Experimental study of terahertz radiation driven by femtosecond ultraintense laser

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.