Optical properties and phase matching conditions of KTiOPO₄ crystals at terahertz frequencies in the temperature range of 5.8 to 300 K

In this paper, the optical properties and phase matching (PM) conditions of KTiOPO₄ (KTP) crystals in the temperature range of 5.8–300 K are studied by terahertz (THz) time-domain spectroscopy. It is found that the absorption coefficient of KTP decreases with the decrease of temperature, and that of the Z axis at 1.2 THz decreases from 19.56 cm⁻¹ at 300 K to 7.83 cm⁻¹ at 5.8 K. Along the Z axis, the refractive index decreases with the decrease of temperature and changes the most, while along the X and Y axes the refractive index is relatively stable with temperature. The optical properties along the Z axis are sensitive to temperature, mainly because the movement of K⁺ is located in the Z axis and slows down with the decrease of temperature. KTP crystals are found to have obvious birefringence characteristics in the frequency range of 0.3–1.6 THz, with the birefringence of approximately 0.67 at 300 K and 0.58 at 5.8 K. The PM angle is calculated using the refractive index dispersion equation to satisfy the PM condition of frequency transformation. When the temperature is lowered from 300 K to 5.8 K, the maximum change in PM angle is 1.08^◦, while the minimum change is 0.10^◦ at the same output wavelength. Due to the small absorption coefficient and the birefringence characteristics at low temperatures, and the relatively stable change of PM angle with temperature, the potential of KTP crystals to efficiently generate THz radiation at low temperatures can be fully realized.