Investigation of backward THz generation in periodically poled stoichiometric LiTaO_3

Abstract

We have investigated characteristics of terahertz (THz) radiation generated by periodically poled stoichiometric LiTaO3 (PPSLT) gratings, based on optical rectification of ultrafast optical pulses in a backward configuration. The highest output power from the PPSLT gratings has reached nearly 100 μW. The advantages of LiTaO3 include its significantly reduced photorefractive effect, significantly lower poling electric field, and absence of two-photon absorption in the vicinity of 800 nm. Based on our analysis, effective second-order nonlinear coefficients are enhanced by factors of from 3.7 to 23, resulting in the enhanced output powers. Such enhancements originate from a polariton resonance at the frequency of 127  cm−1, which can be induced by nonlinear mixing of two transverse-optical phonons through strong anharmonicity of LiTaO3. In the second wafer having significantly shorter poling periods, we have observed the complete resonant peak. The linewidth of the THz output can be reduced by increasing the diameter of the pump beam. When the beam size is sufficiently small, the divergence of the THz radiation can be one of the factors preventing the THz power being efficiently coupled out of the crystal, resulting in the optimal beam diameters. As the diameter is increased, the THz peak frequency is slightly blue-shifted due to weak thermal effect.