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

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