气体等离子体产生太赫兹波的特性研究

Study on the characteristics of terahertz generated by laser-induced gas plasma

基于光电流模型,对不同偏振情况的双色飞秒激光脉冲聚焦产生的气体等离子体中辐射出的太赫兹波特性进行了研究。根据光电流理论,气体分子被电离释放出的自由电子在非对称的激光场的作用下运动形成电子电流,产生在太赫兹波段的辐射。研究结果表明,太赫兹辐射的偏振特性与强度和入射双色激光的偏振特性紧密相关,仅当双色脉冲均为线偏振时,辐射出的太赫兹波才为线偏振,且强度受到双色脉冲偏振方向的夹角的影响;而对于实验中经过倍频晶体后变成椭圆偏振的基频光,太赫兹强度与倍频晶体的具体放置情况有很大关系,并且产生的太赫兹为椭圆偏振。

The characteristics of the terahertz irradiated by gas plasma were studied based on the transient photocurrent model,and this gas plasma was produced by two-color femtosecond laser pulse under the different polarization. According to the photocurrent theory,the gas molecules are ionized,and then liberate free electrons. These free electrons form a photocurrent in unsymmetrical laser field and produce the terahertz. Research results show that polarization property and intensity of terahertz radiation are closely related to polarization property of incident two-color laser. Only when two-color laser is both linear polarized,the irradiated terahertz is linear polarized and the intensity is affected by the angle between the polarization directions of fundamental and second harmonic pulses. For fundamental frequency laser of elliptical polarization after frequency doubling by the crystal,terahertz intensity is related to the place of frequency doubling crystal and the produced terahertz is elliptical polarized.