Wanxia Huang

Photoinduced active terahertz metamaterials with nanostructured vanadium dioxide film deposited by sol-gel method

Applying the photoexcitation characteristics of vanadium dioxide (VO(2)), a dynamic resonant terahertz (THz) functional device with the combination of VO(2) film and dual-resonance metamaterial was suggested to realize the ultrafast external spatial THz wave active manipulation. The designed metamaterial realizes a pass band at 0.28-0.36 THz between the dual-resonant frequencies, and the VO(2) film is applied to control the transmittance of the spatial THz wave. More than an 80% modulation depth has been observed in the statics experiment, and the dynamic experimental results illustrate that this active metamaterial realizes up to a 1 MHz amplitude modulation signal loaded on a 0.34 THz carrier wave without any low noise amplified devices. The electromagnetic properties and photoinduced dynamic characteristics of this structure may have many potential applications in THz functional components, including modulators, intelligent switches, and sensors.

Synthesis and properties of Mo and W ions co-doped porous nano-structured VO2 films by sol–gel process

Porous nano-structured vanadium dioxide (VO2) films doped with Mo and W ions had been synthesized by sol gel process by employing a sol containing ammonium molybdate and ammonium tungstate with the addition of cetyltrimethyl ammonium bromide (CTAB). The effects of molybdenum and tungsten co-doping and CTAB addition on the structure, morphologies, crystalline and optical properties of VO2 films were investigated systematically in this study. The composition and microstructure were detected by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The Mo and W ions co-doped porous nano-structured VO2 films showed excellent infrared transmittance (nearly 70xa0%), large transmittance difference (55xa0%) before and after the phase transition, low transition temperature (35xa0°C), wide hysteresis width (22xa0°C) and fast phase transition. The results suggest that such Mo and W ions co-doped porous nano-structured VO2 film is an ideal fundamental material for optical data storage.

Asymmetric single-particle triple-resonant metamaterial in terahertz band

This paper presents the design, simulation, and measurement of an asymmetric triple-band metamaterial composed of single geometry electric field coupled resonators in the terahertz region. Theoretical and experimental results show that the structure has three distinct and strong absorption frequency peaks near 0.38, 0.58, and 0.74 THz, all of which are related to the inductance-capacitance resonance of the metamaterial. Due to the well-separating of different resonances in the particle, this metamaterial shows potentially application promises in the design of multiband terahertz devices.

Curing of polyester powder coating modified with rutile nano-sized titanium dioxide studied by DSC and real-time FT-IR

Polyester powder coating modified with 2 mass% of rutile nano-sized titanium dioxide (nano-TiO2) was prepared by melt-blend extrusion method. The nano-TiO2 dispersion state in the powder coating matrix was analyzed by field emission scanning electron microscopy (FE-SEM), which presented a well dispersion of modified nano-TiO2 in the polyester powder coating. The effect of nano-TiO2 on the curing of polyester powder coating was investigated by differential scanning calorimeter (DSC). Kissinger and Crane methods were used to study the activation energy (E) and reaction order (n) of the coating. The results indicated that 2 mass% of nano-TiO2 additive played a prompting role in the curing of polyester powder coatings, due to the hydroxyl functional groups existed on the surface of nano-TiO2. Furthermore, real-time Fourier transform infrared (FT-IR) spectroscopy with a heating cell was also employed to record the curing actions, of which results were consistent with the DSC experiments.

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO2) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V2O5 colloid and different surfactants. Porous nano-structured VO2 films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO2 films showed excellent infrared transmittance (nearly 70xa0%), low transition temperature (59.7xa0°C without doping), wide hysteresis width (37.8xa0°C), and different optical transmittance difference before and after the phase transition (39–67xa0%). The results suggest that these porous nano-structured VO2 films have significant importance in practical application in VO2-based optical and electronic devices.

Enhanced hydrophilicity of the Si substrate for deposition of VO2 film by sol–gel method

We have illustrated the role of hydrophilic nature of Si substrate played in the improvement of the contact performance between the vanadium dioxide (VO2) film and Si substrate. The VO2 films were fabricated by sol–gel method on single crystal Si substrate, which was pre-treated with hydrophilic solution and obtained a quite improved hydrophilicity. The bonding of Si substrate with precursor V2O5 gel was interpreted. The morphology and crystalline structure of the films were investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction. It is shown that the surface of the film on Si substrate with enhanced hydrophilicity is quite homogeneous and uniform. The film exhibits the formation of VO2 phase with (011) preferred orientation. Moreover, the optical pump induced phase transition property of the film was studied by terahertz time-domain spectroscopy, which revealed around 70% reduction of transmission at 0.1–1.5xa0THz in the VO2 film across the phase transition.

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO2) and CeOx–VO2 (1.5xa0<xa0xxa0<xa02) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO2 and CeOx–VO2 composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70xa0% though the concentrations of cerium reached 9.77 atxa0%. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10xa0% at λxa0=xa04xa0μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66xa0°C of the pure VO2 and CeOx–VO2 composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO2 composite film with an addition of 7.82 atxa0% Ce showed a largest hysteresis width with about 20.6xa0°C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.

Role of incident polarization in THz transmission through the hole array

The theoretical and experimental study of the role of incident polarization in THz transmission through the hole array is presented in this paper. First, the detailed theoretical investigation and numerical calculations are carried out and the mode expansion theory is applied to analyse the physical process. Therefore, the homogeneous coupled equations governing the transmission are obtained. Second, the experimental study by using terahertz time-domain spectroscopy has shown that not only the p-polarization but also other polarization angles can support the strong transmission, and the high-order modes can be excited in the holes.