Gregory S. Herman Univ. of Arizona, Science Applications International Corp., and NASA Langley Research Ctr. (United States) Gianluigi Bertelli Old Dominion Univ. and NASA Langley Research Ctr. (United States)
Far-infrared (FIR) transmission spectra have been measured for several nonlinear crystals. The results show that both GaP and ZnGeP2 have a fairly narrow Restrahlen band. After the Mid-infrared (MIR) cut off at wavelengths near 12 micrometers, the crystals begin transmitting again at wavelengths of 100 micrometers. The transmissivity is highly dependent on the conductivity of the samples as free carrier absorption will deleteriously affect transmission. These results indicate that these crystals may be used for generation of FIR radiation using difference frequency generation (DFG) from the Near-Infrared (NIR) or MIR. High resistivity GaAs and GaSe should have similar properties, but no high resistivity material was found. Other crystals such as LiNbO3, LiIO3, BBO and KTP may be used at longer wavelengths.