Spectral dynamics of THz pulses generated by two-color laser filaments in air: The role of Kerr nonlinearities and pump wavelength

TL;DR

This study investigates how Kerr nonlinearities, pump wavelength, and phase relationships influence THz pulse generation in air filaments, revealing complex dependencies and limitations of simple scaling laws through theoretical and numerical analysis.

Contribution

It introduces a detailed theoretical and numerical analysis of THz generation, emphasizing the roles of nonlinearities, pump wavelength, and phase, which were not comprehensively addressed before.

Findings

Raman-delayed nonlinearity weakens THz generation.

Four-wave mixing significantly affects THz spectra.

Scaling of THz efficiency with wavelength depends on phase and pulse shape.

Abstract

We theoretically and numerically study the influence of both instantaneous and Raman-delayed Kerr nonlinearities as well as a long-wavelength pump in the terahertz (THz) emissions produced by two-color femtosecond filaments in air. Although the Raman-delayed nonlinearity induced by air molecules weakens THz generation, four-wave mixing is found to impact the THz spectra accumulated upon propagation via self-, cross-phase modulations and self-steepening. Besides, using the local current theory, we show that the scaling of laser-to-THz conversion efficiency with the fundamental laser wavelength strongly depends on the relative phase between the two colors, the pulse duration and shape, rendering a universal scaling law impossible. Scaling laws in powers of the pump wavelength may only provide a rough estimate of the increase in the THz yield. We confront these results with comprehensive numerical simulations of strongly focused pulses and of filaments propagating over meter-range distances.