Narrow-band and wide-band coherent terahertz source generation using 3 wave difference frequency mixing and Cross-Reststrahlen-band dispersion compensation in ultra-high purity III-V semiconductor crystals

We discuss the Cross-Reststrahlen band phasematching technique for narrow-band three-wave interactions. In GaP, assuming a maximum practical crystal length of 100 millimeters, the calculations predict that with a source wavelength of 0.965 micrometers, a tuning range of greater than 50 GHz is possible around the perfectly phasematched 3.0 THz center frequency. When GaP is pumped with a source wavelength of 1.000 micrometers, the coherence length is at least 100 mm for a frequency range of greater than 600 GHz around the perfectly phasematched 630 GHz center frequency. High Resistivity GaP is available in cylindrical boules that are larger than 50 millimeters in diameter and 75 millimeters in length. GaP has a bandgap cutoff wavelength in the Visible at 0.55 micrometers and a simple phonon absorption spectrum with a single fundamental absorption at approximately 27 micrometers (11 THz), which indicates a potential for high transmission in both the NIR and FIR. Any remaining FIR absorption can be attributed to free electron absorption and two phonon absorption processes. In this paper, we report new measurements of GaP regarding FIR absorption, optical damage threshold and optical quality. These measurements indicate that undoped, high-resistivity GaP single crystal can be used to generate THz waves.