Terahertz-wave absorption in GaP crystals with different carrier densities

Using a GaP Raman-based THz-wave (GRT) generator spectrometer in the frequency range 0.8-5 THz, we measured the terahertz (THz) wave absorption spectra of GaP crystals with carrier densities of 10¹⁰ and 10¹⁶ cm^-3. We also investigated the temperature dependence from 20 to 300 K. In the lower-carrier-density GaP, the THz-wave absorption by phonons increased from 1.5 to 7 cm^-1 as the THz frequency increased. In the higher-carrier-density GaP, absorption due to free carriers appeared below 2.5 THz. The THz-wave absorption decreased as a function of crystal temperature in each sample (α<2 cm^-1 at 20 K). We also measured the temperature dependence of the THz-wave output from the GaP crystals. In each crystal, the THz-wave output was enhanced at 90 K, compared with room temperature, due to the decrease in the THz-wave absorption.