静电场调制飞秒激光气体微等离子体的太赫兹辐射增强

Terahertz Radiation Enhancement from Femtosecond Laser Ionized Gas Micro-Plasma Under Static Electric Field

基于光电流模型,通过数值模拟和理论分析,研究了静电场幅值对飞秒激光气体微等离子体中太赫兹辐射的影响。结果显示,随着外加静电场幅值的增大,太赫兹辐射的强度呈线性增大;当外加电场极性相反时,太赫兹波形整体反转;当外加电场与线偏振激发激光的偏振方向呈一定夹角时,太赫兹辐射的偏振方向完全由外加电场方向决定。数值模拟结果与已有的实验结果相符。理论分析表明,这个线性依赖、极性反转以及夹角依赖均源于等离子体产生之后的静电加速的主导作用。

Objective Terahertz(THz)radiation has attracted significant attention and gained wide applications in biology,imaging,sensing,and communications due to its unique characteristics.Femtosecond laser filamentation is one of the important approaches to obtain THz pulses with ultra-broad bandwidth and high electric field.With the rapid development of femtosecond laser technology,plasma based THz generation by a femtosecond laser has been well studied,in which two theoretical models are generally adopted for understanding the physical mechanism of THz radiation generation,namely the photocurrent model and the four wave mixing model,respectively.By applying an external direct-current(DC)electric field to the laser-induced plasmas filament,the enhancement of THz radiation by several orders of magnitude has been reported.However,the physical mechanism of laser-induced THz emission enhancement under the aid of an external DC electric field remains unclear because of the complicated laser-plasmas interaction process.In this paper,based on the photocurrent model,we analyze and numerically investigate the performance of THz radiation from a direct-current electric field biased single-color femtosecond laser ionized gas micro-plasma.The results show that the photocurrent model is effective in interpreting the THz generation of DC-biased laser-induced gas plasmas.Methods The photocurrent model attributes the THz radiation to a quasi-DC electron current produced by electron motion in the laser field.In this work,the THz generation from an 800nm femtosecond laser-induced plasma in Argas is investigated.First,we calculate the ionization rate of Ar-gas interacting with a laser by using the ADK formula,and the density of ionized electron can be obtained by an equation[Eq.(5)].Second,the initial velocities of the ionized electrons are assumed to be zero,which are then accelerated by field.By making use of Newton’s second law,the transient velocity of electron motion is derived.If the electron density and velocity are given,the electron current is then obtained.Finally,the time derivative of electron current leads to THz radiation.The effects of laser field and applied DC electric field on THz generation have also been studied in detail.Results and Discussions The THz radiation from femtosecond laser ionized Ar-gas plasma with an external DC electric field perpendicular to the filament direction is numerically simulated.With the increase of pump laser intensity,the amplitude of THz radiation is enhanced[Fig.1(a)].Furthermore,the THz signals are also amplified by applying the external DC electric field.The dependence of THz peak-to-peak amplitude on laser intensity under different external DC electric fields is revealed in Fig.1(b).In contrast,the generated THz pulse from femtosecond laser ionized Ar-gas plasma without the DC electric field is extremely weak as shown in the inset in Fig.2.The THz radiation can be enhanced by several orders of magnitude when applying an external static electric field on the plasma(Fig.2).The peak-to-peak amplitude of the THz emission from a DC-biased filament is linearly proportional to the external electric field amplitude under the same laser intensity,which agrees well with the experimental observation[Fig.3(a)].Besides,the polarity of the THz waveform is reversed if reversing the polarity of the DC electric field.In order to examine whether the laser or the external DC electric field dominantly contributes to THz radiation,we artificially replace the total field with only the DC electric field in Eq.(6)and one can find that the THz waveforms before and after replacement barely change[Fig.4(a)],which indicates that the DC electric field plays a dominant role in the generation of THz radiation.The critical value of the external DC electric field has also been determined[Fig.3(b)].Finally,when the direction of the applied DC electric field has an angle with the polarization direction of the incident linearly polarized 800nm laser,the polarization direction of THz emission is completely determined by the applied DC electric field direction[Fig.4(b)].Conclusions We have extended the photocurrent model on THz radiation generation from an external DC electric field biased and single-color femtosecond laser induced Ar-gas micro-plasma.The properties of THz radiation under different conditions have been analyzed.The simulation results show that the THz radiation from laser-induced plasma can be significantly enhanced by the introduction of an external DC electric field,which is linearly dependent on DC field.Furthermore,the THz polarization direction follows the direction of the external DC field.The electrons ionized and accelerated by both the laser field and the external DC electric field result in THz radiation,in which the external DC electric field has a dominant contribution.The theoretical and numerical investigations help us deeply understand the THz radiation generation process from DC electric field biased and femtosecond laser ionized gas plasma.