Fig. 1. (a) Experimental setup. PM, parabolic mirror; FS, fused silica plate. (b) Microscopic image of the optical damage traces on the fused silica plate. The laser irradiation time of the upper and bottom traces is about 100 seconds and 1000 seconds, respectively. (c) Schematic diagram of the THz emission from the plasma formed by the laser ablation process of the fused silica plate.

Fig. 2. (a) Waveforms of the THz radiation emitted from the laser-ionized fused silica plate, air, and the BK7 and K9 glass plates, respectively. (b) Corresponding Fourier transform spectra of the THz waveforms emitted from the laser-ionized fused silica plate, air, and the BK7 and K9 glass plates, respectively. The black dashed line represents the THz wave emitted from the fused silica plate under irradiation of 90-fs laser pulses.

Fig. 3. (a) Dependence of the peak THz amplitude emitted from the laser-ionized fused silica plate on the laser incident angle. (b) THz waveforms measured at incident angles of 45 degrees and −45 degrees, respectively.

Fig. 4. (a) Dependence of the THz peak amplitude on the laser pulse duration measured at laser pulse energies of 200, 400, 800, and 1600 µJ. The laser chirp is presented in terms of the group delay dispersion (GDD) imparted to the laser pulses in the top label. The data in the left panel are obtained when laser pulses are positively chirped and vice versa. (b) THz peak amplitude as a function of the laser pulse energy for 92-fs, 200-fs, and 496-fs laser pulse durations.