Revealing the terahertz-laser velocity effect during air filamentation via travelling-wave-antenna model

TL;DR

This paper investigates how the velocity ratio between THz phase velocity and laser group velocity influences THz wave generation during air filamentation, revealing its significant impact on radiation patterns and beam shaping.

Contribution

The study introduces an improved travelling-wave-antenna model to analyze the impact of velocity ratio K on THz radiation, highlighting its role in beam pattern transitions and providing a method to infer K from experimental data.

Findings

K significantly affects THz radiation distribution and emission patterns.

Transition from Bessel- to Cherenkov-type THz emission is observed.

The TWA model can infer K and determine if the ionization front is superluminal or subluminal.

Abstract

During femtosecond laser filamentation in air, the velocity ratio (K) between the terahertz (THz) phase velocity and the laser group velocity plays a crucial role in THz waves generation. However, K is typically assumed to be unity and its impact has been long overlooked due to the more attention paid to the more easily controlled filament length. Here, we investigate the obscured contribution of K to the THz radiation characteristics by using the improved travelling-wave-antenna (TWA) model. It has been found that, under both single- and two-color laser pumping schemes, K significantly determines the far-field spatial distribution of forward or backward THz radiation, as well as a transition from Bessel- to Cherenkov-type THz emission patterns. These results establish the TWA model as a reliable theoretical tool for studying the mechanisms of THz beam shaping via the designed K. Moreover, for cases of K not being controlled, its value can also be inferred by the proposed TWA model, which could be an effective method to confirm whether the laser ionization front is superluminal or subluminal compared with the generated THz waves.