根据光整流效应,利用超快激光脉冲泵浦GaSe晶体实现了0.2～2.5 THz的宽带太赫兹辐射输出.禁带中的电子在两个800 nm光子的作用下激发到导带中形成自由载流子,进而吸收所产生的太赫兹辐射,最终导致太赫兹的输出随泵浦功率的增加而趋于饱和.为了研究双光子吸收对太赫兹输出的影响,测量了800 nm处的GaSe晶体的双光子吸收系数,结果为 0.165 cm/GW.通过对太赫兹输出实验数据的拟合,得到GaSe晶体中自由载流子对太赫兹输出的吸收截面为1×10-15 cm2.本文的研究结果可用于优化GaSe晶体在强激光泵浦下的太赫兹转换效率.

The broadband THz generation from 0.2 to 2.5 THz was measured as a function of pump intensity in a single pure gallium selenide crystal based on the optical rectification of ultrafast laser pulses.Two-photon absorption at 800 nm can generate free charge carriers which can absorb the generated THz radiation,and finally attenuate THz output.The result indicates that the pump intensity dependence of ouput THz radiation changes from square to sub-linear relationship and the saturation of output THz radiation appears at high pump intensity.To study the impact of two photon absorption on THz generation,the two photon absorption coefficient of GaSe at 800 nm is measured to be 0.165 cm/GW,which is determined by the measured nonlinear transmission.The THz output fitting result taking account of the free charge carrier absorption cross section of 1×10-15 cm2 is consistent with the experiment data very well.This estimation result can be used to optimize the conversion efficient of THz generation in GaSe crystal under intense laser pump.