| Popis: |
Thin films of VO2 are capable of reversibly switching from a transparent to a metalstate at high speeds and with high spatial resolution. The result is a dynamic reticleor mask in the IR with large on -off ratios for both reflection and transmission. Threeapplications for which V02 films are a key will be discussed: a high performance realtime coherent optical data processor, a transmit /receive switch and a fast random accessscan laser with a large field of view and high resolution.Review of PropertiesThe optical properties of vanadium dioxide were recognized by several groups(1 -2) assuitable for display applications in the visible. The refractive index of V02 in the twophases differ enough to obtain transmittance and reflectance contrast. Furthermore, theproperties of vanadium dioxide and other transition metal oxides and sulfides are offundamental interest and as a result have been studied extensively by theorists.(3)The material property of interest here is the semiconducting -to -metal phase transitionexhibited by V02 at 65 °C. Extensive work has been done at this laboratory in developingthe techniques required to prepare quality films of these materials such that the phasetransition property could be readily exploited. Below the transition temperature, a VO2film is in a semiconductor phase and transmittance is high. At higher temperatures, thematerial is metallic and highly reflecting in the infrared with very low transmission.Figure 1 shows the IR transmission of the film as a function of temperature, and Figure2 is a similar plot for reflection. Note that in both figures the curves exhibit hyster-esis, i.e., a higher transition temperature for the semiconductor -to -metal transition thanfor the reverse case. Since the curves are double valued over a finite temperature range,it is possible to store spatial imagery, in terms of phase, by raising the whole filmsurface to an operating temperature within the hysteresis loop. Writing is accomplishedby additionally warming a local portion of the surface enough to undergo the transition,and then allowing it to cool back to the operating temperature. Alternatively, dynamicwriting with adjustable fade time occurs if the operating temperature is maintained belowthe hysteresis loop. |
