limited cycling endurance, and trade-offs between modulation depth and speed.

Taken together, the comparative analysis suggests that no single tuning mechanism is universally optimal across all spectral regimes or application scenarios. Instead, hybrid strategies that integrate PCMs with electrical, optical, or mechanical methods appear particularly promising. Such multi-field cooperative tuning can compensate for the deficiencies of individual mechanisms—for example, combining the ultrafast response of optical excitation with the large-range bistability of PCMs. Looking ahead, the convergence of advanced functional materials, nanoscale structural engineering, and synergistic multi-physics control is expected to play a decisive role in pushing metasurface absorbers toward broadband, energy-efficient, and programmable operation.

4. Tunable Absorbers in the Microwave Band

As a critical portion of the electromagnetic spectrum, the microwave band holds significant value for both defense stealth technology and civilian communication systems. The technological development of tunable absorbers in this band is highly diverse [120,121,122,123,124,125,126,127]. This section systematically reviews the current research status, operating principles, representative implementation schemes, and performance characteristics of the main technical approaches for tunable microwave absorbers, providing design and application references.

4.1. Electrically Tunable Absorbers in the Microwave Band

Owing to their rapid response and flexible tuning capabilities, electrically tunable absorbers occupy an essential position in microwave-band dynamic stealth and reconfigurable communication systems. The most widely adopted approach involves integrating active electronic components, primarily PIN diodes and varactor diodes. Technological advancements in this field generally follow two main paths: discrete tuning based on PIN diodes and continuous tuning based on varactor diodes. This section presents a systematic analysis of both approaches.

In 2013, Wangren Xu et al. [128] innovatively proposed a tunable metamaterial reflector/absorber based on PIN diodes. As shown in Figure 3a, the design integrates PIN diodes into the unit cells of an electromagnetically coupled resonator array. By switching the diodes between forward-biased (ON) and reverse-biased (OFF) states, the electromagnetic response of the metamaterial can be dynamically tuned. The experimental results revealed that, when the diode was reverse-biased, the structure exhibited a “cut-off resonance” at 4.1 GHz; when forward-biased, the resonance frequency shifted to 4.785 GHz. This simple electrical control approach demonstrated engineering potential for applications such as dynamic radar cross-section (RCS) control, reconfigurable antennas, and electromagnetic shielding. Following continued research, broadband-tunable absorbers for the microwave band were developed. In 2017, Shaonan An et al. [129] designed and fabricated a polarization-insensitive, broadband-tunable metamaterial absorber based on split semi-ring resonators (SSRRs), as shown in Figure 3b. The absorber comprised four layers: an FR-4 substrate, an active frequency-selective surface (AFSS), a honeycomb isolation layer, and a copper ground plane. Each unit cell contained four copper SSRRs rotated by 90°, with a PIN diode integrated at the gap of each resonator to achieve impedance matching. The absorber achieved a reflectivity below −10 dB in the 2.1–2.5 GHz range, and by adjusting the diode bias voltage, the operational band could be tuned to 3.1–6.8 GHz. Experimental validation confirmed its broadband tunability and potential in stealth and wireless communication applications.

In China, the research group led by Professor Weiren Zhu at Shanghai Jiao Tong University [130] proposed a novel switchable metasurface based on PIN diodes, as shown in Figure 3c. The design combined alternating square copper loops on the top layer with a specially shaped square loop on the bottom layer, enabling nearly perfect reflection, transmission, and absorption states. By independently controlling the bias voltages of top-layer diodes (PIN1) and bottom-layer diodes (PIN2), a three-state switching mechanism was realized: Both PINs OFF → high absorption (94.7% at 7.5 GHz), Only PIN1 ON → high reflection; both PINs ON → high transmission. This triple-state switching offers a new pathway for multifunctional electromagnetic wave control. Unlike PIN diodes, varactor diodes enable continuous tuning by altering the reverse bias voltage, thereby changing the junction capacitance. In 2006, Shadrivov et al. [131] first demonstrated the use of varactor diodes for metamaterial tuning. As shown in Figure 3d, a varactor was integrated in series at the point of maximum current in a split-ring resonator (SRR) on an FR-4 substrate. By varying the DC bias, the depletion layer width—and thus the junction capacitance—was adjusted, enabling both frequency tuning and nonlinear control. Experimental results showed that, when the bias voltage varied from +1 V (forward) to −10 V (reverse), the resonance frequency shifted from 2.27 GHz to 2.9 GHz, covering a tuning range of 26%. This work provided a crucial experimental foundation for tunable and nonlinear metamaterials in dynamic microwave stealth and adaptive devices.

In 2013, Professor Tiejun Cui’s team [132] at Southeast University proposed a polarization-insensitive tunable absorber based on varactor diodes, as shown in Figure 3e. Symmetrical electric–LC (ELC) resonators and varactors were integrated on a dielectric substrate, with two bias lines on the bottom layer supplying voltage via through-holes. By increasing the reverse bias from 0 V to −19 V, the resonance frequency shifted continuously from 4.45 GHz to 5.64 GHz, while maintaining an absorption rate above 90% and polarization insensitivity. This design effectively utilized the nonlinear capacitance of varactors to tune equivalent circuit parameters. Professor Weiren Zhu’s group also developed an electrically tunable metasurface combining graphene and varactor diodes [133], as shown in Figure 3f. The structure featured a graphene–electrolyte–graphene sandwich layer on top, a 2 mm dielectric spacer in the middle, and a varactor-loaded high-impedance surface at the bottom. This asymmetric configuration enabled dual-degree-of-freedom tuning: the graphene layer controlled absorption depth, while the varactor array tuned resonance frequency. The device allowed for resonance tuning between 3.41 GHz and 4.55 GHz and reflection amplitude tuning between −3 dB and −30 dB. By replacing the active phase-change material, this architecture could be extended to the terahertz and optical bands. To summarize briefly, in the microwave band, electrical tuning primarily follows two routes: PIN diode-based discrete switching and varactor diode-based continuous tuning. PIN diodes enable on/off state transitions to switch between absorption and reflection, offering simplicity and ease of control—ideal for dynamic RCS management. In contrast, varactor diodes allow fine-grained frequency control by adjusting the bias voltage, making them especially advantageous for achieving nonlinear responses and broadband tuning.

4.2. Magnetically Tunable Absorbers in the Microwave Band

In addition to electrical tuning, magnetic tuning represents another important approach for microwave absorbers. By applying an external magnetic field, magnetic tuning enables linear and stable frequency adjustment, making it particularly suitable for microwave applications. The most commonly used magnetically tunable materials are ferrites—magnetic oxides composed of iron-group elements combined with one or more additional metallic elements [134,135]. These materials possess considerable application potential in tunable electromagnetic devices. Systematic research on magnetically tunable devices began in 2008, when Kang [136] innovatively proposed a magnetically tunable negative-permeability metamaterial, as illustrated in Figure 4a. In this design, yttrium iron garnet (YIG) ferrite rods were integrated with an array of split-ring resonators (SRRs). By exploiting the ferromagnetic resonance of YIG to actively control the effective environmental permeability, the equivalent inductance LLL of the SRR could be continuously adjusted, thereby achieving bidirectional, continuous, and reversible frequency shifts. Subsequently, in 2009, Zhao et al. [137] further combined YIG ferrite rods with metallic wires, realizing broadband, dynamic, continuous, and reversible magnetic tunability of a left-handed passband, as shown in Figure 4b. In this structure, when a magnetic field was applied along the long axis of the ferrite rods and increased from 1600 Oe to 2300 Oe, the central frequency shifted from 8.2 GHz to 10.7 GHz, corresponding to a tuning rate of 3.5 GHz/kOe. These two pioneering works established the fundamental design concept for magnetically tunable absorbers: replacing structural reconfiguration with the intrinsic magnetic response of materials—a paradigm still central to research in this field today.

Subsequent studies began incorporating ferrite materials directly into absorber designs for magnetic tuning. For example, HUANG Y. J. et al. [138] achieved early dynamic control of microwave-band absorbers, as shown in Figure 4c. This absorber leveraged the synergistic effect of ferrite magnetic resonance and copper-wire electric response to achieve efficient absorption. Under a DC bias magnetic field, the ferrite generated a tunable negative permeability, with resonance frequency linearly adjustable within 2.6–3.4 kOe. Meanwhile, the copper wires exhibited high-loss dielectric characteristics in the 10–13 GHz range, enabling impedance matching in conjunction with the ferrite. Experimental results showed that the single-layer structure achieved 98.2% absorption at 9.9 GHz, with a >90% absorption bandwidth of 2.3 GHz. The double-layer configuration further increased absorption to 99.97%. By varying the magnetic field, the absorption peak was linearly tuned from 6.75 GHz to 11.75 GHz at a rate of 2.625 MHz/Oe, demonstrating excellent broadband tunability. This compact configuration offers a promising strategy for developing dynamically tunable, broadband metamaterial absorbers.

Huang [139] further explored integrating ferrites with conventional absorbers, as shown in Figure 4d. Two configurations were investigated: one using ferrite as the substrate (MA1) and another using ferrite as a cover layer (MA2), both employing electric–LC (ELC) resonator arrays with optimized geometry for impedance matching. Experiments revealed that, with magnetic field adjustment, the MA1 resonance frequency blue-shifted from 10.99 GHz to 11.29 GHz (tuning rate: 0.36 MHz/Oe), while MA2 tuned from 9.52 GHz to 9.66 GHz (tuning rate: 0.18 MHz/Oe), both maintaining high absorption throughout. Further analysis showed that the thicknesses of the ferrite and FR4 layers significantly affected tuning performance: a thinner FR4 layer enhanced sensitivity but reduced the high-absorption bandwidth. Through optimization, the MA2 structure ultimately achieved a relative tuning range of 11.5% (8.6–9.65 GHz) with consistently high absorption.

Li et al. [140] combined ferrite materials with a periodic metallic-strip metasurface to realize magnetically tunable near-perfect absorption over an ultrawide 0.2–7.6 GHz range, as depicted in Figure 4e. This innovative design simultaneously suppressed ferrite-induced polarization conversion and enhanced absorption efficiency via diffraction and interference mechanisms. Both simulation and experimental results indicated absorption peaks exceeding 0.9, with absorption frequency exhibiting a strong linear relationship with magnetic field strength. This “ferrite dynamic response + metasurface field localization enhancement” strategy provides a paradigm for developing multifunctional materials that combine broadband tunability, strong absorption, and polarization stability, with promising applications in electromagnetic stealth, intelligent shielding, and adaptive communication systems. Building on this, the same team designed a magnetically tunable perfect absorber consisting of a ferrite-rod array and metallic ground plane [141], as shown in Figure 4f. By integrating a YIG ferrite-rod array with a conductive backing, they achieved highly efficient, magnetically controlled electromagnetic wave absorption. Both simulations and experiments confirmed that, within the 8–12 GHz range, tuning the applied magnetic field from 2000 Oe to 2400 Oe continuously shifted the absorption peak from 8.97 GHz to 10.02 GHz, with maximum absorption reaching 99.9%, further validating the feasibility of ferrite-based metamaterial absorbers.

4.3. Mechanical Controlled Absorbers for Microbands

Mechanical tuning methods offer unique advantages in the microwave band by enabling dynamic modulation of electromagnetic properties solely through physical deformation, without reliance on complex electronic components or external excitations. In 2019, Dinh Hai Le and Sungjoon Lim [142] first proposed a programmable metamaterial based on a ternary foldable origami structure. This study integrated mechanical reconfiguration with digital coding, achieving four programmable modes using only a two-layer structure composed of dielectric paper and a conductive backing. By precisely controlling the mechanical deformation of the origami, electromagnetic functionality was reconfigured. As shown in Figure 5a, three fundamental folding states were defined: unfolded (code “0”), short fold (code “1”), and long fold (code “2”), constructing a discretized ternary coding system. The spatial coding sequences generated four typical operation modes: a uniform “000…” sequence produced full reflection; periodic “101…” and “202…” sequences resulted in single-band absorption peaks at 8.6 GHz and 4.3 GHz, respectively; and the alternating “201…” sequence yielded dual-band absorption characteristics. This coding-to-function mapping was realized by equivalent circuit parameter changes induced by the folding deformation. Simulations and experiments demonstrated strong agreement, showing that in Mode II, the sample exhibited over 90% absorption at 9.0 GHz; Mode III achieved a peak absorption of 97.5% at 4.5 GHz; and Mode IV realized dual-band absorption with efficiencies of 92% and 88.7% at 4.2 GHz and 8.7 GHz, respectively. This design offers a low-cost, easily manufacturable approach to reconfigurable electromagnetic devices, mechanical metamaterials, and intelligent stealth technologies.

In 2021, Prof. Zou Yanhong’s team at Hunan University [143] proposed an origami-based reconfigurable ultra-wideband microwave absorber. By innovatively combining conductive polylactic acid (PLA) material with origami folding, the system, depicted in Figure 5b, formed four structural modes: a planar thin sheet (M1) with negligible absorption; a single-arched fold (M2) producing a single absorption peak at 16.8 GHz; a double-arched fold (M3) achieving dual peaks at 12.7 GHz and 15.3 GHz due to separate resonances on the inner and outer walls; and a double-arched fold filled with U-shaped strips (N) producing ultra-wideband absorption spanning 3.4–18 GHz. Experimental results closely matched simulations, with M3 maintaining reflection losses below −10 dB over 10.5–18 GHz and N achieving ultra-wideband absorption from 3.7 to 18 GHz. Although fabrication errors caused approximately 1 GHz frequency shifts, all modes maintained peak absorption above 90%. Compared to traditional voltage- or magnetic-driven tunable absorbers, this purely mechanical tuning approach demonstrates significant advantages in bandwidth performance.

Myungjin Chung et al. [144] in 2022 developed a MEMS-based tunable frequency millimeter-wave metamaterial absorber operating in the Ka-band (25.5–28 GHz). As shown in Figure 5c, the absorber consists of symmetric split-ring resonators (SRRs) integrated with MEMS cantilever arrays exhibiting initial out-of-plane deflections due to stress gradients. Applying a 0–15 V DC bias electrostatically actuates the cantilevers, altering the SRR equivalent capacitance and tuning the absorption frequency continuously from 28 GHz (initial state, 72.9% absorption) down to 25.5 GHz (fully pulled-down state, 99.9% absorption). The design was optimized through full-wave electromagnetic simulations to achieve impedance matching and fabricated using microfabrication techniques on glass wafers, including photoresist sacrificial layers and oxygen plasma etching for cantilever release. Although nonuniform bending introduced deviations from ideal tuning characteristics, the device demonstrated significant dynamic absorption modulation in the millimeter-wave regime.

Recently, Xiang Li et al. [145] developed an elastic carbon aerogel microwave absorbing material derived from chitin nanofibers. Its anisotropic honeycomb porous structure provides excellent mechanical resilience and electromagnetic tunability. Prepared by carbonization at 800 °C, the aerogel exhibits optimized N/O doping defect structures, achieving good initial impedance matching and moderate dielectric loss. The material demonstrates up to 80% elastic strain along vertical porous channels and maintains fatigue stability over 60,000 compression cycles. As shown in Figure 5d, mechanical compression modulates thickness to continuously tune the maximum absorption frequency in the X-band from 10.4 GHz to 12.1 GHz, maintaining a high absorption intensity near 40 dB. Further compression switches the material from strong absorption to transmission mode, presenting a promising approach for intelligent adaptive microwave absorbers.

Inspired by the chameleon photonic crystal color-changing mechanism, Dahyun D. Lim et al. [146] developed a tunable microwave metamaterial absorber based on a cross-lattice structure. Figure 5e illustrates the design composed of carbon black/PLA composite lattices connected by polymers, which mechanically rotate to reversibly morph between compressed and expanded states. This enables dynamic switching between broadband absorption and transmission modes over 4–18 GHz. The compressed state features an anisotropic honeycomb porous structure, achieving an average broadband absorption of 96.4%, while the expanded state reaches a peak transmission of 24.2%. A data-driven design combining neural network surrogate models and genetic algorithms effectively optimized complex geometric parameters. Experimental results confirmed a relative tuning range of 101.9% within 5.85–18 GHz, with array synchronization enabled via linkage mechanisms.

Qiao S et al. [147] proposed a gradient-structured rotationally tunable ultra-wideband microwave absorber (GRTMA) fabricated by 3D printing, composed of two gradient-distributed layers of unit cells. Rotation of the upper layer dynamically modulates the electromagnetic response, achieving over 90% absorption across 2.56–18 GHz and a bandwidth of 15.44 GHz. Structurally, ferromagnetic materials A and B fill the lower and upper layers, respectively, with periodic fan-shaped columns of varying height gradients. Rotating the upper layer rearranges the height distribution to enable reversible switching between dual absorption peaks (3.4 GHz and 4.44 GHz) and a single peak (3.84 GHz). Impedance matching analysis indicates that equivalent impedance near resonance approximates free space impedance, with magnetic field localization and quarter-wavelength resonance as primary absorption mechanisms. Experimental validation confirmed stable absorption under TE polarization (θ < 30°) and TM polarization (φ < 75°). This innovative design integrates tunable absorbers with coded metamaterials, producing a 180° ± 37° phase difference through material filling variations and achieving a radar cross-section reduction bandwidth of up to 3.5 GHz in the C-band. Compared to conventional fixed coded metamaterials, this mechanically rotated phase modulation significantly enhances coding flexibility.

5. Tunable Absorbers in the Terahertz Band

Terahertz (THz) waves (0.1–10 THz) occupy the transitional region of the electromagnetic spectrum between microwaves and infrared radiation. The long-wavelength range (0.1–1 THz) significantly overlaps with the microwave band, whereas the short-wavelength range (1–10 THz) closely connects with the infrared regime. Because this frequency span cannot be fully described by conventional optical theories nor effectively addressed by microwave techniques, the THz band has long suffered from the lack of efficient sources, detectors, and tunable mechanisms—an issue widely recognized as the “THz gap” [148,149]. Since the proposal of the metamaterial perfect absorber, remarkable progress has been achieved in this field. Device performance has evolved from the initial single-frequency response to dual-band, multi-band, and broadband absorption characteristics [150,151]. In terms of material platforms, research has expanded from traditional noble metals to novel tunable functional materials such as graphene, liquid crystals, and phase-change media [152,153,154,155,156,157]. These advances provide new technological pathways to address the core challenges of THz technology.

5.1. Electrically Tunable Absorbers in the Terahertz Band

At terahertz frequencies, metasurface feature sizes reach the micron scale, limiting the applicability of conventional diodes. Researchers have found materials such as graphene and Dirac semimetals possess excellent properties, inspiring recent efforts to design high-performance tunable terahertz absorbers using these materials. In 2013, Borislav et al. first proposed a graphene-based tunable metamaterial absorber replacing traditional metal resonators [158]. In 2017, Chen’s research group [159] presented a tunable single-frequency absorber based on a graphene metasurface. As shown in Figure 6a, the top layer consists of periodically arranged composite graphene metasurface units, each comprising a ring structure (outer diameter 1.4 μm) with an embedded cross-shaped resonator. By tuning graphene’s Fermi level, the absorption frequency can be continuously adjusted from 2.06 to 2.71 THz. Further, dual-layer stacking of metasurfaces achieved dual-band absorption at 1.99 THz and 2.69 THz with absorption rates of 98.94% and 99.10%, respectively, both demonstrating excellent tunability. In the same year, Yao et al. [160] proposed a dual-band tunable perfect metamaterial absorber based on a graphene elliptical nanodisk array (Figure 6b). This structure comprises a single layer of periodically arranged graphene elliptical nanodisks and a metal ground plane separated by a SiO₂ dielectric layer. By tuning the graphene Fermi level (0.3–0.5 eV), near-perfect absorption of 99% and 97% was achieved at 8.57 THz and 5.08 THz, respectively, with polarization insensitivity and stable performance for wide incidence angles (±60°). The absorption mechanism originates from the synergistic effect of localized surface plasmon resonance excited along the short and long axes of the elliptical nanodisks and magnetic dipole resonance. Additionally, tuning electron relaxation time independently controls the absorption intensity. This single-layer graphene design simplifies fabrication and highlights graphene’s potential in terahertz sensing and optoelectronic modulation.

Due to bandwidth limitations of graphene absorbers, researchers have sought breakthroughs in wideband, high-efficiency tunable terahertz absorbers for stealth applications. In 2020, Juzheng Han et al. [161] proposed a single-layer graphene metasurface with a hollow square graphene pattern, connected by continuous narrow strips to maintain electrical continuity (Figure 6c). By adjusting the chemical potential (0–0.9 eV), the absorber achieves a fractional bandwidth of 97.5% with peak absorption near 100% over 1.14–3.31 THz. The broadband performance stems from plasmonic coupling between the inner and outer parts of the hollow pattern, with symmetric structure ensuring polarization insensitivity and wide-angle stability. Compared to earlier dual-band designs, this structure simplifies fabrication while improving polarization and angular performance, advancing terahertz stealth and sensing applications.

Despite progress with graphene-based absorbers, their atomic-scale thickness limits terahertz wave absorption and coupling strength, constraining device design freedom and tunability. Dirac semimetals overcome these drawbacks, as they are three-dimensional topological materials often regarded as “3D graphene”. In 2018, Liu et al. [162] proposed a single-band tunable terahertz absorber based on a Dirac semimetal. The structure (Figure 6d) consists of a photonic crystal slab with circular air holes and a Dirac semimetal thin film. By tuning the Fermi level from 50 meV to 80 meV, the absorption frequency is actively tuned from 1.381 THz to 1.395 THz, maintaining absorption above 95%. Subsequently, Zhang et al. [163] designed a dual-band tunable terahertz absorber using a composite structure of a ring dielectric layer and Dirac semimetal reflector (Figure 6e). Absorption rates exceeding 99% were achieved at 2.02 THz and 2.49 THz. By tuning the Dirac semimetal Fermi level, the absorption peak shifted from 1.955 THz to 2.05 THz, with dynamic absorption tuning from 90% to 99%.

Besides graphene and Dirac semimetals, liquid crystal (LC) materials have attracted considerable attention in terahertz tunable absorbers recently. In 2013, Padilla et al. [164] integrated nematic liquid crystals into metamaterial unit cells to realize the first fully electronically tunable terahertz perfect absorber. In 2017, Jing et al. [165] designed and fabricated an LC-based tunable metamaterial absorber in the F band, which partially overlaps with the terahertz range. The absorber uses a quartz/metal resonator/LC/quartz sandwich structure, with a periodic circular hole array (period 1600 μm, diameter 500 μm) serving as the top electrode and a metal layer as the bottom electrode. By applying 0–9 V bias, the LC molecular orientation—and hence dielectric constant—was tuned from 2.61 to 3.03. The device exhibited stable performance under 60° oblique incidence, with absorption above 90% and resonance frequency tunable from 110.9 GHz to 103.8 GHz (6.4% tuning rate). This integrated electrode-resonator design significantly reduced driving voltage and improved tuning performance and process compatibility, offering a promising approach for tunable terahertz absorbers. However, the absorber supported only single polarization (TM waves), leaving polarization dependence unresolved.

In 2018, Yin et al. [166] further addressed polarization limitations by designing a narrowband tunable absorber based on asymmetric ring resonators (Figure 6f). By tuning LC molecular orientation via 0–10 V bias, polarization-dependent frequency tuning was achieved: TE-polarized absorption peak red-shifted from 239.5 GHz to 228.1 GHz (4.7% tuning bandwidth), while TM-polarized peak shifted from 306.6 GHz to 294.0 GHz (4.1% tuning bandwidth). This development provides a novel multidimensional control method for terahertz polarization imaging and multiplexed sensing. These breakthroughs signify the transition of LC-tunable absorbers from basic performance optimization to multifunctional integration, surpassing earlier F-band absorbers in adaptability and integration in complex electromagnetic environments.

Figure 6. Electrically−controlled absorbers in the terahertz band: (a) Chen’s metasurface unit structure based on graphene metasurfaces [159]; (b) Metasurface unit structure based on graphene elliptical nanodisk array [160]; (c) Juzheng Han designed a metasurface unit structure based on graphene [161]; (d) Liu designs structural units based on Dirac semimetals [162]; (e) Zhang designs adjustable absorbers based on Dirac semimetals [163]; (f) Yin design based on liquid crystal narrowband tunable absorber [166].

Figure 6. Electrically−controlled absorbers in the terahertz band: (a) Chen’s metasurface unit structure based on graphene metasurfaces [159]; (b) Metasurface unit structure based on graphene elliptical nanodisk array [160]; (c) Juzheng Han designed a metasurface unit structure based on graphene [161]; (d) Liu designs structural units based on Dirac semimetals [162]; (e) Zhang designs adjustable absorbers based on Dirac semimetals [163]; (f) Yin design based on liquid crystal narrowband tunable absorber [166].

5.2. Temperature-Tunable Absorbers in the Terahertz Band

Research on thermally excited terahertz absorbers has made significant breakthroughs in recent years, with temperature-tuning technologies based on phase-change materials exhibiting unique advantages. Strontium titanate (STO), known for its excellent temperature-sensitive properties, has attracted considerable attention. In early studies, Luo et al. [167] designed a thermally tunable dual-band terahertz absorber based on STO. As shown in Figure 7a, the absorber unit consists of two nested metal square ring resonators, an STO dielectric layer, and a metal backing plate. Through optimized design, excellent absorption performance was achieved. Experimental results indicate that at room temperature, the absorber exhibits absorption rates of 97% and 75% at 0.096 THz and 0.137 THz, respectively. The temperature sensitivity of the STO dielectric layer endows the device with significant tunability: as the working temperature decreases from 400 K to 250 K, the two absorption peaks shift by 25% and 27%, respectively; when further reduced to 150 K, the frequency tuning exceeds 53%.

In subsequent research, the team innovatively improved the original structure [168]. As shown in Figure 7b, by trimming the four corners of the outer square ring, a triple-band absorption characteristic was successfully realized. The improved absorber exhibits three distinct absorption peaks at room temperature located at 0.129 THz, 0.198 THz, and 0.316 THz, with absorption rates of 99.3%, 99.1%, and 94.6%, respectively. Temperature-dependent studies reveal a pronounced blue shift in the three resonant peaks when the temperature increases from 200 K to 400 K, with tuning depths of 67.3%, 37.2%, and 38.3%, respectively. The design was miniaturized using low-temperature co-fired ceramics (LTCC) technology, and its wide temperature tunability offers new solutions for terahertz switches, filters, and stealth applications.

The team led by ZHENG Wei [169] proposed a thermally tunable terahertz metamaterial absorber based on InSb semiconductor material, featuring a metal-dielectric-metal three-layer structure. This absorber demonstrates excellent tuning performance over the temperature range of 160 K to 350 K, with the absorption frequency dynamically tuned from 0.82 THz to 1.02 THz and a maximum absorption rate of 99.8%. The full width at half maximum (FWHM) is only 38 GHz. As depicted in Figure 7c, the structure consists of a 40 μm side-length electric ring resonator on top, a 7.2 μm thick silicon dioxide dielectric layer in the middle, and a 200 nm metal backing plate at the bottom, with a 200 nm thick InSb material embedded in the resonator gap. By temperature tuning the dielectric properties of InSb and using an equivalent LC circuit model to match the resonant frequency, near-perfect absorption is achieved at 320 K. Simulations show that the electric field energy is mainly concentrated in the InSb-filled gap and the four corners of the electric ring resonator, confirming that thermal tuning of material properties enables dynamic frequency modulation.

Among phase-change materials, vanadium dioxide (VO₂) is particularly notable due to its relatively low phase transition temperature (~341 K), making it highly promising for optoelectronic devices. The phase transition in VO₂ is a first-order Mott transition, driven by electron-electron correlations. At the atomic level, heating above the critical temperature causes a structural change from a low-temperature monoclinic (insulating) phase to a high-temperature rutile (metallic) phase. This rearrangement of vanadium atom dimers leads to a drastic increase in electron density near the Fermi level, transforming the material from a band insulator to a correlated metal. Consequently, the real part of the dielectric constant (ε’) changes from positive (dielectric) to negative (metallic), while the imaginary part (ε”) exhibits a significant increase, indicating enhanced ohmic losses. In 2015, Naorem et al. [170] proposed a temperature-tunable metamaterial absorber using VO₂ as the ground plane. The schematic is shown in Figure 7d. The metamaterial unit consists of a top Au layer, a ZnS dielectric layer in the middle, and a VO₂ ground plane at the bottom. VO₂ behaves as a dielectric at low temperatures, rendering the structure a frequency-selective surface. Upon heating above 341 K, VO₂ transitions to a metallic state, forming a metamaterial absorber capable of simultaneously exciting electric and magnetic resonances. At 22.5 THz, the VO₂ phase transition reduces reflectivity from 35% (low temperature) to 8% (high temperature); conversely, at 34 THz, reflectivity increases from 15% to 55%. This clearly demonstrates the temperature-tunable electromagnetic properties of metamaterials achieved by integrating VO₂.

Zhao Zhang’s team [171] innovatively combined InSb with an ultra-thin dielectric layer. As shown in Figure 7e, the structure uses a 1.37 μm ultra-thin polyimide layer as the dielectric, 600 μm thick n-type InSb as the temperature-sensitive layer, and a 200 nm aluminum film as the reflective backing. In the temperature range of 250–320 K, the absorption peak blue shifts from 1.41 THz to 3.29 THz, while maintaining absorption above 99%. Theoretical analysis indicates that impedance matching is achieved via nontrivial phase shifts at the dielectric–substrate interface. The absorption peak position exhibits high tolerance to dielectric thickness variations; even a 100% thickness deviation maintains absorption above 99%. Subsequent experiments verified polarization insensitivity and over 90% absorption efficiency at large incident angles up to 60°. Moreover, the thermal stability of polyimide in the terahertz band ensures tuning reliability.

Zhang et al. [172] proposed a thermally tunable terahertz metamaterial absorber based on ring dipole coupling effects. As illustrated in Figure 7f, the design integrates a 12-fold symmetric trapezoidal metal ring array with VO₂ phase-change material, achieving ultra-broadband absorption from 2.38 THz to 21.13 THz and a maximum absorption tuning depth of 0.57. Multipole analysis indicates that ring dipole scattering power exceeds that of conventional electric/magnetic dipoles by 5–7 orders of magnitude, dominating the absorption process. Impedance matching is maintained across 2.4–21.1 THz, with excellent polarization insensitivity and wide-angle stability. This combination of ring dipole interference and thermal phase-change tuning offers a novel strategy for developing intelligent tunable terahertz absorbers.

Besides VO₂, phase-change material GST also draws attention. The tunability of GST stems from its reversible phase transition between amorphous and crystalline states. In the amorphous phase, GST behaves as a semiconductor with a wide bandgap (~0.7 eV) and high resistivity. Crystallization, induced by thermal annealing, leads to the formation of a metastable rocksalt structure. This process reduces the bandgap and drastically increases the free electron density, thereby enhancing the material’s electrical conductivity and altering its complex permittivity. The change in the dielectric function (Δε) is particularly pronounced in the infrared regime, enabling effective modulation of optical and electromagnetic properties. Unlike VO₂, which requires continuous external stimulation to maintain its phase, GST exhibits nonvolatile behavior—its crystallinity remains stable without external excitation until deliberately changed. This unique nonvolatile property makes GST highly valuable in dynamic tunable devices. During phase transitions, GST’s resistivity changes significantly, enabling control over metasurface electromagnetic responses. Guo Linyang et al. [173] designed a GST-based absorber as shown in Figure 7g, comprising a silicon substrate and a GST cylindrical array. By thermally controlling GST crystallization, dynamic switching between two absorption peaks (1.98 THz and 5.88 THz) and an absorption valley (V1) is realized, overcoming the instability limitations of traditional temperature-sensitive materials. Experiments show that heating to 150 °C increases the P2 absorption from 51% to 65%, with V1 persistently overlapping the P2 resonance, forming a stable “ON–OFF” switch. Structural parameter studies indicate that increasing GST cylinder diameter by 0.4 μm raises P1 absorption to 95%, while thickness variation mainly affects magnetic resonance modes.

Liu Hongyao’s team [174] proposed a switchable dual-tunable terahertz absorber based on patterned graphene and VO₂. As shown in Figure 7h, the three-layer structure comprises patterned graphene, a VO₂-metal composite layer, and a SiO₂ dielectric layer. By tuning graphene’s Fermi level and VO₂ phase state, multifunctional switching is achieved. When VO₂ is in the insulating phase, the device switches from full reflection to ultra-broadband absorption (4.5–10.61 THz) with a relative bandwidth of 80.87% by modulating graphene’s Fermi level from 0 to 0.8 eV. When VO₂ transitions to the metallic phase, single- or dual-band absorption features emerge.

Li Dan’s team [175] designed a temperature-tunable terahertz metamaterial absorber based on STO and VO₂. As shown in Figure 7i, the device employs a metal–dielectric–metal three-layer structure comprising dual open-ring resonators, a silicon substrate, and a metal backplane. The device achieves 99% single-band absorption at 1.23 THz. Utilizing the temperature sensitivity of STO, the absorption peak blue shifts from 1.41 THz to 1.52 THz over 200–400 K, with a sensitivity of 0.00055 THz/K. When the backing plate is replaced with VO₂ and temperature exceeds the 340 K phase transition threshold, absorption rises sharply from 10% to 84%. Innovatively, the device achieves dual-band absorption by rotating the top resonator ring 45°, obtaining absorption rates of 100% and 97% at 1.28 THz and 1.43 THz, respectively.

5.3. Optically Tunable Terahertz Absorbers

In the field of terahertz (THz) modulation technology, although electrical and thermal tuning methods readily achieve high modulation depths, their modulation speeds are often constrained by intrinsic physical mechanisms of the materials, such as carrier drift velocity and thermal diffusion time constants. These limitations hinder meeting the demands for ultra-high-speed dynamic modulation. In contrast, semiconductor modulation techniques based on optical excitation exhibit speed limits primarily governed by carrier recombination lifetimes, enabling ultrafast responses on the order of picoseconds or even sub-picoseconds. By synergistically optimizing semiconductor material systems and metasurface designs, precise THz wave control with multifunctional and multiband capabilities can be realized while maintaining high modulation efficiency, making this an emerging focus in recent research.

In practical studies, several research groups have achieved significant breakthroughs through innovative structural designs. Zhengyong Song’s team [176] proposed a silicon-based sandwich-structured absorber metasurface, as illustrated in Figure 8a. This device employs a silicon thin film patterned with a cross-shaped air hole array as the top layer, where silicon conductivity (ranging from 0 to 2500 S/m) can be modulated by optical excitation or electrical injection. This achieves a continuous tunability of absorption from 1% to 100%. Experimental results demonstrate over 90% absorption across a broad frequency range of 0.497–1.045 THz, with a center frequency at 0.771 THz. The superior performance stems from impedance matching changes induced by silicon conductivity modulation, while the centrosymmetric cross-shaped design ensures polarization independence and excellent angular stability, maintaining above 80% absorption even at a large incident angle of 60° under both TE and TM polarizations.

The team led by Prof. Yongzhi Cheng from Wuhan University of Science and Technology [177] developed a broadband metasurface absorber based on square-ring photoconductive silicon, as shown in Figure 8b. This structure features a planar square-ring photoconductive silicon array as the resonant top layer, forming a sandwich structure with a 35 μm thick polyimide dielectric layer and a bottom gold ground plane. Without optical pumping, the device acts as a broadband reflector with reflectance exceeding 97.2%. Upon 800 nm optical pumping, silicon conductivity increases to 1.5 × 10⁵ S/m, transforming the device into a broadband absorber with over 90% absorption within 0.668–1.532 THz, achieving a relative bandwidth of 86.4% and a modulation depth of up to 97.2%.

In the domain of multistate tunable metamaterials, Li Damin’s group from Shanxi University [178] developed a multistate tunable THz metasurface absorber based on photosensitive semiconductors. Figure 8c depicts the nested square-ring design incorporating gallium arsenide (GaAs) and germanium (Ge) semiconductors within the ring gaps for dynamic control. Experimental data show that without optical pumping, the three rings operate independently, generating triple absorption peaks at 0.518, 0.906, and 1.514 THz with absorption rates exceeding 96%. When 1550 nm pump light excites Ge, the inner and middle rings couple to produce dual-band absorption at 0.524 and 1.106 THz. Under simultaneous 800 nm excitation of both GaAs and Ge, the three rings collectively produce a single absorption peak at 0.706 THz with an absorption rate of 99.91%. Surface current distribution analysis reveals that conductivity modulation of GaAs from 100 to 2 × 10⁵ S/m enables single- and dual-band switching, while Ge introduces tri-state tunability. The absorber’s fourfold symmetry maintains stable performance within polarization angles of 0°–45° and incident angles up to 60°, with absorption above 90% under both TE and TM modes.

Meng Qinglong’s team collaborated with Prof. Bin Zhang from Sichuan University [179] to develop a multi-band optically tunable THz metasurface absorber based on optically controlled GaAs. As shown in Figure 8d, the structure consists of four groups of metallic strip arrays of varying lengths, forming a sandwich structure with a 4.5 μm thick polyimide dielectric layer and a gold reflective bottom layer. GaAs fills the gaps between the metal strips to enable optical control. Without optical pumping, the four strip lengths (26, 30, 32, and 38 μm) generate four absorption bands with over 95% absorption at 2.96, 2.69, 2.43, and 2.15 THz, respectively. Upon 800 nm optical pumping, GaAs conductivity rises from 100 to 2 × 10⁵ S/m, converting the four-band absorption into dual-band absorption at 1.39 and 1.19 THz, with a modulation depth exceeding 90%.

Regarding novel optically controlled structures and fabrication processes, Chenglong Zheng [180] and collaborators developed a high-resistivity silicon-based optically controlled THz metasurface absorber, illustrated in Figure 8e. This device features an array of 45 μm side square silicon pillars forming a resonant structure with 60 μm height and an 80 nm gold reflective layer beneath. Optical control is achieved using a continuous 1064 nm laser. Experimental results reveal that increasing laser power from 0 to 1.46 W raises silicon conductivity, switching the absorber from full reflection to 99% absorption at 0.74 THz, with a 90% absorption bandwidth of 0.5 THz. Impedance matching analysis indicates that optical pumping enables device impedance to match free space impedance. By tuning the silicon pillar dimensions, the operational frequency range from 0.48 to 0.98 THz can be customized. This design avoids complex doping procedures and is fabricated via standard ICP etching technology, suitable for THz sensing and optoelectronic switching applications.

Professor Xiaoguang Zhao’s team, in collaboration with Professor Richard Averitt [181], developed an all-silicon H-shaped array for ultra-broadband THz metasurface absorption, as shown in Figure 8f. The structure features a 140 μm period array of H-shaped silicon pillars forming a resonant system with a 53 μm thick microstructure layer on a 300 μm silicon substrate. Measurements indicate 99.99% absorption at 0.97 THz, with a 90% absorption bandwidth of 913 GHz spanning 0.59 to 1.49 THz. Electric field distribution analysis shows that broadband absorption arises from the superposition of HE and EH hybrid modes. Under 800 nm femtosecond laser pumping, the absorption peak undergoes a 420 GHz blue shift while maintaining above 99% peak absorption. Simulations incorporating graded carrier concentration profiles confirm the optical control mechanism. The device is fabricated by standard CMOS processes, showing significant potential in THz sensing and imaging.

In summary, optically controlled THz modulation technology, through synergistic design of semiconductor materials and metasurfaces, achieves ultrafast response and high-efficiency multifunctional THz wave manipulation. These innovative studies not only deepen understanding of light–matter interaction mechanisms but also provide critical technological support for developing high-performance THz functional devices. Future work may focus on exploring novel semiconductor systems, optimizing metasurface designs, and advancing fabrication techniques to propel optical THz modulation toward higher performance and broader applications.

6. Tunable Absorbers in the Infrared Band

It is worth noting that the discussion of tunable absorbers in the microwave band is intentionally more extensive than that of the terahertz and infrared bands. This emphasis arises from the relative maturity of microwave technologies, the wide variety of tuning mechanisms that have been successfully implemented (including electrical, magnetic, and mechanical control), and the broad spectrum of practical applications ranging from radar stealth to wireless communications. By contrast, research in the terahertz and infrared regimes is still at a comparatively early stage, with progress largely driven by the exploration of emerging materials and novel physical mechanisms. Consequently, the corresponding sections focus on representative studies and key breakthroughs rather than an exhaustive categorization. This difference in treatment reflects the current state of the field rather than an imbalance in academic importance.

In recent years, significant progress has been made in the study of metamaterial-based perfect absorbers in the infrared regime. Researchers have achieved various performance optimizations in metamaterial design, including broadband absorption, highly selective narrowband response, and near-unity absorptivity. Moreover, with the deep integration of tunable functional materials and infrared metamaterials, it has become possible to realize dynamically controllable absorption and emission within specific wavelength ranges, thereby providing essential technological support for applications such as infrared stealth, thermal radiation management, and spectroscopic sensing. Since the five primary tuning mechanisms and their fundamental principles have already been systematically introduced in the preceding sections, this part will focus on representative research advances specifically in the infrared band.

Temperature tuning. Phase-change material GST has been widely applied in optically controlled infrared absorbers due to its pronounced refractive index and conductivity contrast between amorphous and crystalline states, as well as its excellent bistable characteristics. Chen et al. [182] proposed a broadband-tunable absorber based on GST, as illustrated in Figure 9a. The structure consists of metal resonators with two distinct dimensions combined with a GST dielectric layer to form a metal–insulator–metal configuration, enabling dynamic absorption control in the mid-infrared range. In the amorphous state, dual resonance peaks emerge at 7.8 μm and 8.3 μm, generating a broadband absorption window of 7.7–8.5 μm with a peak absorptivity of 88%. Upon crystallization, the absorption peak redshifts to ~10 μm, demonstrating remarkable wavelength tunability. Near-field amplitude and phase distributions of the resonant modes were directly visualized using scattering-type near-field optical microscopy, confirming that the two resonances in the amorphous state originate from different resonator dimensions, whereas in the crystalline state, both magnetic and cavity resonances contribute. This approach, which exploits refractive index modulation of phase-change materials to control resonant modes, offers a promising strategy for low-power, nonvolatile, tunable infrared devices. Similarly, Du [183] reported a double-layer GST thin-film absorber that enables precise thermal emission control in the mid-infrared. As shown in Figure 9b, the device comprises a GST film deposited on a metallic reflector. By switching the phase state, the emissivity can be dynamically modulated: in the crystalline state, the peak emissivity reaches 0.97, approaching that of an ideal blackbody, whereas in the amorphous state it falls below 0.2, yielding an extinction ratio exceeding 10 dB. By varying the GST thickness, wavelength-selective emission can be realized across 3–15 μm, with minimal sensitivity to incident angle. The device can be fabricated using standard thin-film deposition processes, while the metallic layer not only reduces thickness but also enhances resonance effects, making the structure highly promising for infrared thermal management and spectrally selective emission control.

Electrical tuning. Liu et al. [184] designed an electrically controlled multifunctional hybrid absorber based on VO₂, achieving dynamic electromagnetic modulation in the mid-infrared. As shown in Figure 9c, the device adopts a sandwich configuration consisting of a top gold grid, a middle VO₂ active layer, and a bottom gold reflector. The 260 nm thick VO₂ film serves as the core phase-change medium integrated directly into the resonant structure. Driven by Joule heating, the insulator-to-metal transition of VO₂ yields substantial modulation: absolute reflectance changes of 80% at 3.05 μm and 75% at 3.85 μm, corresponding to relative modulation ratios of 75× and 5×, respectively. Its inherent bistability further provides electrically controlled switching and rewritable memory functionality, achieving 10% reflectance contrast under a 0.8 A bias current. A key innovation lies in the monolithic integration of continuous plasmonic structures with the phase-change material, where the gold grid functions simultaneously as an optical resonator and an electrical heating channel. Moreover, by spatially patterning the structure, dynamic control of infrared images was demonstrated, enabling reversible appearance and disappearance of “PSU” letters at 2.67 μm.

Mechanical tuning. Muskens et al. [185] introduced a plasmonic optomechanical coupling system based on a gold–SiN bilayer incorporating nanoslit antenna arrays, achieving synergistic regulation of Fano resonances and mechanical vibrations. The system realizes near-perfect absorption over a 4-THz bandwidth and leverages photothermal backaction to drive mechanical oscillations. Under an input power threshold of 19 μW, mechanical oscillations are excited with a six-order amplitude enhancement, whereas at 210 μW, mechanical cooling reduces the mode temperature to 48 K. This optomechanical paradigm provides a new route toward tunable infrared optomechanical devices.

Optical tuning. Yang et al. [186] proposed a femtosecond-scale polarization switching device based on indium-doped cadmium oxide (CdO:In), achieving ultrafast polarization modulation in the mid-infrared. The device adopts a simple sandwich structure, shown in Figure 9d, with a 75 nm CdO:In film between an MgO substrate and a gold reflector. Leveraging the high electron mobility of CdO, the system forms a Berreman-type perfect absorber with a quality factor of 12. Performance measurements show that at 2.08 μm, the p-polarized reflectance is tuned from 1.0% to 86.3%—an absolute modulation of 85.3% corresponding to a relative modulation ratio of 8.141%—while s-polarized reflectance remains above 90%. Pump-induced transient modification of the effective electron mass enables 53° polarization rotation within 800 fs, offering a new strategy for ultrafast polarization control in the mid-infrared.

Emerging hybrid mechanisms. Zhu et al. [187] proposed a plasmonic metamaterial absorber based on Fano resonances, employing a novel optomechanical coupling strategy for broadband mechanical modulation. The device comprises a gold/SiN bilayer with cross-shaped nanoslit antenna arrays integrated above a metallic reflector, forming an air gap that can be electrically tuned. The design supports both low-Q optical Fano resonances and high-Q mechanical resonances. The optical absorption peak results from hybridization of localized nanoantenna modes with Fabry–Pérot cavity modes, leading to near-perfect absorption. Its key innovation lies in bidirectional mechanical control induced by optothermal forces: under blue-detuned pumping, parametric gain drives coherent mechanical oscillations at powers as low as 19 μW, while red-detuned pumping enables optical cooling of the mechanical mode to an effective temperature of 48 K. This plasmonic–mechanical hybrid system exhibits a working bandwidth up to 4 THz and can be flexibly tuned via electrostatic control, offering a promising pathway toward broadband, low-power infrared dynamic regulation.

Figure 9. Tunable absorbers in the Infrared band: (a) Chen’s proposed GST based temperature controlled absorber unit structure [182]; (b) The temperature controlled metasurface unit structure based on GST proposed by Kai Kai Du [183]; (c) Liu Liu proposed an electrically controlled absorber unit structure based on VO₂ [184]; (d) YANG Yuanmu proposed an absorber unit structure based on indium doped cadmium oxide [187].

Figure 9. Tunable absorbers in the Infrared band: (a) Chen’s proposed GST based temperature controlled absorber unit structure [182]; (b) The temperature controlled metasurface unit structure based on GST proposed by Kai Kai Du [183]; (c) Liu Liu proposed an electrically controlled absorber unit structure based on VO₂ [184]; (d) YANG Yuanmu proposed an absorber unit structure based on indium doped cadmium oxide [187].

The field of artificial intelligence (AI), particularly deep learning (DL) and machine learning (ML), has transitioned from a period of theoretical exploration and “technical prowess” to one of profound practical implementation and integration across scientific disciplines [188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241]. This paradigm shift is largely driven by advancements in algorithmic optimizations, such as Transformer architectures, sparse activation, and quantization compression, which have reportedly reduced training costs for some efficient models by over 40% compared to the previous year [242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273]. Concurrently, the design and characterization of functional materials, especially at the micro- and nanoscale, have long been hampered by the complexity of physical laws, the high dimensionality of design spaces, and the prohibitive cost of experimental and computational methods [274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298]. The integration of AI, DL, and ML is breaking these barriers, enabling a new era of data-driven scientific discovery. The current AI landscape is characterized by several key trends moving beyond mere scale. The pursuit of larger models is being balanced by a critical focus on efficiency. Techniques like neural network pruning (removing redundant weights), knowledge distillation (training smaller models to mimic larger ones), and quantization compression (reducing numerical precision of calculations) are paramount [299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325]. Companies like DeepSeek have demonstrated that ultra-efficient models with a fraction of the parameters of traditional models can achieve equivalent or superior inference performance. This reduction in computational overhead makes advanced AI accessible for resource-intensive tasks like multiscale materials simulations. Generative models, particularly Generative Adversarial Networks (GANs) and Variational Autoencoders (VAEs), are no longer confined to creating images. They are now instrumental in generating plausible molecular structures, synthetic material microstructures, and even designing entirely new physical structures based on desired properties. The integration of multiple data types—text, image, speech—into multimodal models is also a key trend. Tech giants in China are leading in this area: Tencent’s Hunyuan model seamlessly fuses image, speech, and text for diverse user scenarios, while Alibaba’s Qwen3 model employs strategies to “activate few parameters for heavy work,” enhancing application adaptability [326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355].

The application of these advanced AI tools is revolutionizing nanotechnology and materials science. Perhaps the most impactful application is the inverse design of materials—specifying desired electromagnetic or mechanical properties and allowing an AI to generate the optimal structure that delivers them. This bypasses the traditional, inefficient “trial-and-error” approach reliant on researcher intuition and costly simulations [356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373]. Researchers combined AI with two-photon polymerization 3D printing to develop ultra-strong, lightweight carbon nanolattices. Traditional nanolattice designs suffer from stress concentration at sharp junctions. Using a multi-objective Bayesian optimization algorithm, the AI predicted optimal shapes that minimized these stress concentrations. Remarkably, this algorithm achieved this with only ~400 data points, far fewer than typical ML methods requiring tens of thousands. The resulting AI-designed nanolattices were twice as strong as previous models and had a strength-to-weight ratio five times that of titanium, demonstrating the power of AI to discover non-intuitive, high-performance architectures. The work on simple metasurface absorbers lays the groundwork for a transformative leap in designing advanced, wideband, and multifunctional electromagnetic absorbers using AI. The research frontier is moving towards more complex systems. Traditional design of layered composite absorbers is notoriously reliant on expert experience and iterative “trial-and-error,” which is inefficient and easily trapped in local optima. The design space expands exponentially with the number of layers, making it impossible to explore manually. The key challenge is balancing wideband absorption with strong absorption performance across the entire band, especially with the push towards lower radar frequencies.

The field of artificial intelligence and deep learning is undergoing a rapid transformation, characterized by increased efficiency, enhanced reasoning capabilities, and greater practical utility. These advances are proving to be uniquely powerful for addressing the long-standing challenges in micro/nanostructure research [374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390], enabling autonomous imaging, robotic nanomanipulation, and—most significantly—the inverse design of functional materials. The application of these tools in electromagnetic absorber design is particularly promising. As demonstrated by pioneering work on metasurfaces and sophisticated laminated composites, AI-driven approaches like Progressive Bayesian Optimization are not merely incremental improvements; they represent a paradigm shift. They move the field away from experience-dependent intuition and towards a data-driven, physics-informed engineering discipline [391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406]. The AI’s ability to uncover non-intuitive, high-performance designs and reveal novel physical mechanisms positions it as an indispensable partner in scientific discovery. The research frontier is set to expand towards fully integrated autonomous discovery systems, multi-objective optimization for multifunctional absorbers, and the use of explainable and generative AI to unlock new fundamental knowledge and material concepts. As AI models continue to become more efficient and insightful, their integration with computational physics and experimental robotics will undoubtedly usher in a new golden age of innovation in electromagnetic materials science, with profound implications for aerospace, telecommunications, and defense technologies.

7. Summary and Prospects

This study presents a comprehensive review of recent advances in metasurface-based tunable absorbers across the microwave, terahertz, and infrared bands, covering their core performance requirements, implementation strategies, representative architectures, and key applications. The discussion focuses on five mainstream dynamic tuning mechanisms—electrical, magnetic, optical, thermal, and mechanical—providing an in-depth analysis of their operating principles and functional characteristics. Overall, compared with traditional static absorbers, metasurface-based tunable absorbers exhibit significant advantages in terms of structural design flexibility, tuning speed, tuning range, and multifunctional integration, offering practical solutions for real-time responsiveness and cross-band compatibility in complex electromagnetic environments.

In the microwave band, active components such as PIN diodes and varactor diodes form the core of efficient electrical tuning, while the incorporation of magnetic materials expands stable tuning capabilities and enables nonreciprocal functionality. Mechanically reconfigurable structures further provide large-range tunability. In the terahertz band, emerging materials such as graphene, Dirac semimetals, and liquid crystals effectively overcome the bandwidth and tuning amplitude limitations of conventional metallic structures, while thermally driven phase-change materials offer new approaches for wide-temperature-range tunability and enhanced stability. In the infrared band, phase-change materials such as VO₂ and GST, combined with finely engineered nanostructures, enable multi-degree-of-freedom control over both absorption peak positions and absorption amplitudes. Optical tuning mechanisms, on the other hand, offer unique advantages in terms of ultrafast response and non-contact modulation. The relative strengths and limitations of each tuning mechanism vary across bands in terms of response speed, tuning range, structural complexity, and power consumption. These differences not only reflect the constraints imposed by material properties but also highlight the importance of multi-mechanism synergy.

Looking ahead, the development of metasurface-based tunable absorbers is expected to follow several key trends. First, multi-field cooperative tuning will become essential for enhancing overall performance. By organically combining electrical, optical, magnetic, thermal, and mechanical control methods, it will be possible to achieve larger tuning amplitudes and higher modulation precision while maintaining ultrafast response, thereby breaking the inherent “speed–amplitude–precision” trade-off. Second, continued advances in emerging functional materials—such as low-loss two-dimensional materials, broadband-tunable superconductors, and phase-change systems with lower transition temperatures and improved stability—are anticipated to revolutionize device efficiency and operational stability. Third, cross-band integrated design will become a major direction for engineering applications, enabling continuous tunable absorption from the microwave to the infrared via multiscale structural engineering and multiphysics coupling, greatly expanding application boundaries in terms of stealth, protection, communications, and sensing.

Furthermore, low-power, miniaturized, and integrable designs will be essential for practical deployment, particularly for platforms such as UAVs, wearable devices, and satellites, which impose stringent requirements on weight, energy consumption, and environmental adaptability. Meanwhile, intelligent and adaptive functionalities will enhance environmental perception and self-adjustment capabilities; by integrating artificial intelligence algorithms with programmable metasurface technologies, devices will be able to perform real-time recognition and optimal control in complex electromagnetic environments. Finally, advances in manufacturing technologies—including multi-material 3D printing, micro–nano fabrication, and flexible electronics—will provide strong support for the mass production of high-precision, multifunctional tunable metasurfaces.

In summary, metasurface-based tunable absorbers are at a critical stage of rapid transition from laboratory research to engineering-scale, system-level, and intelligent applications. With continued progress in materials science, micro–nano manufacturing, control electronics, and intelligent algorithms, their application potential in national defense, information and communication, energy management, and space exploration will be further unlocked, positioning them to play an irreplaceable role in highly dynamic and complex electromagnetic environments.