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.

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Terahertz spectroscopy from air plasmas created by two-color femtosecond laser pulses: The ALTESSE project Berge, L.; Kaltenecker, Korbinian J.; Engelbrecht, S.; Nguyen, A.; Skupin, S.; Merlat, L.; Fischer, B.; Zhou, Binbin; Thiele, I.; Jepsen, Peter Uhd