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.
Highlights
Laser filaments produced by ultrashort light pulses proceed from the dynamic balance between Kerr self-focusing and plasma generation [1, 2]
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
In summary, we have theoretically studied the influence of long pump wavelengths belonging to the range 0.8-2 μm in THz emissions caused by two-color laser pulses through air photoionization
Summary
Laser filaments produced by ultrashort light pulses proceed from the dynamic balance between Kerr self-focusing and plasma generation [1, 2]. Filamentation of pulses with different frequencies has been proposed as an innovative way to downconvert optical radiation into the THz range [5] and create broadband THz sources remotely [6] In this context, THz generation can proceed from the excitation of plasma currents via longitudinal ponderomotive motions of free electrons [7]. The paper is organized as follows: Section 2 proposes a one-dimensional (1D) approach combining known laser-driven THz sources It recalls that, in the range of intensities reached by two-color filaments in air, photoionization and to a lesser extent the Kerr nonlinearity are the principal players in THz generation.
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