THz time-domain reflection spectroscopy of KTiOPO4

There is an ongoing development of THz sources for high-field applications, especially with the view of potential for THz-driven particle accelerators. For such sources nonlinear materials with high optical damage threshold and low loss are required. KTiOPO4, (KTP) is a well-established material for nonlinear optical applications in the visible and near-IR region thanks to its large transparency, high nonlinear coefficient and damage threshold. Its reasonably low absorption [1, 2] and strong coupling to phonon-polariton degree of freedom [1]. This shows promise for efficient IR to THz conversionprocess. Indeed,Wu et al.[2] recently demonstrated its superiority to LiNbO3and LiTaO3in a back to back experiment. In view of this, it is important to determinethe spectrum of complex refractive index over larger spectral range and with higher accuracy than has been do so far. KTP is an orthorhombic crystal belonging to mm2 point group with a rather complex vibrational mode structure containing 192 degrees of freedom. Earlier spectroscopic studies of KTP have either lacked the possibility of directly extracting the complex refractive index [3] or been limited by absorption of the transmitted signal due to the strong resonances of KTP, limiting the spectral extenttoabout 2.5 THz [4]. In this work we utilize reflection THz time-domain spectroscopy (TTDS) to determine the spectrum of the refractive index, n, and the absorption coefficient, , between 0.5 THz and up to 7 THz for two eigenpolarizations on the (001) plane.QC 20191217Not duplicate with diva2:1445597 diva2:1379156