Plasma Micro-Cavity of Terahertz Wave During Laser Filamentation

In this work, we focused on the resonant cavity effect of the air plasma, formed during femtosecond laser filamentation, on the near- and far-field profiles of the generated terahertz (THz) radiation. Via the method of ray tracing, it has been demonstrated that both on-axis propagation and conical forward emission of THz wave could be qualitatively interpreted in case of considering the filament as a Fabry-Perot-like micro-cavity with radial step refractive index distribution in THz domain. Our theory might potentially solve the contradictions of different experimental observations on THz spatial profiles reported in publications, and renew the understanding of THz wave propagation inside the filament plasma column.