强激光与固体靶相互作用中的太赫?#26085;?#26029;

封面文章:H. Liu, et al. Study of backward terahertz radiation from intense picosecond laser–solid interactions using a multichannel calorimeter system[J]. High Power Laser Science and Engineering, 2019, 7(1): e6

强太赫兹辐射源由于在物质、光以及电?#37038;?#30340;相干和非相干调控中的应用而受到众多研究人员的关注。借助太瓦(1012 W)和拍瓦(1015 W)激光装置,近年来强激光等离子体相互作用过程中产生的太赫兹辐射已经发展成为一?#20013;?#22411;的强太赫兹辐射源。在强激光等离子体相互作用过程中,有许多粒子或者物理过程都可以产生太赫兹辐射,比如超热电子和等离子体波?#21462;?/p>

频谱测量?#26434;?#29702;解太赫兹辐射的产生机制非常有帮助。但是目前的太赫兹频谱测量方法?#26434;?#24378;激光等离子体太赫兹辐射源并不适用:

首先,目前基于强激光等离子体相互作用的太赫兹辐射源的重复频率都比?#31995;停话?#23567;于1 Hz,甚至是单发运?#23567;?#22914;果使用电光采样这类基于多发扫描的太赫兹频谱测量技术表征强激光等离子体太赫兹辐射源,不仅效率低而且准确率也不高。

其次,强激光等离子体太赫兹辐射源属于宽带辐射源,带宽可达几十太赫兹,而常见电光晶体比如ZnTe和GaP的工作带宽都比较窄,只有几个太赫?#21462;?/p>

因此,很有必要开发能够进行单发和宽带测量的太赫兹辐射频谱诊断仪器或者技术。中国科学院物理研究所?#26412;?#20957;聚态物理国家研究中心联合研制了一种多通道太赫兹辐射能量测量系统。

他们利用该系统在高功率皮秒激光与固体靶相互作用实验中单发表征了靶前太赫兹辐射的频谱,研究了靶前太赫兹辐射的能量和频谱与激光能量靶厚度以及预等离子体尺度的关系。

用于解释靶前太赫兹辐射的产生机制有多种,其中包括相干渡越辐射、线?#38405;?#24335;转换和靶前表面瞬态电流辐射。通过?#21592;?#27979;量结果与理论上这些机制产生太赫兹的频谱特征,研究人员认为:

靶前太赫兹辐射的低频分量(<1 THz)?#19978;?#24178;渡越辐射机制产生;

当靶前的预等离子体尺度较大时,线?#38405;?#24335;转换机制逐渐成为主导,其主要贡献是靶前太赫兹辐射中的高频分量(>3 THz)。

该课题组的李玉同教授表示:“我们展示了一种单发测量太赫兹辐射频谱的可靠方法。这套谱仪将在基于大?#22270;?#20809;装置的强激光等离子体太赫兹辐射源的研究和应用中发挥重要作用。”

将来这套系统可以进一步升级,以提高系统的谱分辨率。

实验中多通道太赫兹谱仪?#23395;?#31034;意图

A multichannel THz spectrometer for diagnosing terahertz radiation from intense laser-solid interactions

Intense THz radiation sources have attracted increasing research interest due to their applications in coherent and incoherent control of matter, light and electron beams. With terawatt and petawatt laser systems, THz radiation from intense laser-plasma interactions (ILPI) has been demonstrated as a novel intense THz source. The measurement of THz spectrum is very important to determine the generation mechanisms of the THz sources.

However, the existing THz spectrum measurement techniques do not work for ILPI-based THz sources. This is because the repetition rate of ILPI-based THz sources is quite low at present, typically below 1 Hz, even in single shot. As a result, multi-shot scanning methods, such as electro-optic sampling and autocorrelation measurement with a Michelson interferometer, are almost impossible to characterize the ILPI sources. Moreover, the bandwidth of ILPI THz sources can reach tens of THz, which also limits the application of electro-optic sampling techniques since the effective bandwidth is only several THz for common electrooptical crystals such as ZnTe and GaP.

New diagnosing methods or techniques, which can deliver single-shot and broad-band spectral measurements, should be developed. A research group from Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences developed a multichannel calorimeter system which was used in a single-shot way to characterize the spectrum of THz radiation in high-power picosecond laser-solid interaction experiment. THz radiation from target front surface propagates backward relative to the incident laser, which is referred as backward THz radiation (BTR). A number of mechanisms are proposed to be responsible for the BTR generation, such as coherent transition radiation, linear mode conversion, and surface fast electron currents. In the experiment, the dependence of the BTR energy and spectrum on laser energy, target thickness and pre-plasma scale length is studied. By comparing the experimental results with theoretical mechanisms, it is concluded that coherent transition radiation is responsible for the low frequency component (< 1 THz) of BTR. The Linear Mode Conversion mechanism starts to work when a large-scale pre-plasma is formed in the target front surface, which enhances the high frequency components (> 3 THz). The research results are published in High Power Laser Science and Engineering, Volume 7, Issue 1, 2019 (H. Liu, et al,Study of backward terahertz radiation from intense picosecond laser–solid interactions using a multichannel calorimeter system).

“The method of THz radiation spectrum measurement used in the article is not novel, but it provides a valid way to characterize the ILPL sources. And the spectrometer will play an important role in the studies and applications of ILPI THz sources based on large-scale laser facilities.” said Profess Yutong Li.

The multichannel calorimeter system developed in this work provides a convenient single-shot method to study the generation mechanism of the broad-band THz radiation generated in large laser facility-based experiments. The instrument should be updated in the future to improve its spectral resolution.

The schematic layout of the multichannel calorimeter system in the experiment