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VO2, which exhibits semiconductor–metal phase transition characteristics occurring on a picosecond time scale, holds great promise for ultrafast terahertz modulation in next-generation communication. However, as of now, there is no reported prototype for an ultrafast device. The temperature effect has been proposed as one of the major obstacles. Consequently, reducing the excitation threshold for the phase transition would be highly significant. The traditional strategy typically involves chemical doping, but this approach often leads to a decrease in phase transition amplitude and a slower transition speed. In this work, we proposed a design featuring a highly conductive MXene interfacial layer between the VO2film and the substrate. We demonstrate a significant reduction in the phase transition threshold for both temperature and laser-induced phase transition by adjusting the conductivity of the MXene layers with varying thicknesses. Our observations show that the phase transition temperature can be decreased by 9 °C, while the pump fluence for laser excitation can be reduced by as high as 36%. The ultrafast phase transition process on a picosecond scale, as revealed by the optical-pump terahertz-probe method, suggests that the MXene layers have minimal impact on the phase transition speed. Moreover, the reduced phase transition threshold can remarkably alleviate the photothermal effect and inhibit temperature rise and diffusion in VO2triggered by laser. This study offers a blueprint for designing VO2/MXene hybrid films with reduced phase transition thresholds. It holds significant potential for the development of low-power, intelligent optical and electrical devices including, but not limited to, terahertz modulators based on phase transition phenomena.

The urgent requirement for ultrahigh Terahertz (THz) transmittance in sixth-generation (6G) communication demands specifically developed low-dielectric material with low THz signal loss, and the most efficient method is to take full advantage of ultralow dielectric air to reduce the THz loss of the dielectric material. Therefore, finding a way to introduce air into the matrix and maintain the stability of the corresponding air structure is the key issue for the preparation of ultralow THz loss dielectric material. In this work, low-dielectric poly(tetrafluoroethylene) (PTFE) (Dfof only 0.0004) was selected to form in situ nanofibers to regulate the viscoelasticity of polyphenylene sulfide (PPS) with excellent comprehensive performance in fifth-generation (5G) communication, and the nonresidual supercritical CO2foaming method was applied to introduce a large amount of ultralow dielectric air into the PTFE@PPS nanocomposite. The obtained PTFE@PPS nanocomposite microcellular foam had excellent low-dielectric properties (Dk= 1.19, Df= 0.000255) at THz frequency and, correspondingly, an ultrahigh THz transmission (≥90%). For a THz patch antenna, the THz signal transmission was merely 53.5 m when solid PPS was used as the substrate. When the substrate was changed to the PTFE@PPS nanocomposite microcellular foam specifically developed in this work, the available distance for THz signal transmission increased 9 times to 456.1 m. Furthermore, compared with the contact angle of solid PPS (84.6°), the obtained PTFE@PPS nanocomposite microcellular foam has a superior hydrophobic performance with a contact angle of 135.4°. The strategy applied in this work thus effectively guides the fabrication of low THz signal loss material for 6G communication.

Ti2O3with an ultranarrow bandgap of 0.1 eV, exhibiting an intriguing thermally induced metal–insulator transition (MIT), has drawn significant attention in photothermal conversion, photocatalysts, and mid-infrared photodetection. However, the ultrafast photoexcitation dynamics of Ti2O3film and whether the laser excitation can trigger the MIT of Ti2O3were rarely discussed. Here, a typical time-resolved optical pump terahertz probe (OPTP) spectroscopy was used to investigate the ultrafast terahertz response of Ti2O3film excited by a femtosecond laser. We found that a transient photoinduced carrier generation process occurred within 4 ps. A fast relaxation with a time constant of approximately 2 ps followed, which can be attributed to the recombination of the photogenerated carriers. Subsequently, a slow transition process was observed after the fast recovery process, and this phenomenon can be ascribed to the ultrafast photothermal conversion through nonradiative relaxation. Our work will provide new sights for investigating MIT characteristics of Ti2O3and open a route toward achieving ultrafast dynamic modulation optoelectronics operating in the terahertz regime.

Optical physical unclonable function (PUF) is one of the most promising hardware security solutions, which has been proven to be resistant to machine learning attacks. However, the disordered structures of the traditional optical PUFs are usually deterministic once they are manufactured and therefore exhibit fixed challenge–response behaviors. Herein, a reconfigurable PUF (R-PUF) is proposed and demonstrated by using the reversible phase transition behavior of VO2nanocrystals combined with TiO2disordered nanoparticles. Both the simulation and experiment results show that the near-infrared laser speckle pattern of the R-PUF can be almost completely altered after the phase transition of VO2nanocrystals, resulting in a reconfigurable and reproducible optical response. The similarity of the response speckles shows an obvious hysteresis loop during the rise and drop of temperature, providing a simple way to regulate and control the response behaviors of the R-PUF. More importantly, the hysteretic characteristic provides a new dimension to describe the challenge–response behavior of the R-PUF besides the laser speckle, providing an effective way to improve the security and encoding capacity of the optical PUFs. The proposed R-PUF can be employed as a promising security primitive for high robustness and high-security authentication and encryption.

Tunable focusing planar lens possesses more flexible characteristics to facilitate the expansion of its function and application. This study presents a planar meta-lens based on the phase gradient vanadium dioxide composite metasurface controlled by an external thermal stimulus that enables the switchable focusing. By utilizing the insulator-metal phase transition characteristics of the vanadium dioxide components, the composite metasurface can show a different phase gradient distribution that could lead to various focuses. Both simulated and experimental results demonstrate that without the external stimulus, the vanadium dioxide components display dielectric characteristics that are a phase-transparent response to THz wave. While applying an external thermal stimulus, the phase transition leads to the metallization of vanadium dioxide components that could induce the redistribution of the phase gradient of the metasurface to realize a tunable focusing. The measurements show that the results are in line with our expectations and achieve three times the tunable focal length variation from 2.5 mm with a numerical aperture (NA) of 0.78 to 7.5 mm with an NA of 0.39 at 0.65 THz, which was reversible. The proposed switchable planar metalens is expected to deliver a promising new design for electromagnetic wave dynamic manipulation and enable active applications, such as dynamic imaging or other optical systems at THz frequencies.

The modulation of the terahertz (THz) wave is fundamental for its applications in next-generation communications, biological imaging, sensing, and so forth. Searching for higher efficient modulation is still in progress, although plenty of materials have been explored for tuning THz wave. In this work, optical-transparent self-assembled MXene films are used to modulate the THz reflection at the SiO2/MXene/air interface based on the impedance matching mechanism. By adjusting the number of stacked MXene layers/concentrations of MXene dispersions, the sheet conductivity of the MXene films will be changed so that the impedance at the SiO2/MXene/air interface can be tuned and lead to a giant modulation of THz reflection. Particularly, we demonstrate that the MXene films have highly efficient THz modulation from antireflection to reflection-enhancing with a relative reflection of 27% and 406%, respectively. This work provides a new pathway for developing the MXene films with the combination of optical-transparency and high smart THz reflection characteristics, and the films can be applied for THz antireflection or reflection-enhancing.

Dynamic tuning of terahertz (THz) wave has a great potential application as smart THz devices, such as switches, modulators, sensors, and so on. However, the realization of flexible THz modulation with high efficiency is rarely observed, which is nearly absent from the booming development and demands on flexible electronics. Here, we report a flexible THz modulation based on conductive polymer composites composed of thermoplastic polyurethane (TPU) and conductive particles (Ni). By designing the additive content of Ni particles, such a flexible layer exhibits resistivity change of 6–7 orders under tensile strain due to the formation of an electron-transport channel provided by the in situ evolution of the Ni network. It could be used to dynamically control the THz transmission with a giant modulation depth of around 96%, at a high strain operation (up to around 58.5%). Moreover, these characteristics are demonstrated to be available for highly tension sensitive THz spectroscopy and imaging. This work opens up a connection between flexible polymer-based composites and THz dynamic devices. It proposes an unprecedented flexible THz modulation with giant tuning efficiency and provides a scheme for contactless and passive tension sensors.

Temperature can control the degree of hydrogenation of titania (i.e., black TiO2), determining the defect chemistry and hence its optical absorption and electrical conductivity that are key to the photocatalytic activity. However, how pressure affects the two key factors is unknown. In this work, we used a diamond anvil cell to produce the required high pressure (HP) and studied the pressure dependences of the structure change, the electrical conductivity, and the light absorption of black titania using HP X-ray diffraction, Raman/UV–vis spectroscopy, and electrical transport measurements. Results reveal that accompanying the HP phase transition the electrical conductivity exhibits complex variation with pressure, in good accord with the band gap changes of involved HP phases as a function of pressure. This confirms the assumption that pressure affects the electrical conductivity of black titania via controlling the number of free electrons (holes) distributed in the conduction (valence) band, which is inversely proportional to the exponent of the band gap that scales almost linearly with the pressure. This work provides a fundamental understanding of the pressure-induced structure–property relationship in black titania and will have important implications for tuning the photocatalytic activity via pressure and for developing new applications such as pressure sensors.

A novel and scalable method combining surfactant template and electrophoresis deposition was used to prepare mesoporous titanium dioxide/reduced graphene oxide (TiO2/RGO) nanocomposite film with the assistance of sodium dodecyl benzene sulfonate using tetrabutyl titanate and graphene oxide (GO) as raw material. The electrochromic mechanism of TiO2/RGO nanocomposite film was investigated by XPS technique. The TiO2/RGO nanocomposite film showed that TiO2 nanocrystals can be anchored on the surface of RGO via chemical bonding such as Ti[?]O[?]C and Ti[?]C. The TiO2/RGO nanocomposite film exhibited shorter coloration/bleaching response times (3.9 s for coloration and 6.6 s for bleaching), higher coloration efficiency (34.9 cm2 C[?]1) and better coloration/bleaching cycle stability compared with pure TiO2 film. The electrochromic mechanism of TiO2/RGO nanocomposite film was accorded with valence transition model, and an island-bridge model was proposed to interpret the enhancement mechanism of electrochromic performance. This enhancement is mainly due to a larger specific surface area, higher electrical conductivity and better structural stability in TiO2/RGO nanocomposite film, which facilitates faster transport of Li+ ions and electrons, and longer coloration/bleaching cycle life. This work provides an effective approach and inspiration to improve the electrochromic performance of transition metal oxides, as well as explore other high-performance electrochromic materials.

The dynamic control of terahertz (THz) wave transmission on flexible functional materials is a fundamental building block for wearable electronics and sensors in the THz range. However, achieving high-efficiency THz modulation and low insertion loss is a great challenge while maintaining the excellent flexibility and stretchability of the materials. Herein, we report a Ti3C2TxMXene/waterborne polyurethane (WPU) membrane prepared by a vacuum-assisted filtration method, which exhibits excellent THz modulation properties across stretching. The hydrophilic Ti3C2TxMXene and WPU enable the uniform 3D distribution of Ti3C2TxMXene in the WPU matrix. Particularly, the stretchability with the maximum strain of the membranes can reach 200%, accompanied by dynamic tuning of THz transmittance for more than 90% and an insertion loss as low as −4.87 dB. The giant THz modulation continuously decreases with MXene content per unit area, accompanied by a lower density of the MXene interface and diminished THz absorption during stretching. Such a design opens a pathway for achieving flexible THz modulators with a high modulation depth and low insertion loss, which would be used for THz flexible and wearable devices.

Vanadium pentoxide/reduced graphene oxide (V2O5/rGO) nanocomposite film was fabricated by a facial sol-gel method. X-ray diffraction demonstrated that the interlayer distance of V2O5 increased from 1.05 to 1.36 nm due to the introduction of rGO. Raman spectra confirmed the co-existence of V2O5 and rGO in the V2O5/rGO nanocomposite film. Scanning electron microscopy revealed that V2O5 nanoparticles were well distributed on the surfaces of rGO sheets. And cyclic voltammograms and in-situ optical transmittance measurements confirmed that the V2O5/rGO composite film had better cyclic stability, faster switching speed, larger optical modulatory range and higher coloration efficiency compared with the V2O5 film. Moreover, the electrochemical impedance spectroscopy revealed that the V2O5/rGO composite film evinced the lower resistance due to the good interaction between the rGO sheets and the V2O5 nanoparticles. These improvements can be ascribed to the synergistic effect of rGO sheets and V2O5 nanoparticles. This study would be significant for optimizing the electrochromic performances of other transition metal oxide electrochromic materials by the incorporation of rGO.

VO2films were fabricated on high-purity single-crystalline silicon substrate by the sol–gel method, followed by rapid annealing. The composition and microstructure of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The results indicated a polycrystalline nature with high crystallinity and compact nanostructure for the films, and the concentration of +4 valence vanadium is 79.85%. Correlated with these, a giant transmission modulation ratio about 81% of the film was observed by terahertz time domain spectroscopy. The experimentally observed transmission characteristics were reproduced approximately, by a simulation at different conductivities across the phase transition. According to the effective-medium theory, we assumed that it is important to increase the concentration of +4 valence vanadium oxide phases and improve the compactness of the VO2films for giant phase transition properties. The sol–gel-derived VO2films with giant phase transition properties at terahertz range, and the study on their composition and microstructure, provide considerable insight into the fabrication of VO2films for the application in THz modulation devices.

A novel and scalable method combining surfactant template and electrophoresis deposition was used to prepare mesoporous titanium dioxide/reduced graphene oxide (TiO2/RGO) nanocomposite film with the assistance of sodium dodecyl benzene sulfonate using tetrabutyl titanate and graphene oxide (GO) as raw material. The electrochromic mechanism of TiO2/RGO nanocomposite film was investigated by XPS technique. The TiO2/RGO nanocomposite film showed that TiO2nanocrystals can be anchored on the surface of RGO via chemical bonding such as Ti−O−C and Ti−C. The TiO2/RGO nanocomposite film exhibited shorter coloration/bleaching response times (3.9 s for coloration and 6.6 s for bleaching), higher coloration efficiency (34.9 cm2C−1) and better coloration/bleaching cycle stability compared with pure TiO2film. The electrochromic mechanism of TiO2/RGO nanocomposite film was accorded with valence transition model, and an island-bridge model was proposed to interpret the enhancement mechanism of electrochromic performance. This enhancement is mainly due to a larger specific surface area, higher electrical conductivity and better structural stability in TiO2/RGO nanocomposite film, which facilitates faster transport of Li+ions and electrons, and longer coloration/bleaching cycle life. This work provides an effective approach and inspiration to improve the electrochromic performance of transition metal oxides, as well as explore other high-performance electrochromic materials.

Vanadium pentoxide/reduced graphene oxide (V2O5/rGO) nanocomposite film was fabricated by a facial sol–gel method. X-ray diffraction demonstrated that the interlayer distance of V2O5increased from 1.05 to 1.36 nm due to the introduction of rGO. Raman spectra confirmed the co-existence of V2O5and rGO in the V2O5/rGO nanocomposite film. Scanning electron microscopy revealed that V2O5nanoparticles were well distributed on the surfaces of rGO sheets. And cyclic voltammograms and in-situ optical transmittance measurements confirmed that the V2O5/rGO composite film had better cyclic stability, faster switching speed, larger optical modulatory range and higher coloration efficiency compared with the V2O5film. Moreover, the electrochemical impedance spectroscopy revealed that the V2O5/rGO composite film evinced the lower resistance due to the good interaction between the rGO sheets and the V2O5nanoparticles. These improvements can be ascribed to the synergistic effect of rGO sheets and V2O5nanoparticles. This study would be significant for optimizing the electrochromic performances of other transition metal oxide electrochromic materials by the incorporation of rGO.