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

A Nguyen (1) , Pedro González de Alaiza Martínez (1) , J Déchard (1) , Illia Thiele (2) , Ihar Babushkin (3) , Stefan Skupin (2) , Luc Bergé (1)

1 DPTA - Département de Physique Théorique et Appliquée
2 CELIA - Centre d'Etudes Lasers Intenses et Applications
3 Leibniz Universität Hannover=Leibniz University Hannover

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.

Domaines: Optique [physics.optics]; Physique Atomique [physics.atom-ph]; Physique des plasmas [physics.plasm-ph]

A Nguyen, Pedro González de Alaiza Martínez, J Déchard, Illia Thiele, Ihar Babushkin, et al.. Spectral dynamics of THz pulses generated by two-color laser filaments in air: the role of Kerr nonlinearities and pump wavelength. Optics Express, 2017, 25 (5), pp.4720-4740. ⟨10.1364/OE.25.004720⟩. ⟨hal-01545413⟩