Phase-matching in KTP crystal for THz wave generation at room temperature and 81 K

Optical properties of high-quality flux-grown potassium titanyl phosphate (KTiOPO4, KTP) crystals were studied at 81 K in the spectral range of 0.2-2.4 THz by terahertz time-domain spectroscopy and compared with the room temperature results. No strong phonon peaks were found at frequencies below 2.15 THz in the absorption spectra of alpha(x) approximate to alpha(y) components. At 81 K they do not exceed that of widely used GaSe crystal. Experimental data on the dispersion of refractive index at 81 K for THz waves polarized parallel to optical axes were approximated in the form of Sellmeier equations. Phase matching for collinear difference-frequency generation of visible and near-IR emission into the THz domain was found possible, as well as second harmonic generation within THz region that can be a basis for the new THz sources designs. Phase matching in KTP was found insensitive to temperature variation. These properties together with the exceptional set of other known physical properties render KTP crystal amongst the most prospective crystals for high-power THz wave generation under intense laser pumping.