Optical Phonon Resonance of KTiOPO_4 Crystal Characterized by THz Time-domain Spectroscopy

Utilizing ultrafast femtosecond pulses to pump photoconductive switches in the terahertz time-domain spectroscopy system,the optical phonon oscillation characteristics of KTiOPO4 crystal were studied in the 0.5~2.0THz range.An obvious absorption peak was found when the z-axis of the crystal was parallel to the polarization of the terahertz electric field.With the crystal lattice vibration selective absorption principle and pseudo-harmonic dielectric model,the complex dielectric constant curves fit well and the spectroscopic parameters of a weak optical phonon resonance were achieved.The optical external mode at ω1/2π=1.76THz,which attributed to the K ions vibration relative to the screw of PO4 tetrahedral and TiO6 octahedral,was confirmed.These results show that the THz time-domain spectroscopy is more sensitive and precise to weak lattice vibrations.