Bandwidth-tunable THz absorber based on diagonally distributed double-sized VO2 disks.

Terahertz absorbers combined with phase-changing VO2 are a class of stealth materials with adjustable absorptance. However, such absorbers still suffer from insufficient absorption bandwidth. We propose a three-layer terahertz (THz) absorber, consisting of an array of diagonally distributed double-sized VO2 disks on a silica-coated gold film. We find this structure can generate the superposition of three resonant absorption peaks to broaden the absorption band. The finite element simulation (FES) results show that the absorption bandwidth can be adjusted from 2.63 to 5.04 THz by simply changing the sizes of the VO2 disks. In addition, the peak absorptance can be continuously regulated from 9.8% to 96% by varying the conductivity of VO2. Finally, the absorber is polarization-insensitive and has wide-angle absorption. The wide absorption band and adjustable bandwidth of the absorbers have important applications potentially for intelligent stealth materials.

Localized surface plasmon resonance modulation of totally encapsulated VO2/Au/VO2 composite structure.

We design and fabricate a totally encapsulated VO2/Au/VO2 composite structure which is aimed to improve the tunability of the localized surface plasmon resonance (LSPR) peak. In this work, the structure will ensure all the Au NPs' resonant electric field area is filled with VO2. The modulation range of the totally encapsulated structure is larger than that of the semi-coated structure. To further improve the modulation range, we also explore the VO2 thickness dependence of the structure's LSPR modulation. With the increase of the top layer VO2 thin film thickness, the modulation range becomes larger. When the thickness is about 80 nm, the absorption peak achieves a largest shift of 112 nm. FDTD solution and equivalent model of series capacitor are used to explain the phenomenon. These results will contribute to the area of metamaterial electromagnetic wave absorber and other fields.

Geometric constraints on phase coexistence in vanadium dioxide single crystals.

The appearance of stripe phases is a characteristic signature of strongly correlated quantum materials, and its origin in phase-changing materials has only recently been recognized as the result of the delicate balance between atomic and mesoscopic materials properties. A vanadium dioxide (VO2) single crystal is one such strongly correlated material with stripe phases. Infrared nano-imaging on low-aspect-ratio, single-crystal VO2 microbeams decorated with resonant plasmonic nanoantennas reveals a novel herringbone pattern of coexisting metallic and insulating domains intercepted and altered by ferroelastic domains, unlike previous reports on high-aspect-ratio VO2 crystals where the coexisting metal/insulator domains appear as alternating stripe phases perpendicular to the growth axis. The metallic domains nucleate below the crystal surface and grow towards the surface with increasing temperature as suggested by the near-field plasmonic response of the gold nanorod antennas.

All-Optically Controlled Artificial Synapses Based on Light-Induced Adsorption and Desorption for Neuromorphic Vision.

Artificial synapses with the capability of optical sensing and synaptic functions are fundamental components to construct neuromorphic visual systems. However, most reported artificial optical synapses require a combination of optical and electrical stimuli to achieve bidirectional synaptic conductance modulation, leading to an increase in the processing time and system complexity. Here, an all-optically controlled artificial synapse based on the graphene/titanium dioxide (TiO2) quantum dot heterostructure is reported, whose conductance could be reversibly tuned by the effects of light-induced oxygen adsorption and desorption. Synaptic behaviors, such as excitatory and inhibitory, short-term and long-term plasticity, and learning-forgetting processes, are implemented using the device. An artificial neural network simulator based on the artificial synapse was used to train and recognize handwritten digits with a recognition rate of 92.2%. Furthermore, a 5 × 5 optical synaptic array that could simultaneously sense and memorize light stimuli was fabricated, mimicking the sensing and memory functionality of the retina. Such an all-optically controlled artificial synapse shows a promising prospect in the application of perception, learning, and memory tasks for future neuromorphic visual systems.

NO2 Gas Sensing Performance of a VO2(B) Ultrathin Vertical Nanosheet Array: Experimental and DFT Investigation.

A VO2(B) ultrathin vertical nanosheet array was prepared by the hydrothermal method. The influence of the concentration of oxalic acid on the crystal structure and room-temperature NO2 sensing performance was studied. The morphology and crystal structure of the nanosheets were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Room-temperature gas sensing measurements of this structure to NO2 with a concentration span from 0.5 to 5 ppm were carried out. The experimental results showed that the thickness of the vertical VO2(B) nanosheet was lower than 20 nm and close to the 2 times Debye length of VO2(B). The response of the sensor based on this structure to 5 ppm NO2 was up to 2.03, and the detection limit was 20 ppb. Its high response performance was due to the fact that the target gas could completely control the entire conductive path by forming depletion layers on the surface of VO2(B) nanosheets. Density functional theory was used to analyze the adsorption of NO2 on the VO2(B) surface. It is found that the band gap of VO2(B) becomes narrower and the Fermi level moves to the valence band after NO2 adsorption, and the density of states near the Fermi level increases significantly. This ultrathin vertical nanosheet array structure can make VO2(B) detect NO2 with high sensitivity at room temperature and therefore has potential applications in the field of low-power-consumption gas sensors.

[Electrical and optical phase transition properties of nano vanadium dioxide thin films].

Nano-vanadium dioxide thin films were prepared through thermal annealing vanadium oxide thin films deposited by dual ion beam sputtering. The nano-vanadium dioxide thin films changed its state from semiconductor phase to metal phase through heating by homemade system. Four point probe method and Fourier transform infrared spectrum technology were employed to measure and anaylze the electrical and optical semiconductor-to-metal phase transition properties of nano-vanadium dioxide thin films, respectively. The results show that there is an obvious discrepancy between the semiconductor-to-metal phase transition properties of electrical and optical phase transition. The nano-vanadium dioxide thin films' phase transiton temperature defined by electrical phase transiton property is 63 degrees C, higher than that defined by optical phase transiton property at 5 microm, 60 degrees C; and the temperature width of electrical phase transition duration is also wider than that of optical phase transiton duration. The semiconductor-to-metal phase transiton temperature defined by optical properties increases with increasing wavelength in the region of infrared wave band, and the occuring temperature of phase transiton from semiconductor to metal also increases with wavelength increasing, but the duration temperature width of transition decreases with wavelength increasing. The phase transition properties of nano-vanadium dioxide thin film has obvious relationship with wavelength in infrared wave band. The phase transition properties can be tuned through wavelength in infrared wave band, and the semiconductor-to-metal phase transition properties of nano vanadiium dioxide thin films can be better characterized by electrical property.