Nonlinear Dielectric Metasurfaces for Terahertz Applications

Abstract

The terahertz (THz) region of the electromagnetic spectrum, spanning from 0.1 to 30 THz, represents a prospering area in photonics, with transformative applications in imaging, communications, and material analysis. However, the development of efficient and compact THz sources has long been hampered by intrinsic material limitations, inefficient conversion processes, and complex phase-matching requirements. Recent breakthroughs in nonlinear optical mechanisms, resonant metasurface engineering, and advances in the fabrication processes for materials such as lithium niobate (LN) and aluminum gallium arsenide (AlGaAs) have paved the way for innovative THz generation techniques. This review article explores the latest theoretical advances, together with key experimental results and outlines perspectives for future developments.

1. Introduction

The terahertz spectrum bridges the gap between microwave and infrared (IR) frequencies, enabling unique functionalities that leverage the best of both domains [1,2,3]. Its non-ionizing radiation is safe for biological tissues, making it an attractive tool for medical imaging and diagnostics. Unlike X-rays or higher-frequency radiation, THz photons do not carry enough energy to ionize atoms, thereby avoiding DNA damage and other harmful effects. Moreover, its ability to penetrate non-metallic and non-conductive materials such as polymers and textiles has spurred interest in security screening applications. For example, THz scanners can reveal concealed objects under clothing or inside packages without exposing individuals to hazardous radiation [4]. Beyond imaging, the THz range is sensitive to molecular vibrations, enabling high-resolution spectroscopic fingerprinting for material characterization. Many molecules and solids have characteristic absorptive or emissive features in the THz band, allowing the identification of substances—from pharmaceutical compounds to explosives—by their spectral “fingerprints” [5,6]. Despite these benefits, the THz spectrum has historically been underutilized due to the lack of efficient sources and detectors. Electronics struggle to reach THz frequencies and conventional photonics typically operate at much higher frequencies, leaving a gap in available technologies. However, recent advances in nonlinear optics, particularly optical rectification and difference frequency generation from metasurface-based devices, have begun to address these challenges. Such nonlinear techniques bypass traditional limitations by using ultrafast optical pulses and engineered nanostructures to directly generate THz waves, mitigating issues of materials and phase-matching requirements that previously constrained THz sources efficiency [7,8,9]. Metasurfaces offer several concrete advantages for THz generation, detection, and related applications due to their ability to engineer electromagnetic waves at sub-wavelength scales. One of the main advantages of metasurfaces is their ultrathin and compact dimensions which allow to reduce and miniaturize the geometrical dimension of the final platform. Apart from the geometrical form factor, metasurfaces can achieve dynamic and reconfigurable control of the emitted THz radiation. In particular, optimized metamaterials and metasurfaces have been suggested to be beneficial for the development of various novel optical THz components [10]. Polarization conversion, active phase modulation, engineered absorption, and nonlinear effects have been recently demonstrated in THz meta-structures [11,12,13,14,15]. Moreover, engineered metasurfaces have been shown to significantly enhance THz emission from photoconductive antennas [16]. Notably, nanostructuring of a nonlinear crystal into resonant meta-atoms can dramatically enhance the local field intensity, boosting frequency conversion efficiency. For instance, in [17] an optimized 160 nm-thick GaAs metasurface emits THz radiation comparable to that of a 650 $\mathsf{\mu }$m GaAs crystal, despite its significantly smaller material volume. Several papers have recently presented the potential of nonlinear metasurface for THz generation such as reported in [18,19,20,21]; however, the use of fully dielectric material for realizing the metasurface is a new concept which we address in detail in this manuscript.

This review is organized as follows: Section 2 presents a concise introduction to the prospective applications and implications of the THz frequency range; Section 3 addresses THz generation in dielectric slabs, while Section 4 illustrates the potential offered by dielectric metasurfaces for efficient nonlinear THz generation. Crystalline defect-free condition is assumed in the reported results. Finally, Section 5 proposes some innovative solutions and applications for the previously described thin surfaces. This structured approach allows readers to first understand the broad context and applications of THz photonics, then delve into specific mechanisms of THz generation, and finally explore forward-looking concepts and conclusions.

2. Overview

Terahertz radiation, often referred to as the bridge between electronics and photonics, has gained significant attention in recent years because of its unique properties and the vast potential for applications in sensing, wireless communication, healthcare, spectroscopy, security, quality control, and imaging, as described in Figure 1 [22,23,24,25]. Exploring and harnessing this frequency range necessitates a multidisciplinary strategy that encourages close collaboration between high-frequency semiconductor technology for radio frequencies (RF) electronics and alternative photonic-based solutions. In practice, this means engineers working on microwave/millimeter-wave devices must team up with optical scientists to develop new hybrid approaches that operate at THz frequencies. One of the main fields in which this effort is already yielding promising results is data transfer. With the growing demand for faster data rates and enhanced connectivity, the THz spectrum is gaining attention as a candidate for next-generation wireless communication systems. THz waves promise ultra-high bandwidths that could support data rates exceeding 100 Gbps, enabling technologies such as 6G and beyond [26,27,28,29]. In this context, research efforts are focused on overcoming challenges related to signal attenuation and hardware development, including efficient THz sources and detectors [30,31]. Recent studies have deeply highlighted the potential of the THz range for communication applications [32,33]. In addition to this research topic, we also underline that THz imaging and spectroscopy have introduced a significant breakthrough in material characterization through non-destructive testing and biomedical imaging [34,35,36,37,38,39]. Indeed, the ability of THz waves to probe materials without causing damage is particularly promising in recognizing defects in semiconductors, investigating pharmaceutical compounds, and detecting concealed objects. Furthermore, their interaction with molecules and biological tissues makes them ideal for medical diagnostics [40,41,42]. Moreover, it is relevant to stress that the penetration capabilities of THz waves through fabrics and plastics make them suitable for security screening [4,43,44]. As previously mentioned, differently from X-rays, THz radiation is non-ionizing, guaranteeing safety for routine processes in airports and other high-security environments. Additionally, THz sensing enables for the detection of explosives and hazardous substances through their spectral fingerprints [45,46], while in industry, THz systems are being integrated into quality control processes in many advanced material manufacturing [47,48]. For instance, THz quality control scanners can monitor plastic welds or inspect food products and pharmaceutical tablets for uniformity without physical contact or damage, illustrating the breadth of THz applicability [49,50]. Several comprehensive reviews on metasurfaces operating in the THz range were recently published [51,52,53,54]. Here, we intend to briefly highlight a variety of possible THz metasurfaces designs and applications. For instance, in Ref. [55], a Silicon–Silica metasurface with a graphene layer on a teflon substrate demonstrated high-Q resonances under THz excitation whereas in Ref. [56] a high-Q Silicon–Silica metasurface on gold was suggested for sensing applications. Additionally, a liquid crystal–silica–graphene metasurface THz absorber was developed in [57], while a ceramic microsphere metasurface designed as a THz reflector was fabricated in [58]. Let us also mention that in [59], a silicon cubes-based metasurface was designed and fabricated to function as a magnetic reflector in the THz.

Despite its promise, the development of efficient, compact, and reliable THz sources remains a critical challenge, particularly due to the inherent limitations of conventional materials and techniques. This difficulty in creating THz emitters and detectors has historically been known as the THz technology gap. Indeed, for THz transmitters, achieving modulated THz generation with a high repetition rate across different spectral regions is essential. Although certain spectral gaps remain where suitable THz sources are lacking, various mechanisms now enable THz generation. The most widely explored methods are THz-Quantum Cascade Lasers (THz-QCLs), photoconductive antennas (PCAs), and nonlinear optical rectification process (NORP) in bulk crystals. These approaches, which are particularly well-suited for spectroscopy and imaging applications, rely on large generation volumes and typically operate at frequencies below 5 THz [60,61,62,63,64,65,66]. In brief, THz-QCLs are semiconductor lasers with cascading quantum well transitions that can be lasing at THz frequencies (often around 3–4 THz), though they usually require cryogenic cooling to function. PCAs, on the other hand, use an ultrafast laser (with pulse energy of the order of 10 nJ [67]) to excite carriers in a biased photoconductive material, producing broadband THz pulses; they are common in time-domain THz spectroscopy but their output power and bandwidth are limited by, among others, carrier mobility and lifetime, phonon absorption or the applicable bias voltage [68]. Regarding NORP, difference frequency generation in the crystals (with zinc telluride and lithium niobate being the most popular) occurs between the different spectral components that comprise the optical probe resulting in a spectrally limited THz pulse (0.1–3 THz) [69]. Although widely used for time-domain THz spectroscopy, this approach requires laser pulses with pulse energy higher than 1 $\mathsf{\mu }$J (i.e., much larger compared to PCAs) [67].

More recently, to achieve the highest performance per unit volume using nanoscale resonant platforms, THz generation through optical rectification in plasmonic Split-Ring Resonators (SRRs) has been demonstrated. In this method, nanoscale SRRs nonlinearly generate THz signals by exciting magnetic dipole modes in the infrared regime at the pump wavelengths [70,71,72,73,74]. By confining the optical excitation to the surface of metallic ring structures, these plasmonic schemes significantly enhance local electromagnetic fields, which in turn strengthens the nonlinear polarization and THz emission. Such resonant plasmonic approaches have succeeded in creating THz bursts in sub-wavelength volumes, albeit often with trade-offs in efficiency due to metal losses. Importantly, a novel approach for generating structured single-cycle THz wavepackets using engineered nonlinear plasmonic metasurfaces has been reported in [75] where the generation of propagating spatiotemporal quadrupole and few-cycle THz pulses with engineered angular dispersion is demonstrated. In the report, the THz output was not a simple Gaussian beam but had a complex field pattern—a quadrupole-like spatial distribution—and only a few oscillation cycles in time, achieved by carefully tailoring a plasmonic metasurface. This spatiotemporally structured THz emission showcases the level of control possible with designed nonlinear surfaces.

A promising alternative to plasmonic THz emitters is represented by all-dielectric THz metasurfaces, which have already attracted significant attention in nonlinear optics as key components for second- and third-harmonic generation [76,77,78]. Such platforms may outperform plasmonic metasurfaces in nonlinear generation efficiency because of the significantly lower optical losses, as they avoid the ohmic losses inherent in metallic structures. In dielectrics, incident light does not induce the same kind of free-carrier currents that dissipate energy as heat, so more of the input optical energy can be converted into the desired THz radiation. Moreover, in high refractive index dielectrics, the bulk nonlinear contribution is mostly the dominant one, opposite to metallic structures where surface currents are responsible for the harmonic generation. This difference arises because many non-centrosymmetric dielectric materials (like certain semiconductors or ferroelectrics) possess strong second-order nonlinearities throughout their volume, whereas centrosymmetric metals have no bulk second-order response and produce even-order nonlinearities only at surfaces, where inversion symmetry is broken.

Conventional phase matching in nonlinear optics relies on satisfying strict momentum conservation conditions over a macroscopic propagation length via birefringence or periodically poled structures. In contrast, metasurfaces exploit localized, resonant nonlinear interactions at the nanoscale [55,79], where phase accumulation is governed by sub-wavelength unit cell engineering rather than bulk coherence lengths. In other words, resonant structures with sub-wavelength scale mode volumes do not require strict phase matching, as their conversion efficiency depends on the modal overlap between fundamental modes and higher harmonics. Of course, the overall small volume limits the total efficiency of the harmonic process involved. To circumvent this drawback, hybrid nonlinear integrated photonic devices consisting of phase gradient metasurfaces patterned on top of a nonlinear waveguide have been recently proposed [80]. In this context, the metasurfaces can provide additional momentum to compensate for the phase mismatch between the fundamental and the harmonic waves. Distributed phase matching in nonlinear metasurfaces is another mechanism that contributes to efficient THz generation via constructive interference across a metasurface without requiring strict bulk phase matching [81]. Also, high conversion efficiency can be achieved with high-Q quasi-bound states in the continuum (quasi-BICs) and other (Mie, Fano) resonances in metasurfaces [82,83]. Furthermore, adjusting the geometric parameters of the metasurfaces enables additional functionalities, such as polarization control, dynamic modulation, beam shaping, and steering of the emitted THz waves (usually achieved by adding an extra metasurface [51,52,53,79,84]). Notably, the ENZ materials (e.g., ITO) reduce phase-matching constraints due to near-zero permittivity. Recent advances in ENZ-metasurfaces [79,85] further bridge the gap between material limitations and phase-matching requirements. By patterning ENZ materials into sub-wavelength resonators, these platforms combine the field enhancement of ENZ modes with the momentum engineering of metasurfaces, achieving efficient THz generation without bulk constraints. Notably, they surpass LN in conversion efficiency per unit thickness while enabling dynamic tuning [85].

For all the abovementioned reasons, this review explores the latest advances in THz generation methods, with a focus on nonlinear optical mechanisms, in resonant dielectric metasurfaces, with a specific concentration on materials such as LN and AlGaAs. The discussed innovations address key challenges such as limited efficiency, scalability, and phase-matching constraints, paving the way for next-generation photonic devices. By synthesizing findings from experimental and theoretical studies, this article provides a comprehensive evaluation of the current state of the field, highlighting the critical role of material science and nanotechnology in driving breakthroughs in THz technologies. In addition, future perspectives are examined, including the integration of dielectric thin-film platforms and metasurface-based systems, which hold promise for transforming scientific and industrial applications. Integrating ultrathin nonlinear films with resonant metasurfaces could lead to hybrid devices that combine the advantages of each, potentially enabling on-chip THz emitters or sensors. This brief overview delves into the key challenges and motivation behind recent research efforts in this rapidly evolving domain.

3. THz Generation in Dielectric Slabs

In this section, we present recent results about THz generation through optical rectification in lithium niobate (LN) slabs. The investigation of LN’s material properties dates back to the 1960s. Its crystal structure falls within the 3 m point group, characterized by a three-fold rotational symmetry around the [0001] c-axis (typically referred to as the z-axis) and mirror symmetry across three planes spaced 60° apart [86,87]. In simpler terms, LN has a trigonal crystal symmetry that lacks inversion symmetry, a crucial attribute for second-order bulk nonlinear effects. LN has a wide transparency range from 350 nm to 5 $\mathsf{\mu }$m, spanning across the visible, near-infrared, and mid-infrared spectra. Its relatively high refractive index (roughly 2.2 at 1550 nm) enables the formation of high index contrast waveguides on various amorphous and crystalline substrates, such as silica and sapphire. With a high Curie temperature (1210 °C), LN maintains a stable ferroelectric phase, making it compatible with several fabrication processes and operating conditions. Unlike silicon, LN is a non-centrosymmetric crystal with large second-order nonlinear coefficients, making it ideal for optical wavelength conversion and photon-pair generation. Moreover, its exceptional Pockels coefficient ($r_{33}$ = 31 pm/V) makes LN the preferred material for electro-optic modulators, a key component in telecommunication networks [88]. These combined features—broad optical transparency, strong nonlinearity, and robust fabrication tolerance—explain why LN is ubiquitous in nonlinear optical devices such as frequency converters and modulators. These properties make also LN an ideal material for studying THz emission through optical rectification (OR) mechanism in dielectrics. Indeed, LN strong second-order nonlinearity and polar lattice vibrations offer a promising platform for converting optical pulses into THz pulses via OR, which will be discussed next. OR, a second-order nonlinear optical process, is one of the most widely used methods for generating THz radiation [89,90]. OR is a nonlinear optical process in which an intense optical beam (labeled as the pump) is utilized to generate a low-frequency charge motion in a nonlinear medium, emitting in turn THz radiation. In practical terms, when a femtosecond laser pulse passes through a nonlinear crystal like LN, the pulse is an ultrafast intensity envelope that can induce a DC or very low-frequency polarization wave in the crystal; this time-varying polarization at THz frequencies radiates as a burst of THz electromagnetic waves. Thus, OR can be viewed as a special case of difference frequency generation where the difference between frequency components of a broadband pulse produces a continuum of THz output. Thin-film lithium niobate (TFLN) platforms have demonstrated significant advantages in this context, mainly related to the reduction of the generated THz signal self-absorption with respect to bulk crystals in which strong intrinsic absorption significantly reduces the detected THz [81,91,92]. In bulk LN, the emitted THz wave must traverse a long path in the material and is heavily attenuated by LN phonon absorption bands; a thin-film geometry minimizes this propagation loss, allowing more of the generated THz to escape. LN exhibits vibrational lattice modes (i.e., optical phonons) which can interact with incident electromagnetic radiation in the THz range. This interaction alters the dispersion properties of the electromagnetic wave propagating through the crystal, resulting in a hybrid mode known as a phonon–polariton (PhP). It has recently been demonstrated that optical phonons are responsible for a substantial improvement of THz generation between 2 and 8 THz in LiNbO₃ (LN) samples [93]. Interestingly, the optimal thickness to benefit from the PhP enhancement of the nonlinear response is estimated to be less than 2 $\mathsf{\mu }$m. This finding indicates that there is a sweet spot in LN thickness: a sub-micron film is thick enough to efficiently generate THz radiation but thin enough to avoid excessive absorption, leveraging phonon–polariton resonances to boost the output in the 2–8 THz range.

The generation and detection of THz radiation require precise experimental setups. Figure 2 depicts the experimental configuration for THz generation using x-cut LiNbO₃, highlighting the use of pump lasers, parabolic mirrors, and electro-optic detection via gallium phosphide (GaP) crystals, which characterize the generated THz radiation via electro-optical sampling technique [94]. The time domain traces are shown in the bottom panel of Figure 2a. In particular, in [93] the polarization of the pump signal and of the probe pulse are tuned to achieve polarization-resolved measurements, thus addressing the various tensorial components of the nonlinear optical response of LN. Four combinations for the pump and THz polarization have been considered to isolate the contributions of the individual nonlinear coefficients. By measuring THz signals for various polarization alignments (denoted in compact notation as $zzz$, $zyy$, $yyy$, $yzy$ where the first letter indicates the THz signal polarization, while the others identify the pump polarization relative to the LN crystal axes), the experiment could disentangle which tensor components of LN second-order susceptibility were involved in the THz generation. Interestingly, the emitted THz radiation polarized along the z-axis (in compact notation $zzz$ and $zyy$) possesses a broad spectrum, while THz waves polarized along the y-axis (i.e., $yyy$ and $yzy$) display a sharper peak around 4 THz and a dip at 6 THz, as shown in Figure 2b. These features are attributed to the ionic enhancement of the nonlinear response in the LN film. In other words, when the THz field is aligned with the crystal optical axis (z), the phonon–polariton effects broaden the emission spectrum. Conversely, for THz polarization along y (perpendicular to the polar axis), the interaction with specific optical phonon modes causes a resonance peak (near 4 THz) and an attenuation (near 6 THz), revealing how the lattice vibrations selectively amplify or suppress certain THz frequencies. To confirm the significant impact of ionic contributions on the nonlinear response, frequency-domain numerical simulations using the finite-element method in COMSOL have been conducted in [93], focusing on the optical-to-THz conversion mechanism. By slightly adjusting the magnitude scale of the nonlinear coefficients an excellent agreement between experiments and simulations has been obtained. This close match between simulation and experiment, achieved after accounting for the frequency-dependent behavior of LN’s nonlinear coefficients, reinforces the interpretation that phonon dynamics strongly influence the THz generation process. The analysis in [93] also reports OR measurements at several different pump powers and on LN samples with different thicknesses to provide additional validation that the detected THz signal in the experiments originates from the optical rectification process. As a characteristic of second-order nonlinear processes, the generated signal power scales quadratically with the pump power. However, when keeping the pump power constant, increasing the LN sample thickness leads to a significant deviation from this quadratic trend due to THz absorption in LN. As the LN thickness grows, the spectral power density in the lower frequency range becomes dominant, ultimately suppressing THz generation above 4 THz. This is consistent with stronger re-absorption of higher-frequency THz components (near LN phonon resonance) in thicker samples—longer path lengths inside LN allow the material’s intrinsic absorption to attenuate the high-frequency portion of the THz pulse. Notably, in [93] also the case of a LN rod (with 12 $\mathsf{\mu }$m length, 2 $\mathsf{\mu }$m width and 500 nm height) has been reported demonstrating that the THz emission is boosted by almost a factor of five compared to the unstructured film. Patterning the LN into a discrete rod introduces geometrical resonances or improves out-coupling that significantly enhances the THz output beyond the uniform thin-film case. These findings highlight the powerful capability of phonon-resonance engineering in precisely tuning the characteristics of generated THz radiation at sub-wavelength dimensions. Moreover, the observed enhancement in the rod-structured system sets the stage for the next section of this review, which explores in detail the THz generation from metasurfaces. If a single isolated LN rod can increase emission fivefold, an array of such many resonant structures—i.e., a metasurface—could potentially yield even larger enhancements and additional control over the THz waves, as we discuss in the next Session. We recall that hybrid gold THz antennas integrated with thin-film lithium niobate circuits have shown promising potential for tailored THz sources operating in the lower THz frequency range (<1 THz) [81]. With advancements in nano-structuring LN, recent theoretical proposals for THz radiation generation have emerged in hybrid LN/Si coupled-waveguide structure, [95] alongside experimental approaches leveraging topological confinement in laser-written LN slabs [96]. Apart from LN and AlGaAs that are deeply presented in this manuscript, conventional semiconducting crystals used for THz-wave generation and detection include Zinc Telluride (ZnTe) and Gallium Phosphide (GaP). The latter have much lower electro-optic coefficients compared to LN, but are widely used because of the possibility for phase matching that is not directly possible with LN and other inorganic crystals with a high electro-optic coefficient [97,98]. Let us also mention that Perovskite, Barium Borate, and Quartz may find applications in THz generation such as those reported in [99,100,101]. Alongside widely recognized inorganic nonlinear optical crystals commonly utilized as THz emitters [102], organic nonlinear electro-optic crystals have gained growing attention due to their higher macroscopic nonlinearity and lower dielectric constant, making them promising candidates for efficient and broadband THz wave generation. In this context, in recent decades, extensive research has been conducted on various electro-optic crystals; however, only a limited number of crystals exhibiting an electro-optic response exceeding 50 pm/V have been identified [103,104,105,106,107].

4. THz Generation from Resonant Dielectric Metasurfaces

In this section, we introduce the concept of dielectric metasurfaces for efficient THz generation via difference frequency generation (DFG) process. DFG is a nonlinear process generating beams with differing frequencies of the input beams. For example, if two infrared laser beams at frequencies ${\omega }_1$ and ${\omega }_2$ (with ${\omega }_2>{\omega }_1$) interact in a nonlinear medium, DFG can produce radiation at the THz frequency ${\omega }_3={\omega }_2-{\omega }_1$. Indeed, a metasurface can be defined as an engineered material composed of a repetition of sub-wavelength scale structures usually called meta-atoms, designed to manipulate and control electromagnetic waves [108,109,110,111]. In practice, a metasurface is an ultrathin (often planar) array of nanoantennas or resonators whose individual scattering responses can be tailored. By arranging these meta-atoms in a specific pattern, one can achieve desired effects on an incoming wavefront, such as focusing light, altering its polarization, or in our context, facilitating nonlinear frequency conversion. Among the others, AlGaAs-based metasurfaces have earned significant acclaim in photonics due to their optical properties, high versatility, and sizable nonlinear optical response [112,113,114,115]. With a broad transparency range spanning from the visible to the infrared spectrum, AlGaAs is adaptable to a multitude of photonic applications, while its high refractive index facilitates precise control over light, enhancing the manipulation of amplitude, phase, and polarization. Additionally, its low absorption losses, particularly in the near-infrared region, ensure efficient light interaction, improving the final device performance. Another advantage of AlGaAs lies in its compatibility with standard CMOS fabrication techniques, enabling scalable manufacturing and accessible integration into existing systems [116,117,118,119,120]. In other words, AlGaAs nanostructures can be fabricated using processes similar to those employed for silicon microelectronics, which is important for mass production and integration. These attributes position AlGaAs metasurfaces as key players in different research areas such as nonlinear optics, sensing, and communications [121,122,123,124]. Indeed, AlGaAs metasurfaces have already shown efficient second-harmonic generation (frequency doubling) and other nonlinear effects, highlighting their potential to serve as active elements in sensors or as compact frequency converters in optical communication links. Interestingly, a recently published study has meticulously explored how an AlGaAs meta-atom can be employed for THz applications, achieving a generation efficiency several orders of magnitude higher than that of previously proposed metallic structures [7]. In particular, a fully dielectric transceiver is proposed. Through the use of a near-IR pump beam, the optical-domain information signal is converted into the THz frequency band. Notably, adjusting the frequency of the pump beam allows for coverage of the entire THz spectral range. Crucially, this approach directly translates the information into THz radiation without requiring additional components, ensuring compatibility with various modulation schemes and signal formats. This is significant because it implies a very simple transmitter: an optical data signal is fed into the nonlinear meta-atom and comes out as a broadcast THz signal carrying the same data, with no need for electronic oscillators or separate mixers. The optimized AlGaAs nanoantenna can achieve a conversion efficiency of up to 10⁻⁷ W⁻¹ at approximately 11 THz, see Figure 3, while an efficiency on the order of ${10}^{-7}$ is still quite low in absolute terms, it represents a substantial improvement relative to earlier nanoscale THz emitters. This result showcases the potential of a single sub-wavelength AlGaAs resonator to serve as a highly miniaturized THz source. In more detail, in [7] an AlGaAs nanodisk with diameter and height equal to 400 nm supporting a magnetic dipolar resonance around 1550 nm is considered as the meta-atom. Moreover, the AlGaAs permittivity and the ${\chi }_{ijk}^{\left(2\right)}$ in the THz range are modeled. These quantities are indeed necessary to perform the nonlinear DFG simulation at ${\omega }_3={\omega }_2-{\omega }_1$ where the nonlinear currents in the AlGaAs nanocylinder volume can be written as follows:

$$J_i\left({\omega }_3\right)=-i{\omega }_3{ϵ}_0{\chi }_{ijk}^{\left(2\right)}\left({\omega }_3\right)[E_j\left({\omega }_2\right)E_k^{\ast }\left({\omega }_1\right)+E_k\left({\omega }_2\right)E_j^{\ast }\left({\omega }_1\right)]$$

(1)

where the subscripts $i,j,k$ are associated with the Cartesian coordinates. Instead, the DFG efficiency is defined as follows:

$${\eta }_{DFG}=P_{rad}^{{\omega }_3}/P_i{P}_s$$

(2)

where the numerator is the power radiated at THz, and the denominator is the product of the incident powers of the information signal and pump beam, respectively. Figure 3b displays the AlGaAs nanodisk ${\eta }_{DFG}$ from 4 to 14 THz. The results are obtained by fixing ${\omega }_2$ and by varying ${\omega }_1$ to cover the desired THz range at the generated ${\omega }_3$. Interestingly, one can observe four peaks in the THz emission: two of them are located at the spectral position of the AlGaAs transverse-optical (TO) phonon frequencies, whereas another couple of peaks are in exact correspondence with the localized surface phonon-polaritons (LSPhP) resonances of the nanopillar. This result demonstrates the key role of surface phonon–polaritons in the THz generation from all-dielectric nonlinear nanoantennas. In other words, when the difference frequency matches either a bulk lattice vibration mode (TO phonon) or a surface-confined phonon–polariton mode in the nanodisk, the THz output is significantly enhanced, producing a peak in efficiency. The interested reader may find more information in [7]. It should be highlighted that the obtained THz efficiency can be useful in many communication applications. As an example, the inset of Figure 3b reports the capability of the proposed platform to convert an information signal in the IR region with a bandwidth of 160 GHz into the THz region, around 11 THz, without significant distortion. This approach is equally applicable to other materials that exhibit second-order nonlinearity, which allows access to different spectral regions [8]. Using a different material for the meta-atom, one could target lower THz frequencies—for instance, LN meta-atoms might be optimized for the 1–5 THz range, thus expanding the versatility of the DFG approach.

After having presented the THz generation mechanism via DFG in a single meta-atom, we continue our analysis considering the THz generation from a dielectric metasurface made of AlGaAs nanodisks. In this context, recent efforts have been reported in [17,75,125]. It should be noted that, so far in this paragraph discussion, we considered two independent sources with an infinitesimally narrow frequency band as input signals that nonlinearly combine to generate the THz radiation. From a theoretical perspective, this is certainly the easiest scenario to consider. However, real sources typically have pulses with non-negligible spectral bandwidth. Therefore, it is relevant to consider the different spectral components of the incident signal which are responsible for the so-called intra-pulse DFG, i.e., OR. An experimental demonstration of this concept is reported in [125], where different metasurfaces composed of AlGaAs nanodisks with a different elliptical basis are investigated. The pump consists of a femtosecond laser (80 MHz repetition rate with 140 fs pulse duration) that delivers p-polarized light on the samples with an angle of incidence of $\pi /4$ rad. The measured THz power spectral density (PSD) is in strong agreement with the theoretical simulations, as reported in Figure 4. For the modeling of the incident beam, the authors consider a Gaussian pulse centered around 234 THz (1280 nm) with a bandwidth of 2 THz. For estimating the total THz emission behavior coming from the metasurface, a discretized version of the incident Gaussian beam spectrum is considered. This spectrum is sampled with a finite number of frequency steps N (N = 18) thus allowing the DFG calculation due to the weighted spectral components of the incident beam. In other words, the broadband pulse was discretized into 18 narrowband slices, and all pairwise mixes of those slices that differ by a given THz frequency were computed. In this way, the simulation of the total THz generation is divided into several DFG problems, which can be modeled following the same procedure reported in Equation (1), where only the bulk nonlinearities are considered. The final THz efficiency at a specific ${\omega }_n$ is obtained as the sum of all the DFG processes that combine all the possible spectral components which are ${\omega }_n$ distant in frequency. Interestingly, the excellent agreement with the experiment is achieved by considering only the bulk nonlinear contributions in the simulations. Therefore, for these pillar dimensions, the contribution of surface nonlinearities is negligible. However, as observed in thinner GaAs membranes, surface nonlinear contributions have also been shown to play a role in THz generation. For instance, in a 160 nm thick GaAs metasurface excited above the material bandgap, a significant portion of the THz field is reported to be likely generated at the surfaces [17]. In that scenario, pumping above the bandgap likely creates ultrafast photocurrents or depletion field dynamics at the GaAs surface, which become a source of THz radiation. This serves as a reminder that, depending on material and excitation conditions (especially in centrosymmetric media or at high photon energies), surface effects might dominate.

Thus, all-dielectric metasurfaces can act as an efficient and customizable platform for THz pulse generation. The design, based on cylindrical AlGaAs resonators, achieves a remarkable 40-fold enhancement in THz emission efficiency compared to a bare substrate. This enhancement factor underscores how nanostructuring a material into resonant elements can dramatically improve THz yield by concentrating optical energy and engineering emission properties, rather than using an unpatterned film. Beyond the aspect of efficiency, the metasurface allows precise control over THz radiation characteristics by adjusting the elliptical cross-section of the nanocylinders. Additionally, tuning the optical excitation wavelength enables direct manipulation of the THz phase [125]. This unique capability for spatio-temporal structuring of the THz wavefront underscores the potential of all-dielectric metasurfaces for nonlinear frequency conversion, overcoming the limitations of plasmonic and non-transparent systems. With further optimization, these engineered nanoresonator surfaces could replace traditional bulk crystals for THz generation, paving the way for advancements in THz photonics. One can envision ultrathin THz emitters integrated onto photonic chips, using metasurfaces to produce and direct THz beams without the need for centimeter-thick nonlinear crystals. This would represent a significant step toward compact, on-chip THz systems.

5. Perspectives

The experimental demonstration of THz generation from a metasurface shown so far is mainly limited in efficiency by constraints imposed by the spectral width of the used pulsed source. Indeed, the finite and relatively narrow bandwidth of the experimental pulses (around 2 THz) does not allow for exploiting the AlGaAs ${\chi }^{\left(2\right)}$ resonances (located around 8 and 11 THz). A possible future development that may improve the efficiency of the nonlinear process could be to explore these spectral regions using sources with sufficiently wide pulses. For instance, using an even shorter pump pulse or a specially shaped multi-color pump that covers 8–11 THz in difference-frequency content could drive the AlGaAs metasurface at its peak nonlinear response, thereby producing stronger THz output. State-of-the-art ultrafast laser technology (e.g., optical parametric amplifiers or chirped-pulse difference frequency generation) can already reach such broadband excitation. However, in [9] it was demonstrated that the zeros of nonlinear susceptibility can also have important implications in the field of analog computing. Curiously, as highlighted in Figure 5, the AlGaAs ${\chi }^{\left(2\right)}$ shows a ‘zero-crossing’ and almost linear frequency dependence of its real part around 5.6 THz with a negligible imaginary part. In [9], a fully analytic analysis is reported when an AlGaAs thin metasurface is excited by two different signals. It is shown that under certain conditions and, in particular, when the bandwidth of the input electric field envelope is smaller than the THz carrier frequency $f_{THz}$, the DFG output signal is proportional to the first derivative of the product of the incident fields, and it oscillates at frequency $f_{THz}$. This analog computation capability reveals that the metasurface is not just generating THz for communications or spectroscopy, but actually processing information (in this case, detecting the temporal edges or rate-of-change of the input signals’ envelopes) in the THz domain. Such a functionality is reminiscent of an optical analog computer that can; for example, detect motion or transitions (since taking a derivative accentuates rapid changes in a signal), which is indeed noted as the main function required in motion detection applications.

To further validate the analytical framework, a Comsol Multiphysics simulation is also presented in [9] where one optical pump is a continuous-wave (CW) signal, while the second, representing the information signal, has a specific modulation bandwidth. If the CW signal is frequency separated from the information signal by approximately 5.6 THz—which is the AlGaAs zero-crossing frequency—the nonlinearly emitted THz electric field corresponds to the time derivative of the information signal. The proposed metasurface is composed of a square array of nanodisks with a radius of 160 nm, a height of 400 nm, and a period equal to 400 nm, positioned over a low refractive index substrate. In the simulation, a CW source at 1030 nm and an optical signal modulated by a rectangular pulse illuminate the device at an oblique incidence of $\pi /4$ rad. For these design parameters, the metasurface has a magnetic dipolar resonance around 1030 nm, leading to a two-fold enhancement of the event detection nonlinear THz field compared to the unpatterned AlGaAs slab. Remarkably, the output THz field exhibits two distinct peaks corresponding to the steps of the information signal. All the presented discussion demonstrates that dielectric metasurfaces serve as a viable flat-optic platform for efficiently generating and modulating THz radiation. This opens up a new paradigm where a THz nonlinear metasurface could be engineered to process information at the speed of light, potentially detecting motion or performing edge enhancement in imaging, entirely via optical means. Additionally, we have briefly reported their capability to perform analog computing operations such as the first derivative, which is the main function required in motion detection applications. This discussion demonstrates how the future of THz technology lies in the mixture between metasurface engineering and material properties together with the possibility of integration of multifunctional platforms. Materials such as hybrid perovskites, transition metal dichalcogenides, and engineered dielectric surfaces appear to be very promising for nonlinear generation, offering sizable efficiency, and suitable spectral coverage [126,127,128,129,130]. For example, halide perovskites can be tailored in composition to enhance their second-order or third-order nonlinearities and can be processed into microstructures, making them attractive for THz applications. Similarly, two-dimensional materials and other emerging semiconductors might be incorporated into metasurfaces to extend THz generation into frequency ranges or functionalities not accessible with LN or AlGaAs alone. Future metasurfaces are expected to integrate tunable functionalities for the dynamic control of the scattered light in terms of efficiency, polarization, and shape. By combining THz generation, manipulation, and detection on a single chip, these technologies might be able to make significant progress in many different fields such as medical diagnostics, security screening, and high-speed communications. Imagine a single chip that can emit a tailored THz pulse, detect the reflected signal, and analyze it, all within an integrated metasurface framework. Such a device could revolutionize THz imaging scanners, making them more portable and sensitive, or enable ultra-fast data links at THz frequencies by integrating transmitters and receivers in one photonic package. In summary, the trajectory of research suggests that versatile, efficient THz metasurface devices will likely bridge the gap between fundamental laboratory demonstrations and real-world applications in the near future.

6. Conclusions

The convergence of nonlinear optics, advanced materials, and metasurface engineering has ushered in a new era of THz technology. Innovations in optical rectification, resonant metasurfaces, and LSPhP have addressed traditional challenges while unlocking new possibilities for compact, efficient THz devices. With applications ranging from communication to analog computing, the THz spectrum is set to become a cornerstone of next-generation photonic systems, while THz waves were once confined to niche laboratory setups, the developments surveyed in this review indicate that broadly tunable, powerful, and integrable THz sources may soon be feasible, enabling practical systems in various domains. Despite significant progress, challenges remain, including enhancing conversion efficiency, minimizing losses, and integrating metasurfaces with compact and scalable technologies. Future research will further refine these structures, paving the way for practical and high-performance THz devices. For example, achieving higher THz output power will require not only materials with larger nonlinear coefficients or broader bandwidth lasers, but also clever thermal management and out-coupling strategies to handle high optical intensities and extract THz radiation efficiently. Similarly, incorporating THz metasurfaces into portable systems will demand innovative packaging and possibly co-integration with electronics for drive and read-out. Through ongoing advancements, THz metasurfaces are destined to become key elements of next-generation THz technologies, bridging the gap between fundamental research and real-world applications. As researchers continue to improve device performance and address engineering hurdles, we anticipate that the concepts outlined here—from thin-film nonlinear crystals to flat nonlinear optics and analog computation—will possibly translate into tangible THz components in communication networks, imaging devices, and sensing systems, thus strengthening the role of THz photonics in the technological landscape.