1.Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2.College of Materials Sciences and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China 3.School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China 4.Songshan Lake Materials Laboratory, Dongguan 523808, China
Fund Project:Project supported by the Science Challenge Project, China (Grant No. TZ2016005), the National Natural Science Foundation of China (Grant Nos. 92050106, 11827807), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB16010200, XDA25010000)
Received Date:17 March 2021
Accepted Date:12 April 2021
Available Online:14 April 2021
Published Online:20 April 2021
Abstract:Powerful terahertz (THz) radiation sources are crucial to the development of THz science. High-energy strong-field THz pulses have many significant applications such as in the ultrafast control of matter and the THz-driven electron acceleration. In recent years, ultraintense laser-plasma interactions have been proposed as a novel approach to strong-field THz generation. In this paper, the experimental results are presented about the generation of THz radiation from a solid foil irradiated by a 10-TW femtosecond laser pulse. The THz energy as a function of laser energy and defocusing amount is studied. It is found that both the THz energy and the laser-to-THz conversion efficiency increase nonlinearly with the laser energy increasing. At maximum laser energy ~270 mJ, the measured THz pulse energy is 458 μJ, corresponding to a laser-to-THz energy conversion efficiency of 0.17%. No indication of saturation is observed in the experiment, implying that a stronger THz radiation could be achieved with higher laser energy. By simultaneously monitoring the backward scattered laser light spectrum, it is qualitatively understood that the observed THz radiation as a function of laser energy and laser defocusing distance is closely related to the electron heating mechanisms at different laser intensities. The THz spectrum and polarization are characterized by using different band-pass filers and a wire-grid polarizer, respectively. The THz radiation covers an ultrabroad band ranging from 0.2 THz to 30 THz, and shows a radially polarized distribution. By fitting the measured THz spectrum with the theory of coherent transition radiation, the THz pulse duration is inferred to be about 30 fs. At the THz focal spot of ~1 mm in size, the THz field strength is evaluated to be 3.68 GV/m. Such a strong-field THz source will enable the study of extreme THz-matter interactions. Keywords:ultraintense laser-plasma interaction/ strong terahertz radiation/ coherent transition radiation
3.实验结果与讨论实验首先测量了太赫兹能量随激光能量的变化情况, 如图2(a)所示. 可见, 随着激光能量的增加, 太赫兹能量和激光-太赫兹能量转换效率均呈现非线性的增长趋势. 在实验最大激光能量270 mJ时, 太赫兹能量为458 μJ, 相应激光-THz转换效率为0.17%, 且仍未出现任何饱和迹象. 这表明, 如果继续增加激光能量, 太赫兹能量将以更快的速度继续增强. 这是相较于晶体太赫兹源的一个最明显的优势[6]. 图 2 太赫兹能量与激光能量及激光背向散射光的关系 (a) 太赫兹能量(红色方块)和激光-太赫兹能量转换率(蓝色方块)随激光能量的变化; (b) 不同激光能量下的背向散射光光谱 Figure2. Relationship between THz energy, laser energy and laser back scattered light: (a) Dependence of THz energy (red square) and THz-laser efficiency (blue square) on the laser energy; (b) laser back scattered light spectra at different laser energy.
图 1 实验布局示意图
图 2 太赫兹能量与激光能量及激光背向散射光的关系 (a) 太赫兹能量(红色方块)和激光-太赫兹能量转换率(蓝色方块)随激光能量的变化; (b) 不同激光能量下的背向散射光光谱
图 3 太赫兹能量、激光光强及激光背向散射光与激光离焦量的关系 (a) 太赫兹能量(红色圆点)和激光光强(黑色方块)随激光离焦量D的变化; (b) 不同离焦量对应的激光散射光光谱
图 4 太赫兹频谱、偏振及光斑表征 (a) 使用带通滤片测得的太赫兹频谱(棕色方块)以及根据CTR理论使用实验参数拟合得到的理论频谱(黑色实线); (b) 使用太赫兹偏振片测得的太赫兹偏振分布; (c) 使用太赫兹相机测得的太赫兹光斑(左), 旋转相机阵面90°后测得的太赫兹光斑(右), 图中黄色箭头为太赫兹相机偏振敏感方向
