Nonlinear Refraction and Increasing Absorption in HgCdTe Optical Bistability at Room Temperature: an Experimental Study

Abstract

Many authors have experimentally examined the nonlinearities of HgCdTe at resonance with the CO2 laser wavelength [1]. At room temperature non1inearities of electronic origin, arising from band filling, are weak because of Auger recombination and are masked by thermal effects. In this communication we extend the analysis of ref. 1 on thermally induced optical bistability at low power: we show, for the first time in HgCdTe, the appearance of clockwise hysteresis cycles in transmission as input power Pi is increased, due to nonlinear absorption. We can account for both absorptive and refractive nonlinear contributions by simultaneous monitoring of transmission and reflection. The Hg0.815Cd0.185Te plates (280 um in thickness, with doping pa =1*1016 cm-3, prepared and polished by Elettronica SpA) were placed without heatsinking in a thermally controlled environment. In Fig. 1 the transmitted (Pt) and reflected (Pr) power is shown as a function of Pi: the three data sets differ in cavity mistuning. The thermooptic effect was measured directly, by a close focus optical pyrometer, and indirectly by taking data at different thermal bath temperature T. The values of Pi at the hysteresis associated to a given cavity interference order (indicated with the symbols of Fig. 1c) drift with T (Fig. 2): the maximum induced temperature increase is inferred to be 30-35°C, with steps of 2.5-3.5°C at the switching points, and a value of dn/dT = -2.8*10-3 K-1 is deduced. Fig. 3 presents numerical results (to be compared with Fig. 1b) obtained parametrically solving the nonlinear Fabry Perot equations using the absorption measured after AR coating one sample face. This nonlinear absorption will be discussed in terms of a free carrier origin.