Search Results (235)

Time-resolved visualization of local structural dynamics driven by external fields is essential for understanding structure–property relationships in functional materials and devices. Conventional ultrafast methods primarily capture femtosecond-to-picosecond photoinduced dynamics, yet they lack real-space access to spatially inhomogeneous processes occurring at their intrinsic mesoscopic timescales that govern material and device performance—particularly electrically driven processes that closely mimic actual device operating conditions. Here, we report a multifunctional ultrafast transmission electron microscopy (UTEM) platform targeting reversible structural dynamics spanning nanoseconds to microseconds under stroboscopic multi-field excitation. Our system employs photoelectron pulses generated by nanosecond UV laser illumination as the probe, alongside optical and electric pulses as pump excitation. A unified electronic synchronization scheme based on a high-speed photodiode and a digital delay generator enables precise timing control among the optical pump, electrical pump, and photoelectron pulses across the nanosecond-to-microsecond range. Using vanadium dioxide (${\mathrm{VO}}_2$) as a model system, we demonstrate a combined spatiotemporal resolution with measurable signals on the order of 10 nm–10 ns, allowing real-space mapping of spatially inhomogeneous dynamics. Electrical-pump experiments further reveal Joule-heating-induced non-uniform structural phase transitions and thermal-shock-excited megahertz-range mechanical oscillations. These results establish the developed multi-field UTEM platform as a practical tool for probing local structural dynamics in functional materials under optical and electrical excitation. Full article

Vanadium dioxide (VO₂) is a phase-change material, which changes its properties under thermal or optical stimuli. Thanks to the fact that the material phase transition appears at conditions which are close to environmental ones, VO₂ has been widely used in diverse structures, including metasurfaces, that acquire switching and reconfigurability capabilities. In this paper, we numerically study the functionality-enabling properties of dielectric resonator-based nondiffractive meta-atoms that comprise small VO₂ components, i.e., covers or drops, in switchable coding and wavefront-manipulating scenarios at THz frequencies. The goal is to unveil the potential of these meta-atoms in switching the reflected wave’s phase coverage under temperature variations. The main attention is paid to how the shape and size of the VO₂ components affect the functionality switching that is enabled by the changes in coverage. It is shown that metallic and insulator states of VO₂ can play different roles in diverse switching scenarios. Different resonance regimes exert different influences on the resulting capability of switching, while contributing to multifunctional operating scenarios. Possible roles of state-dependent absorption are clarified. Full article

This study investigates the electronic and photocatalytic properties of greenly fabricated rutile-phase titanium dioxide (bTiO₂) modified with iron vanadate (Fe/bTiO₂/VO₄) and vanadium-substituted goethite (Fe/bTiO₂/VOOH) by detailed experimental assay and density functional theory (DFT) calculations. Our analysis of the density of states (DOS), band structure, and work function reveals that both dopant systems significantly modify the electronic structure of pure rutile bTiO₂. The dye methyl orange (MO) was used as the model pollutant. During photodegradation tests, parameters such as the reaction time, solid-to-liquid ratio, initial concentrations of the photocatalyst and dye, as well as distance of the lamp from the reactor and pH were varied. Degradation kinetics follows the equation of the pseudo-first order law for both photocatalysts (k_VO4 = 0.058 min⁻¹ and k_VOOH = 0.065 min⁻¹), while degradation efficiencies of 92% and 99% were observed after 120 min at pH 3, respectively. Specifically, the DOS analysis highlights the contribution of Fe 3d and V 3d orbitals, which create new electronic states within the bandgap, facilitating charge transfer. These insights provide a strong foundation for the rational design of novel, highly efficient Fe/bTiO₂-based photocatalysts for the degradation of organic pollutants in water. Full article

Vanadium dioxide (VO₂) films have attracted extensive attention for their pronounced metal–insulator transition (MIT) and multifunctional responses, holding great promise for smart windows, infrared stealth, memristive devices, and advanced sensors. However, conventional approaches for tuning the transition temperature, such as elemental doping or heterostructure engineering, often suffer from complicated processing, impurity phases, and poor device uniformity. Here, we use a dopant-free, high-vacuum annealing (9 × 10⁻⁴ Pa, ≈9 × 10⁻⁶ mbar) strategy to regulate the intrinsic structural evolution of VO₂ films via oxygen-vacancy engineering and to clarify its influence on electrical switching contrast and infrared emissivity modulation. As the annealing temperature increases under low oxygen partial pressure, oxygen vacancies gradually accumulate, converting V⁴⁺ to V³⁺ and driving the films through three distinct structural stages: low-temperature lattice expansion with preserved M1 framework, critical structural collapse at 550 °C, and high-temperature defect rearrangement with local recrystallization. Consequently, the electrical MIT temperature continuously decreases, but the switching ratio collapses at the critical point and only partially recovers after high-temperature reorganization, while the infrared emissivity response transitions from abrupt, phase-transition-dominated switching to a continuous, tunable modulation at elevated temperatures. Notably, the infrared response begins continuous tuning earlier (≈450 °C) than the collapse of electrical MIT, reflecting the different sensitivities of optical and electronic responses to local lattice defects. These results reveal the coupling among oxygen-vacancy evolution, structural stability, electrical contrast, and infrared modulation in compositionally simple VO₂ films. Compared with conventional doping, this high-vacuum annealing strategy avoids impurity phases, preserves compositional simplicity, and provides a scalable defect-engineering route to design VO₂-based devices with reconfigurable electrical and infrared response modes. Full article

This article describes a study on the synthesis and annealing processes of thin-film coatings of vanadium oxide on flat, parallel substrates made of quartz glass, sapphire, and silicon, as well as optical fibers using an organometallic precursor, triisopropoxy vanadium (V) oxide. For the first time, optical constants of nanomaterials were estimated in real time during synthesis and subsequent annealed using the lossy-mode resonance effect. The coatings produced in an inert atmosphere after deposition were amorphous, comprising a mixture of VO₂, V₂O₅, V₆O₁₃, and V₃O₅. This method allowed for accurate determination of the threshold temperature for the transformation of oxide mixtures into a monocomponent phase. Optimal conditions for synthesis and annealing were determined for the production of vanadium dioxide (VO₂) and pentoxide (V₂O₅). Morphological changes in coated surfaces were observed as a result of heat treatment. The composition and properties of these samples were studied using optical, terahertz and Raman spectroscopy, as well as temperature-dependent analysis of electrical resistance. The morphology of the coating surface was determined using a scanning electron microscope and an atomic force microscope. The reduction of VO_x to VO₂ was studied in an atmosphere of hydrogen and argon during annealing after deposition, with its effectiveness being compared. It was shown for the first time that the reduction of higher vanadium oxides is due to the presence of elemental carbon in the volume of the material formed from a metalorganic precursor during growth of vanadium oxide. Coatings obtained by annealing in hydrogen had a smaller hysteresis loop width (~5 °C) during phase transition compared to coatings obtained by argon annealing (~9 °C). Both types of coatings demonstrated a 50–60% increase in transmission at 1 THz frequency and in the IR region, accompanied by a 10³–10⁴-fold change in electrical resistance. Full article

A terahertz multifunctional metasurface based on vanadium dioxide (VO₂) and graphene that can switch between waveplate and absorber functionalities is proposed. As the temperature is below 300 K, by electrically controlling the Femi energy of the graphene it can realize half-wave plate (HWP) and quarter-wave plate (QWP) functionalities in the operating bandwidths of both 1.39–2.34 THz and 0.92–2.68 THz, respectively. While the temperature is above 340 K, the dipole resonance between VO₂ and a gold reflector induces absorption. Furthermore, by applying the voltage to graphene, dual-parameter modulation of the amplitude of the transverse electric (TE) waves and the resonance frequency of the transverse magnetic (TM) waves is achieved, the absorption bandwidths of which are 3.65–3.78 THz and 1.41–3.12 THz, respectively. The operating frequencies for HWP, QWP, TE and TM waves can be tuned by changing the electrical field and working temperature. In addition, the incident angles are not sensitive to the performance of the metasurface, confirming its effectiveness even under large-angle incidence. The metasurface with simplicity in design, mature fabrication processes, and comprehensive functionality, has certain promising applications in terahertz optical switches, terahertz spectroscopy systems, modulators, and communication systems. Full article

This study aims to reduce the phase transition temperature (PTT) of W-doped vanadium dioxide (VO₂) multilayer thin films. We designed and fabricated two asymmetric multilayer thin film structures; namely, TiO₂/VO₂-5%W/ITO and ITO/VO₂-5%W/TiO₂. The W-doped VO₂-based Trilayer thin films were deposited using an electron beam evaporation combined with the ion-assisted deposition (IAD) technique. An experimental study was conducted on the temperature-dependent residual stress and optical properties of the two asymmetric VO₂-based three-layer structures. The VO₂-based thin films were characterized using UV–Vis–NIR spectrophotometry, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and an improved Twyman–Green interferometer combined with fast Fourier transform (FFT) analysis for residual stress measurement. The trilayer structures incorporated a ~60 nm thick W-doped VO₂ middle layer, which plays a critical role in modulating thermochromic behavior and residual stress evolution. The results show that both trilayer thin films demonstrated excellent optical performance in transmission spectra. Raman spectral analysis revealed a blue shift in the characteristic W-doped VO₂ peaks, accompanied by a decrease in peak intensity as the temperature increased. Heating experiments on asymmetric W-doped VO₂ trilayer thin films revealed that the critical transition temperature of the ITO/VO₂-5%W/TiO₂/B270 trilayer film structure was significantly reduced to 45 °C. This demonstrates the effectiveness of our proposed multilayer film design in improving the PTT of W-doped VO₂ thin films. Analysis of the changes in residual stress of the trilayer thin films during heating experiments revealed that the residual stress shifted from compressive to tensile in the temperature range of 40 °C to 50 °C. The thermal expansion coefficient and biaxial modulus of the TiO₂/VO₂-5%W/ITO trilayer film structure were 5.37 × 10⁻⁶ °C⁻¹ and 295.7 GPa, respectively. In addition, the thermal expansion coefficient and biaxial modulus of the ITO/VO₂-5%W/TiO₂ trilayer film structure were 6.65 × 10⁻⁶ °C⁻¹ and 745.0 GPa. Full article

Various metal-modified titanium dioxide (TiO₂) nanotubes have been widely investigated for water purification due to their large surface area, stability, and photocatalytic activity. In this context, this study investigates the deposition of vanadium-based nanoparticles (V NPs) on TiO₂ nanotubes via immersion in aqueous dispersions of V NPs synthesized by picosecond and nanosecond pulsed laser ablation in liquid at four different output energies (picosecond: 15 and 30 mJ; nanosecond: 120 and 250 mJ), with the aim of improving their photocatalytic performance. By optimizing the concentration of V NPs in the dispersions and the immersion time, the degradation efficiency of Acid Yellow 23 under photocatalytic conditions was enhanced for TiO₂ modified with V NPs synthesized at output energies of 30 and 250 mJ, whereas no improvement was observed for TiO₂ modified with V NPs synthesized at 15 and 120 mJ. A series of V-TiO₂ photocatalysts was fabricated by depositing laser-synthesized V NPs of various sizes on TiO₂ nanotubes prepared by electrochemical anodization of a titanium mesh. Full article

A bidirectional infrared absorber leveraging the Fabry–Perot resonance within a cascaded metal-dielectric nano-film structure is proposed. The absorber integrates a top Ag–VO₂–SiO₂ film stack, an intermediate thin Ag metal layer, and a bottom Al₂O₃–Ti–Al₂O₃ layer, enabling switchable narrowband and broadband absorption under forward and backward illumination, respectively. Under front illumination, the structure exhibits a high narrowband absorption peak of 98% at a wavelength of 1110 nm when VO₂ is in its metallic state. In contrast, when VO₂ transitions to its insulating state, the absorption peak shifts to 1165 nm. Additionally, under back illumination, ultra-broadband absorption is achieved, covering a wavelength range of 1000–2760 nm with an average absorption of 98%. The proposed absorber demonstrates excellent absorption performance with structural simplicity and low manufacturing cost, offering great potential for applications in solar photovoltaic devices, photodetectors, and related fields. Full article

A hybrid graphene-VO₂ reconfigurable terahertz metamaterial absorber is proposed for broadband radar cross-section (RCS) reduction and high-performance sensing. The designed structure leverages the phase transition property of VO₂ and the electrostatic tunability of graphene to achieve dynamic switching between ultra-broadband and narrowband absorption states. When VO₂ is in the metallic state and graphene is unbiased, the absorber exhibits over 90% absorption across 0.82~3.50 THz, corresponding to a relative bandwidth of 124%. In the narrowband mode, with VO₂ in the insulating state and graphene biased (E_f = 1 eV), a sharp absorption peak exceeding 60% is achieved at 1.48 THz. The symmetrical design ensures polarization insensitivity and wide-angle stability. Applications in broadband RCS reduction higher than 10 dB and refractive index sensing with a sensitivity of 24.86 GHz/RIU are demonstrated, surpassing conventional terahertz sensors. This work provides a promising platform for adaptive terahertz stealth and sensing systems. Full article

A reconfigurable THz metasurface (MS) capable of independent reflection amplitude and phase modulation is designed and analyzed. The tunability is achieved in a simple few-layer structure by control over the chemical potential of a graphene monolayer patterned in square patches and over the bulk conductivity of an overlying vanadium dioxide (VO₂) patch array; these impart control over the reflection phase and magnitude, respectively. To design and analyze the MS unit cell, we employ intuitive equivalent circuit and transmission line modeling, which is validated against full-wave simulations, showing good agreement in the regime of interest, i.e., on the first resonance for normal plane wave incidence. The simulated phase modulation approaches $250°$, enabling binary-encoded digital metasurface designs, while the magnitude modulation spans more than 20 dB, from $-3$ dB almost down to perfect absorption. The flexibility of dynamic phase and amplitude control can unlock the full potential of such THz MS hybrid designs for future wireless communications (6G and beyond) and for sensing applications. Finally, the analytical modeling can be extended to polarization-dependent, anisotropic, or non-local EM responses and/or to include aspects of the multiphysical control mechanisms. Full article

This paper introduces a vanadium dioxide-integrated broadband metamaterial absorber designed for the terahertz frequency range. The simulation results for the proposed structure demonstrate a wide 90% absorption bandwidth of 8.23 THz, corresponding to a fractional bandwidth of 89.5%. By leveraging the phase-transition properties of VO₂, the absorber demonstrated dynamic adjustability by modulating the absorption from 3% to 98.74%. The absorption mechanism was analyzed through the impedance matching theory and electromagnetic field distributions, confirming the role of magnetic resonance and interference. Furthermore, machine learning algorithms, specifically Linear Regression, Support Vector Regression, and Random Forest (RF), were applied to accelerate the design process and optimize the structural parameters. Among these, the RF model demonstrated superior prediction accuracy. The machine learning-assisted optimization successfully extended the effective absorption bandwidth to 9 THz, representing an improvement by 9.4% compared to the traditional optimization methods. These results validate the efficacy of combining electromagnetic simulation with data-driven techniques for advanced metamaterial design. Full article

Vanadium dioxide (VO₂) is a highly promising material for infrared laser protection due to the pronounced optical switching effect during its metal–insulator transition (MIT). However, due to the relatively high MIT temperature of VO₂ and the low transmittance contrast before and after the MIT, practical applications face challenges in modulation depth and response time. In this study, we address these issues using a wafer-scale VO₂/GaN/Al₂O₃ heterostructure fabricated by oxide molecular beam epitaxy. The conductive GaN interlayer enables local Joule heating of the VO₂ film, permitting direct control of the MIT via an external bias with a threshold of 4.7 V. This structure exhibits a substantial resistance change of four orders of magnitude and enables adaptive limiting of a 3.7 μm laser, reducing transmittance from 60% to 10%. Our work demonstrates a practical, wafer-scale laser-protection device and introduces a pre-excitation strategy via external biasing to enhance response performance. Full article

Chiral metasurfaces exhibit enormous potential in optical applications, and their integration with phase-change material vanadium dioxide (VO₂) provides a novel pathway for dynamic regulation. In this study, a chiral absorptive metasurface based on VO₂ is designed. By tuning the VO₂ conductivity around the operating frequency of 2.81 THz, the circular dichroism (CD) can be continuously adjusted from 0.06 to 0.95, realizing a high-contrast chiral switch. On this basis, the Pancharatnam–Berry (PB) phase is introduced to construct a chirality-dependent phase gradient: when the VO₂ conductivity is 200,000 S/m, only the left-handed circularly polarized (LCP) wave is subjected to periodic phase modulation, enabling controllable deflection of the reflected beam, while the right-handed circularly polarized (RCP) wave is selectively absorbed. This “chiral phase encoding” strategy simultaneously achieves absorptive CD tuning and reflective beam shaping on a single metasurface, significantly enhancing the flexible manipulation capability of circular polarization states in the terahertz band. It provides a compact and efficient solution for reconfigurable imaging, unidirectional communication, and integrated photonics systems. Full article