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的产生实验，系统的最大功率为1TW，脉冲宽度为100fs，波长为800 nm。激光聚焦在气压为80pa的氮气中。激光可以立即产生等离子体，从这个区域可以观察到强烈的辐射。为了探测到电磁波，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100ps。频率由飞行时间技术确定，该技术利用了波导中的频率色散。对于所有频率范围，检测到的辐射脉冲具有200ps的持续时间。从激光产生的具有径向偏振的等离子体发出30°圆锥方向的辐射，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：由具有焦距直径和瑞利长度的强激光脉冲产生的等离子体柱。电子靠激光脉冲的有重量的动力排出。另一方面，离子仍然是我们感兴趣的时间尺度上的原始位置。因此，电子受到离子电场产生的电场力的径向振荡。该辐射在波导中具有TM01模，并被发射到自由空间。计算给出了30度角的辐射方向和径向偏振，与实验结果吻合较好。这些实验数据给出了产生更高辐射频率的等离子体参数，即太赫兹辐射。

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.