Thermal modelling of laser damage in 8–14-μm HgCdTe photoconductive and PbSnTe photovoltaic detectors

Abstract

Crystalline Hg0.8Cd0.2Te and Pb0.8Sn0.2Te materials were exposed to intense 10.6-μm laser radiation for irradiation times varying by more than six orders of magnitude. Laser damage thresholds were measured and found to vary by approximately four orders of magnitude over the range of irradiation times studied. Three thermal models describing thermally induced damage in irradiated crystals are presented and discussed. Damage thresholds were calculated using a simple one-dimensional model for a uniformly irradiated semi-infinite solid. Threshold values calculated using this model are in good agreement with experimental values for irradiation times between 10−6 and 10−2 s. For long times (greater than 10−2 s), specific details of detector construction and irradiation conditions, ignored by the one-dimensional model, have a significant effect on damage thresholds. Therefore, two thermal models are presented which take into account the finite detector thickness and radial heat conduction. It is found that predictions of permanent-damage thresholds using these models are in good agreement with experimental data for all irradiation times involved. The advantages and limitations of these models are discussed. As a function of irradiation time the damage thresholds for photoconductive and photovoltaic detectors can exhibit three distinct regions of behavior. For short times (τ<10−6 s) E0 is constant and P0 is inversely proportional to τ, for intermediate times E0∝τ1/2 and P0∝τ−1/2, and for long times P0 asymptotically approaches a constant. The long-time behavior in which P0 is constant is due to the thermal profile having reached a steady-state distribution. This is brought about either because of radial heat conduction or finite detector thickness.