激光等离子体光丝中太赫兹频谱的调控  

Control of the terahertz spectra generated from laser induced plasma

作　　者：李晓璐 白亚[1] 刘鹏[1,3] Li Xiao-Lu;Bai Ya;Liu Peng(State Key Laboratory of High Field Laser Physics,Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences,Shanghai 201800,China;University of Chinese Academy of Sciences,Beijing 100049,China;Collaborative Innovation Center of IFSA(CICIFSA),Shanghai Jiao Tong University,Shanghai 200240,China)

机构地区：[1]中国科学院上海光学精密机械研究所,强场激光物理国家重点实验室,上海201800 [2]中国科学院大学,北京100049 [3]上海交通大学IFSA协同创新中心,上海200240

出　　处：《物理学报》2020年第2期130-135,共6页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:11874373);中国科学院战略性先导科技专项(批准号:XDB16000000)资助的课题~~

摘　　要：研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.Broadband terahertz(THz) emission generated from laser induced gas plasma provides an effective tool for studying nonlinear spectrum, imaging and remote sensing. Recently, the contribution of plasma oscillation to the THz emission was revealed from the nitrogen molecules pumped by intense two-color laser pulses. Plasma oscillation contributes only to the THz emission at relatively low plasma density due to negligible plasma absorption. More generally, with the THz emission generated from the ionizing gaseous medium, the surrounding plasma is expected to play an important role in the generation process. For the THz radiation from laser filament, the plasma region is extended in the laser propagation direction, and the effect of surrounding plasma on the emitted THz spectrum needs studying. In this work, we investigate the relation between pump power and filament length from THz spectrum emitted by air filament driven by two-color laser pulse. The time domain spectrum of THz field is recorded by an electro-optic(EO) sampling technique. In our experiments,significant frequency shifts are observed as the pump power and the filament length increase, and we find that the center frequency of the THz radiation is shifted towards longer wavelength, which is the so called red-shift of the THz spectrum. This red-shift is independent of THz radiation angle. The observations are explained by the plasma absorption inside the air filament. Our theoretical model is based on three mechanisms: the ionization-induced photocurrent, the plasma current oscillation and the plasma absorption. We coherently add up all the local THz fields inside the air filament, and simultaneously consider the plasma absorption induced correction of the THz spectrum. The simulation well reproduces the experimental observation. The skin depth decreases as the plasma density increases, thus the plasma absorption dominates the red-shift process. If the skin depth is larger than the filament length, the plasma oscillation contributes to the THz spectrum dom

关 键 词：太赫兹辐射 激光等离子体 等离子体频率 等离子体吸收 

分 类 号：TN24[电子电信—物理电子学] O53[理学—等离子体物理]