Terahertz Wave Generation From Two-Color Laser-Excited Air Plasma Modulated by Bichromatic Laser Fields

Terahertz (THz) waves were generated using a two-color femtosecond laser with a fundamental wavelength of 1500 nm; the waves were modulated using two-color modulation plasma with a fundamental wavelength of 800 nm in a collinear configuration. The properties of the THz waves including intensity, polarization, and spectrum, were systematically investigated by changing the energy ratio, relative phase, and the polarization direction of the pump and modulation beams. The results demonstrate that the THz intensity has a superimposed effect when the two beams simultaneously ionize the ambient air. When the energy ratio of the beams was changed, the intensity ratio of the high- and low-frequency components of the THz spectrum changed continuously, and the THz polarization gradually transitioned from elliptical to linear. Moreover, the center frequency of the THz spectrum can be blue shifted by shortening the optical length of the modulation beam. Besides, an arbitrary elliptically polarized THz wave can be obtained by controlling the relative polarization angle of the two-color modulation lasers. The findings of this article will facilitate practical applications involving the generation and selection of specific THz radiation.