Measurement of Time Dependent Reflection, Transmission and Absorption in Laser Driven Silicon and GaAs Switches for 250 GHz Radiation

The reflectance (R) and transmittance (T) of Si and GaAs wafers irradiated by a 6 ns pulsed, 532 nm laser have been studied for s- and p-polarized 250 GHz radiation as a function of laser fluence and time. The measurements were carried out using precision timing of the R and T signals, allowing an accurate determination of the absorptance (A) where A=1-R-T. Both wafers had a maximum reflectance above 90% for a laser fluence ≥8 mJ/cm2. Both also showed an absorptance peak of ~50% lasting ~2 ns during the risetime of the laser pulse. Experimental results were compared with a stratified medium theory using the Vogel model for the carrier lifetime and the Drude model for permittivity. Modeling showed that the large absorptance at the early part of the rise of the laser pulse was due to the creation of a lossy, low carrier density layer. For Si, the measured R, T and A were in very good agreement with theory on both the nanosecond time scale and the microsecond scale. For GaAs, the agreement was very good on the nanosecond scale but only qualitatively correct on the microsecond scale. These results may be useful for planning applications of laser driven semiconductor switches.