Jianjun Lai

Characterization of nanostructured VO 2 thin films grown by magnetron controlled sputtering deposition and post annealing method

By magnetron controlled sputtering system, a new nanostructured metastable monoclinic phase VO2 (B) thin film has been fabricated. The testing result shows that this nanostructured VO2 (B) thin film has high temperature coefficient of resistance (TCR) of -7%/K. Scanning electron microscopy measurement shows that the average grain diameter of the VO2 (B) crystallite is between 100 and 250 nm. After post annealed, VO2 (B) crystallite is changed into monoclinic (M) phase VO2 (M) crystallite with the average grain diameter between 20 and 50 nm. A set up of testing the thin film switching time is established. The test result shows the switching time is about 50 ms. With the nanostructured VO2 (B) and VO2 (M) thin films, optical switches and high sensitivity detectors will be presented.

Wide-band achromatic flat focusing lens based on all-dielectric subwavelength metasurface

A new method for realizing achromatic flat focusing based on all-dielectric silicon subwavelength metasurface is presented. The designed subwavelength silicon-air slits waveguide array with varied widths can provide desired phase shift of beam focusing and has the non-dispersive characteristic when the period of each unit cell is far less than the wavelength of incident electromagnetic wave (about λ/10) in mid-infrared and far-infrared spectral range. Numerical simulation of an achromatic flat focusing lens in wide spectral range from 8μm to 12μm is performed by the finite difference time domain method and the results show agreement with theory analysis results. This work indicates an effective solution for wide-band achromatic flat optical elements and potential application in integrated achromatic infrared optical systems.

NANOSTRUCTURE AND DROPPING PHASE TRANSITION TEMPERATURE IN VANADIUM DIOXIDE THIN FILMS

A conventional microstructure of vanadium dioxide film with grain size of 1–2 μm exhibits a phase transition from a lower-temperature semiconductor phase to a higher-temperature metal one with a phase transition temperature of 68°C and with resistivity changes two to three orders of magnitude. Here, we report a nonconventional nanopolycrystalline thin film structure of vanadium dioxide with an average grain of 8 nm and with a phase transition temperature of about 35°C, indicating more promising device applications such as smart window coatings for solar energy, uncooled infrared imaging technology, optical and electronic switching devices, etc.

Research on hybrid integration technology between charge-coupled devices and diffractive microlens arrays

Optical concentration using a microlens with high diffractive efficiency is an important method to improve the fill factor and performances of visible charge-coupled device (CCD) sensors. In this paper, a 516 × 516 element diffractive microlens array (MLA) on the quartz substrate is designed based on scalar diffraction theory and fabricated by submicrometer photolithography technology and magnetically enhanced reactive ion etching. The integration procedures between the MLA and CCD chip are presented. The fill factor of the visible CCD with the MLA increases by a factor of 2.4 in comparison with that of CCD without the MLA. The measuring results show that the large-scale diffractive MLA is able to improve the detecting sensitivity of CCD sensors.

Patterning of visible/infrared dual-band microstrip filter arrays for multispectral imaging application

Visible/infrared dual-band microstrip filter arrays have been developed to be integrated with 512 × 512 PtSi CCD imaging sensor chips for multispectral imaging when it operates in the front-illumination mode. A high visible transmittance and high infrared reflectance ZAO (ZnO:Al) based coating for visible passband and an interference absorbing filter film for a mid-infrared passband have been designed and deposited on sapphire substrates. An effective double-layer lift-off technique that is compatible with high temperature deposition has been developed to create thick microstrip infrared film. The infrared passband film using germanium and yttrium fluoride as high and low refractive indices materials have been deposited by ion-beam-assisted electron beam evaporation. Tested optical performance results reveal that the visible and near-infrared transmittance of the infrared passband film is very low, which makes it ideal for mid-infrared imaging. Environmental durability testing shows that the microstrip arrays have good mechanical and thermal performances for practical applications.

Ultra-narrowband infrared thermal emitter based on Fabry–Perot-like vacuum resonance cavity

Coherent infrared thermal emitters are useful in various applications, especially in the spectral region where laser and LED light sources are lacking. The present study investigates a vertical resonant cavity narrowband infrared thermal emitter (VRCITE) with a Fabry?Perot-like vacuum resonance cavity separating the one-dimensional photonic crystal and the gold layer. This configuration overcomes the thermal stress problem, which is the biggest inherent obstacle in similar structures previously proposed by other workers, and could achieve an even narrower emission spectrum (about 4?nm). Theoretical analysis with a transfer matrix method (TMM) is given and the finite-difference time domain method (FDTD) has been adopted to simulate this structure. The results obtained by TMM and FDTD methods are consistent and thus its feasibility is confirmed.

UV-visible broadband wide-angle polarization-insensitive absorber based on metal groove structures with multiple depths

A new periodic Al groove structure coated by SiO2 thin film is designed and numerically investigated for wide-angle and polarization-insensitive broadband absorption. A metal groove array presents optical absorption enhancement due to the cavity mode resonance, with the absorption peak capable of being shifted by controlling the depth of the metal groove. Broadband absorption can be realized by a periodic array of metal grooves with different depths combined in one single period. A two-dimensional Al structure with four different grooves in each period is designed to realize polarization-insensitive broadband absorption from 220 nm to 800 nm with average absorption efficiency over 80% within the incident angle of 40°. These wide-angle and broadband absorption structures can be applied in UV/visible-related biochemical sensors, solar cells, or photocatalysts.

Vanadium dioxide nanomaterial for smart thermochromic glazing of windows

Nanostructural vanadium dioxide thin films are investigated as intelligent window coatings. The films are fabricated using reactive sputtering and post annealing. A reversible semiconductor to metal phase transition for as-deposited VO2 nanostructure films with grain size of ~8nm takes place at a temperature of 35°C, which lowers about 33°C in comparison with a phase transition temperature of 68°C in conventional VO2 films with a grain size of 1-2 μm. The results indicate that the nanostructural VO2 films are more suitable to the application for smart thermochromic glazing of windows than that of conventional VO2 films.

Simulation of semiconductor nanowire photodetectors with high photoconductive gain

In this paper, first we simulate the light absorption of individual cylindrical nanowire with the diameter ranging from 100 to 300 nm, it is found that the absorption peak has a red-shift along with the increased diameter. Then some numerical simulations have been done to elucidate the high gain mechanism and investigate the dependence of photoconductive gain on various parameters, such as doping, surface state density, and structure. The results show that optimizing these parameters appropriately can lead photoconductive gain up to 106, and give a reliable guiding to the actual device design.

Design and fabrication of visible/mid-infrared dual-band microfilter array

Integrated visible/infrared dual-band filter array is the key component of compact, lightweight, rigid miniature dual-band CCD sensing system. Interference cut-off filter array and interference absorbing filter array have been designed for infrared and visible pass band respectively. A simple, effective and compatible with high temperature deposition process lift-off technique for striping thick infrared film was investigated. Integrating photolithography, ion beam assisted electron beam physical vapor deposition and improved lift-off process, dual-band microfilter array with good performance was fabricated on the same sapphire substrate consecutively. Details of design and fabricating procedure are elucidated, and experimental results are presented.