Generation of broadband THz beams with azimuthally modulated phase and intensity by femtosecond laser pulses in air plasma

Paper in proceeding, 2019

We report efficient generation of the broadband terahertz (THz) radiation by bichromatic femtosecond laser pulses with flat phase fronts or with a phase singularity focused in air. As a pump source, the fundamental and second harmonic radiations of a femtosecond Ti:sapphire laser were used. While the fundamental laser beam had Gaussian intensity distribution and a flat phase front, the second harmonic beam had either flat or helical phase front forming an optical vortex. In both cases the angular spectra of THz radiation and azimuthal phase distributions of THz beams were investigated. It was found that when both the fundamental and second harmonic laser pulses had Gaussian profiles, the resulting THz radiation was generated as an axially symmetric cone with a flat phase front. However, when the Gaussian second harmonic beam was replaced by the one with optical singularity, the intensity and the phase of generated THz cone became modulated along the azimuthal angle. The spectrum of generated THz pulses spanned up to 50 THz in both cases. Our theoretical analysis based on the photocurrent model was in good agreement with the experimental data.