Short Electromagnetic Wave Generation by Interaction between Short Pulse Laser and Mangetized Plasma

利用超高功率短脉冲激光系统进行了短电磁波产生实验，其最大功率为1 TW，脉冲宽度为100 fs，波长为800 nm。激光在气压为80 Pa的N2气体中聚焦。激光可以立即产生等离子体，并从该区域观察到强辐射。为了检测电磁波，我们使用了几个具有特定频率的探测器。这些探测器的响应范围为30 ~ 300 GHz，时间分辨率小于100 ps。频率通过使用波导中的频率色散的飞行时间技术来确定。探测到的辐射脉冲在所有频率范围内的持续时间为200 ps。激光产生的等离子体辐射沿30 °圆锥方向发射，辐射具有径向偏振，可以通过旋转波导和喇叭天线来测量。为了解释这些实验数据，我们建立了等离子体模型：由强激光脉冲产生的等离子体柱，其直径为焦距，长度为瑞利长度。电子被激光脉冲的有质动力驱逐。另一方面，离子保持在我们感兴趣的时间尺度中的原始位置。因此，电子在离子电场产生的电场力的作用下径向振荡。辐射在波导中具有TM 01模式，并被发射到自由空间中。计算结果表明，辐射方向为30度角，辐射方向为径向偏振，与实验结果吻合较好。实验结果还表明，方向角是辐射频率的函数，为产生更高频率的辐射，即太赫兹辐射提供了等离子体参数。

We conducted the short electromagnetic wave generation experiments by use of ultra-high power, short pulse laser system, whose maximum power is 1TW with pulse duration of 100 fs at the wavelength of 800 nm. The laser focused in the N2 gas with the pressure of 80 Pa. The laser can created plasma immediately and the strong radiation is observed from this region. To detect the electromagnetic wave, we used several detector which has specific frequency. These detectors can response the range from 30 to 300 GHz with the time resolution less than 100 ps. The frequency was determined by the Time of Flight technique which uses the frequency dispersion in the waveguide. The detected radiation pulse has the time duration of 200 ps for all of the range of the frequecies. The radiation emitted the conical direction with 30 degree from the laser created plasma with the radial polarization which can be measured by the rotating the waveguide and horn antenna. The cone angle decreases as the radiation frequency increases.To explain these experimental data, we created the plasma model : The plasma column created by the intense laser pulse with the diameter of the focal size and the length of the Rayleigh length. The electrons expel by the ponderomotive force of the laser pulse. On the other hand, the ions remain the original position in the time scale which we are interested. Thus the electrons radially are oscillated by the electric force which created by the ion electric field. The radiation has TM01 mode in the waveguide and is emitted into free space. The calculation indicates the direction of the radiation with the angle of 30 degree and the radial polarization, which is good agreements with the experiments. Furthermore, it indicates the direction angle is the function of the radiation frequency.These experimental data suggest the plasma parameters for generation of higher radiation frequency, ie, the terahertz radiation.