Thin Film Thermochromic Materials for Non-Linear Optical Devices.
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Semiconductor to metallic phase transitions (SMPT) Free carrier effects - thermal runaway processes Non-linear refraction
Non-linear refraction processes are well understood and a number of papers have addressed the levels at which effects can be found. Materials such as InSb exhibit non-linear refractive effects in the IR as a result of band-filling processes. In such cases, the refractive index changes by an amount directly proportional to the number of generated carriers. When high laser intensities are focussed on the materials, the transmitted beam is defocussed so reducing the power density incident on the detector focal plane. The best known example [1] is cadmium mercury telluride CdxHgl-xTe, whose intrinsic carrier lifetime C is dependant on composition. For x = 0.183, , is 4 nsec, increasing to 31±sec at x = 0.29. At high radiance levels, the value of An increases but is accompanied by a decrease in Auger recombination time. In the limit of high fluences this can result in the saturation of the refractive index non-linearity. It is most serious at low cadmium concentrations (x = 0.183) corresponding to one-photon absorption of 10.6gm radiation. Thus the potential of CMT as a non-linear optical material is greatest at higher cadmium concentrations where 2 or 3 photon absoprtion processes are used to induce the non-linearity. Whilst these effects can be of considerable theoretical interest, they are difficult to exploit in thin film structures. Devices such as the Fabry-Perot etalon can be configured to demonstrate the effects, but at wavelengths between the fundamental absorption edges in the material (ie bandgap and multiphonon edge) would require spacer thicknesses (ca 200gim) more typical of bulk material. 105 Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
At short pulses in the nanosecond regime, intraband absorption can also occur in these materials, resulting in hot electron generation which leads to an avalanche of excess carriers with a large cross-section for intervalence band absorption for holes at 10.6jtm. An intensity dependent transmission is therefore observed, similar to that in InAs and InSb. The onset of such limiting is at comparitively low power densities eg 200kW/cm 2 for CMT (x = 0.23) at 20K, 2 mW/cm 2 for InSb at 20K and 30mW/cm 2 for InAs at 300K [2]. Amorphous chalcogenide semiconductors are well known for their switching properties and have been examined at various times for potential application as memory and optical switching elements. Most of these materials are characterised by order-disorder transitions in the range of 100 - 200"C. In the case of the chalcogenide glass T12SeAs2Te3 (TSAT), the order to disorder transition occurs near 200°C, but the glass begins to soften at about 80°C. The electrical conductivity is thermally activated above room temperature with an activation energy of 0.35eV. The properties of the material are best exploited by incorporating the material between supporting IR transparent windows (eg Ge, ZnSe) by hot pressing. This c
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