Infrared reflective optical modulator

摘要

Abstract: An IR reflective optical modulator has been fabricated. This has been achieved by coating a thick film resistor network with a thin film of vanadium dioxide via a reactive sputtering process. Vanadium dioxide undergoes a semiconductor to metal phase transition at approximately 68 degrees C, therefore to switch the reflective optical modulator the thick film resistors in the network are driven electrically. As the resistors heat up to beyond the transition temperature, the vanadium dioxide undergoes its transition from a transparent semiconductor state to a highly reflective metallic state. Provided the thick film heater network, underneath the vanadium dioxide, is non- reflective, then a significant change in reflectivity is observed upon undergoing the transition, and hence the modulation of reflected IR radiation is achieved. The useful waveband of operation of the device encompasses the region 2-25 $mu@m, this is primarily limited by the transparency of the semiconductor state of the vanadium dioxide. Correct stoichiometry of the thin film of vanadium dioxide is critical in producing good modulation depth in the reflective mode. Several devices have been fabricated and tested. They show a reflectivity increase of approximately 13:1 upon switching. The devices to date have demonstrated switch on speeds of 0.1 s and switch off speeds of 0.2 s. This has been achieved without any form of substrate temperature control apart from that produced by the electrical drive. Very slight changes in the stoichiometry of the vanadium dioxide thin film can greatly increase the temperature range and hysteresis of the semiconductor to metal phase transition. This has been utilized to allow partial phase transitions to occur, yielding partial increases in reflectivity, and hence the ability to generate grey levels in the reflected IR radiation. !6