Hong Ma

Nanostructured vanadium dioxide thin films with low phase transition temperature

A low phase turning temperature of 35°C has been observed in the semiconductor-to-metal transition of nanoscale vanadium dioxide (VO2) thin films. The thin films are prepared by reactive ion beam sputtering deposition and subsequent thermal annealing. Both scanning electron microscopy and transmission electron microscopy measurements show that the grain size of the fabricated VO2 thin films are several tens of nanometers. The average height of the crystallite is 20nm and the grain diameter is between 20 and 100nm. The low-temperature phase transition is accompanied by a significant change in the thin film’s infrared transmission property.

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.

A novel method of fabrication of microlens arrays

Abstract A new self-alignment process to fabricate microlens arrays is introduced. By this method, during the fabrication process, the rigorous alignment is avoided which has great effect on diffraction efficiency in the conventional multi-photolithography process. The large arrays of 1500×640 element silica microlens are manufactured by this method. The measurement results show that the eight-phase-level microlens arrays diffraction efficiency is as high as 93%.

1.55 µm spot-size converter integrated polarization-insensitive quantum-well semiconductor optical amplifier with tensile-strained barriers

A 1.55 µm polarization-insensitive lateral tapered spot-size converter integrated semiconductor optical amplifier (SSC-SOA) with tensile-strained barriers was investigated. The optical amplifier structure used a conventional ridge guide for the active layers and a second larger ridge for the passive waveguide. Low beam divergence of 12° × 15° results in about 3.1 dB coupling losses with −1 dB positional tolerances of ±2.3 µm and ±1.6 µm in horizontal and vertical directions using an anti-reflection coated flat-ended single-mode fibre. The active layer of SSC-SOA consisted of a tensile-strained barrier multiple-quantum-well structure. The SSC-SOA exhibited a signal gain of 25.5 dB and a saturation output power of 11.2 dB m with excellent polarization insensitivity (less than 0.5 dB) at 200 mA.

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.

Phase Transition VO2 Thin Films for Optical Switches

This paper presents a method to make vanadium dioxide (VO2) crystallites on silicon substrates by reactive ion beam sputtering. The thickness of the thin film is about 100nm. The phase transition temperature of VO2 is 65°C. The transmittance of the semiconducting phase VO2 is about 50% and it is reduced to as low as 3% in metal phase at the infrared wavelenghth spectrum. The extinction ratio of the optical switches is 12dB. and the insertion loss is of 1-2dB. The switching time is about 1ms.

1310 nm AlGaInAs–InP polarization insensitive multiple quantum well optical amplifier

Abstract A polarization insensitive AlGaInAs–InP semiconductor optical amplifier was realized at a wavelength of 1310 nm. The active layer consisted of three tensile strained wells with a strain of 0.35%. The amplifiers were fabricated into a ridge waveguide structure, which showed excellent polarization insensitivity (less than 0.3 dB) at 1310 nm with a gain of 22.5 dB at 200 mA. the AlGaInAs–InP optical amplifier shows good temperature characteristics, less than 3 dB reduction in the gain when the temperature is raised from 25 to 65 °C, the polarization sensitivity remains less than 0.8 dB over the entire range of wavelength.

Hybrid Integration Between Long Focus Microlens Array and IR Detector Array

A special method, named step simulation method, is proposed for fabricating Si microlens array to improve the performance of infrared focal plane array (IR FPA). The focus length of rectangle-based multistep microlens array with element dimension of 40 µm×30 µm is 885.4 µm by the method, which is much longer than the focus length of microlens array fabricated by conventional Fresnel binary optics technique., The large-scale 256×256 element microlens array is hybridintegrated with the PtSi Schottky-barrier IR FPA by optical adhesive. The test results show that diffractive spot size of the microlens is 17 µm×15 µm and the average optical response of the IR FPA is increased by a factor of 2.4.

A new type NRD-guide receiving front-end in Ka-band

A new type millimeter-wave receiving frond-end using NRD-guide as internal transmission line is presented in this paper. The receiving frond-end consists of a NRD-guide crossbar balanced mixer, a NRD-guide LSE mode local oscillator and two impedance-converters at two input ports of the mixer circuit. Firstly, the propagation characteristics of some transmission modes often used in NRD-guide are given, and the conditions which these modes are applied as the operating modes are discussed. Secondly, the principle of the balanced mixed which is regarded as the key circuit unit in the receiving frond-end is analysed by using vector Greens function, and the conversion loss of the mixer is computed with an equivalent circuit in Ka-band. And then, from the view of obtaining the optimum performances of the whole receiving frond-end, some methods of designing local oscillator are expound. Some experimental results of the receiving frond-end at Kaband are given in the paper lastly. Because the frond-end have the impedance matching and converting circuits, it possesses better electrical performance. The integrated frond-end which have compact size and excellent shock strength is suitable for varied millimeter-wave system.

High power polarization-insensitive 1.3 µm InGaAsP–InP quantum-well superluminescent emission diodes grown by MOVPE

Polarization-insensitive 1.3 µm InGaAsP–InP multi-quantum-well (MQW) superluminescent diodes (SLDs) were grown by MOVPE. Both compressively-strained wells and tensile-strained wells were employed in the active region. The SLDs were fabricated into ridge waveguide structures with 7° tilted cavity and buried-window end facets. The two facets were coated with four layer anti-reflection TiO2/SiO2 films and residual facet reflectivity was found to be less than 0.02%. The SLDs exhibited up to 16.9 mW optical output power and less than 1 dB polarization dependence of output power with less than 0.2 dB optical spectrum modulation at 200 mA.