Abstract
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.
Highlights
The generation of terahertz (THz) wave from a single-color femtosecond laser filament in air has been studied for nearly three decades [1]–[12]
This coincides the theory of THz wave radial emission [2]–[5], which could be the origin of THz oscillations inside the plasma column
The proposed micro-cavity model might suffer the lack of more precise quantitative results, compared with thorough mathematical calculations based on theoretical physics, such as particle-in-cell (PIC) model [44]–[46]
Summary
The generation of terahertz (THz) wave from a single-color femtosecond laser filament in air has been studied for nearly three decades [1]–[12]. The well-known transition-Cherenkov theory [6], [7] cannot well account for many other far-field THz properties in the literature, such as THz radial [2]–[5] and on-axis emission [11], or even the combination of conical and on-axis THz propagation [12], as listed in the column of “Property” in the following Table 1. Note that, these diverse properties of THz emission resulted from a similar experimental frame, i.e., single-color laser pumping THz wave generation. Near-field properties in the literature could be interpreted by the modulation of the micro-cavity’s step-index on the guided THz wave
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