Theoretical study of the influence of laser pulse chirp on terahertz emission characteristics of gas induced by two-color laser field

Abstract

With the development of terahertz (THz) wave research, the demand for controllable THz sources is increasing. How to obtain the regulated THz waves has been one of the research hotspots and key problems in the field of THz science. There have been researchers who modulate the resulting THz waves by changing the wavelength, relative phase, energy, or chirp of the laser produced by a two-color laser. In this paper, we establish a three-dimensional theoretical model of THz wave generation and subsequent propagation induced by two-color laser. And we investigate the influence of chirp modulation of different laser on THz waves by chirp modulation of fundamental wave (FW) and the second harmonic wave (SHW) respectively of two-color laser. Including THz waves amplitude, THz waves center frequency and spectrum width, and analyze the physical mechanism of related phenomena. At the same time, the effects of different orders of magnitudes of laser chirp parameters (femtosecond and picosecond) and initial phase of laser pulse on THz waves parameters are also studied. The results of study show that: 1) In the two-color laser, the chirp of FW mainly affects the shape of THz waves when the initial phase is unchanged. Chirp modulation of SHW can cause significant changes in the amplitude of THz waves, and affect the center frequency and spectrum width of THz waves. 2) In the case of laser pulse width of femtosecond order, 40 fs is taken as an example. When the chirp parameter is femtosecond magnitude, the chirp parameter has a great influence on the THz waves generation efficiency of two-color laser filament. At the picosecond magnitude, the chirp parameter has a weak effect on the THz waves energy and mainly affects the phases of the THz waves. 3) The initial phase of the two-color laser can be aided in chirp modulation of THz waves to optimize the energy generated. 4) The initial phase of two-color laser can be assisted in the process of chirped laser modulation of terahertz waves to optimize the energy generated. Our study shows that the chirp modulation, as a controllable parameter, has multiple regulatory effects on the properties of radiated THz waves. The related study results provide a new idea and basis for the study on the generation and regulation of THz waves.