摘要： Terahertz (THz) radiation has been of increasing interest in the last decades, for security, manufacturing, quality control, medical and security imaging applications. The field requires more efficient and affordable sources and nonlinear upconversion schemes point in this direction. KTiOPO, (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 polariton resonances [1] show promise for efficient IR to THz upconversion and Wu et al. [2] recently demonstrated its superiority to LiNbO3 and LiTaO3 in a back to back experiment. However, the complex refractive index needs to be measured before further power scaling is realistic. Terahertz time-domain spectroscopy (TTDS) allows for direct extraction of both the phase and amplitude of the THz electric field and grants the possibility of extracting the optical parameters n and α. In this work, we report on THz time-domain reflection spectroscopy of flux grown KTP from 0.5 to 7 THz at room temperature. The THz-TDS setup used air-filament plasma for generation of the THz radiation and the Pockel's effect in 300um thick GaP for detection. The a and b-axis of a KTP crystal were probed using an HDPE polarizer and rotation of the sample. The geometry of the setup is based on the work of Pashkin et al. [3], where the THz was focused onto the GaP crystal after the 25x15x0.5mm3 sample. Fig.1 shows the refractive index and the absorption coefficient from left to right, respectively. Blue and red color depict the two axes of the KTP crystal. Errors are estimated to: ?n < ±0.025 and ?α <±5 cm?1 (0.5?3THz), ?n < ±0.05 and ?α < ±25 cm?1 (3?5THz), ?n < ±0.1 and ?α < ±50 cm?1 (5?6THz), ?n < ± 0.25 and ?α < ±150 cm?1 (6?7THz). The accuracy in frequency was ≈ 90 GHz or 2 cm?1, limited by the scan time of 11ps. To the best of our knowledge, the convoluted frequency dependent nature of the refractive index has not been measured by direct time-resolved THz measurement before. Earlier spectroscopic studies of KTP have either lacked the possibility of directly extracting the complex refractive index [4] or been limited by absorption of the transmitted signal due to the strong resonances of KTP, limiting the bandwidth to about 2.5 THz [5]. In the regions where the bandwidth of this and older studies overlap, the measurements are similar, giving credibility to both measurements over the whole bandwidth. Besides, the measured resonances in Fig.1 can be related to IR active phonon modes, as seen in, e.g. [6]. This study will be of utmost importance when constructing new devices for optical to THz conversion. We also report a possible new phonon mode at 1.4 THz.