Fig. 1. (a) Experimental setup: PM1 and PM2, parabolic mirrors; EOS, electro-optic sampling. (b) Schematic diagram of the THz generation process from colliding laser pulses.

Fig. 2. (a) Typical waveforms of the THz radiation and the corresponding Fourier transform spectra (inset). Each THz spectrum shown in the inset is normalized to its maximal value for comparison. Black, red, and blue solid lines represent the THz signals by colliding pulses (pulse A and pulse B), pulse A, and pulse B, respectively. (b) Pseudo-color image of THz electric field versus the time delay for THz waveform scan and the relative delay between pulse A and pulse B. (c) Peak THz amplitude as a function of laser pulse width with pulse energies set at 800 μJ and 200 μJ. Negative chirp is applied to stretch the laser pulses shown in the right half, and vice versa. (d) The dependences of peak THz amplitude on laser pulse energy measured at the optimal negative chirp, optimal positive chirp, and zero chirp, respectively.

Fig. 3. (a) Peak THz amplitude as a function of the relative polarization angle between pulses A and B. The error bars are shown in red. (b) Peak THz amplitude as a function of the polarization angle of co-polarized pulses A and B. The error bars are shown in red. (c) THz radiation angular distribution. 90 and −90  deg represent the propagation direction of pump pulses A and B, respectively. (d) Reconstructed dipole-like radiation pattern. The arrows represent the laser propagation directions.

Fig. 4. (a) Fourier transform spectra of the THz radiation generated by laser pulses with different energies. (b) Peak THz amplitude as a function of the pump delay between the colliding pulses and additional pump pulse used to generate pre-plasma.

Fig. 5. (a) THz waveforms. (b) Corresponding Fourier transform spectra.