Jun Dai

Bio: Jun Dai is an academic researcher from Huazhong University of Science and Technology. The author has contributed to research in topics: Optical switch & Nanocrystalline material. The author has an hindex of 1, co-authored 1 publications receiving 12 citations.

Design of Dual-Band Uncooled Infrared Microbolometer

Abstract: -This paper describes the design and modeling of a smart uncooled infrared detector with wavelength selectivity in the long-wavelength infrared (LWIR) band. The objective is to enhance the probability of detecting and identifying objects in a scene. This design takes advantage of the smart properties of vanadium dioxide (VO2): it can switch reversibly from an IR-transparent to an IR-opaque thin film when properly triggered. This optical behavior is exploited here as a smart mirror that can modify the depth of the resonant cavity between the suspended thermistor material and a patterned mirror on the substrate, thereby altering wavelength sensitivity. The thermistor material used in the simulation is vanadium oxide (VOx). The simulation results show that, when VO2 is used in the metallic phase, it reflects IR radiation back to the suspended VOx and enhances IR absorption in the 9.4-10.8-μm band. When the film is switched to the semiconductor phase, it admits most IR radiation, which is then reflected back to the suspended VOχ by a patterned gold thin film under an SiO2 spacer layer. The spacer layer is used to increase the resonant cavity depth underneath the microbolometer pixel. Thus, the peak absorption value is shifted to 8-9.4 μm, creating the second spectral band. The detector is designed with a relatively low thermal conductance of 1.71 X 10-7 W/K to maximize responsivity (Rv) to values as high as 1.27 X 105 W/K and detectivity (D*) to as high as 1.62 x 109 cm-Hz1/2/W, both at 60 Hz. The corresponding thermal time constant is equal to 2.45 ms. Hence, these detectors could be used for 60-Hz frame rate applications. The extrapolated noise equivalent temperature difference is 14 and 16 mK for the 8-9.4- and 9.4-10.8-μm bands, respectively. The calculated absorption coefficients in the two spectral bands were 59% and 65%, respectively.

Study of the structural, thermal, optical, electrical and nanomechanical properties of sputtered vanadium oxide smart thin films

Abstract: Vanadium oxide thin films were grown on both quartz and Si(111) substrates, utilizing a pulsed RF magnetron sputtering technique at room temperature with the RF powers at 100 W to 700 W. The corresponding thicknesses of the films were increased from 27.5 nm to 243 nm and 21 nm to 211 nm as the RF power was increased from 100 W to 700 W for the quartz and silicon substrates, respectively. X-ray diffraction and field emission scanning electron microscopy were carried out to investigate the phase and surface morphology of the deposited films. The electronic structure and the vanadium oxidation states of the deposited films were investigated thoroughly by X-ray photoelectron spectroscopy. The as-grown films show only stoichiometric vanadium oxide, where vanadium is in V5+ and V4+ states. The phase transitions of the vanadium oxide films were investigated by the differential scanning calorimetric technique. The reversible i.e. smart transition was observed in the region from 337 °C to 343 °C. The average hemispherical infrared emittance of the deposited vanadium oxide films was evaluated by an emissometer in the wavelength range of 3 μm to 30 μm. The sheet resistance of the deposited films was measured by a two-probe method and the data were in the range of 106 to 105 Ω per square. The optical properties of the films, such as solar transmittance, solar reflectance and solar absorptance, as well as optical constants e.g. optical band gap, were also evaluated. Finally, mechanical properties such as the hardness and the Young’s modulus at the microstructural length scale were evaluated by employing a nanoindentation technique with a continuous stiffness mode.

Incorporation of vanadium oxide films in optical fibers for temperature sensing and optical switching applications

Abstract: Fiber optic temperature sensors were fabricated by depositing vanadium oxide thin films on the tips of optical fibers, and by incorporating vanadium oxide materials into the core of optical fibers. It was found that the properties of the initially amorphous vanadium oxide can be controllably converted to those of crystalline VOx compounds via the plasma arc of a fiber fusion splicer. These crystalline VOx compounds can then be over-coated with SiO2, and subsequently fused with another fiber to form an in-line fiber optic sensor. It was found that a well defined optical absorption edge was formed when the vanadium oxide (VOx) thin films were annealed using the plasma arc of a fusion splicer, suggesting the formation of crystalline VOx. Moreover, it was observed that the spectral position of this absorption edge varied with temperature in a reproducible way. The optical fiber devices described in this paper could also be employed for optical switching applications. Based on the spectral position of the band edge and the Raman spectra of the VOx films, deposited on the fiber optic tips, it was found that these annealed VOx films contained a mixture of different phases of vanadium oxide (VOx), in particular V2O5 and VO2. Furthermore, similar in-line optical fiber switches, based only on the insulator to metal phase transitions of VO2, can be fabricated by following the techniques described in this paper.

Reversible phase transition in vanadium oxide films sputtered on metal substrates

Abstract: Vanadium oxide films, deposited on aluminium (Al), titanium (Ti) and tantalum (Ta) metal substrates by pulsed RF magnetron sputtering at a working pressure of 1.5 x10−2 mbar at room temperature are found to display mixed crystalline vanadium oxide phases viz., VO2, V2O3, V2O5. The films have been characterized by field-emission scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy, and their thermo-optical and electrical properties have been investigated. Studies of the deposited films by DSC have revealed a reversible-phase transition found in the temperature range of 45–49 °C.