THz Generation from Relativistic Plasmas Driven by Near- to Far-Infrared Laser Pulses.

Terahertz pulse generation by ultraintense two-color laser fields ionizing gases with near- to far-infrared carrier wavelength is studied from particle-in-cell simulations. For a long pump wavelength (10.6 µm) promoting a large ratio of electron density over critical, photoionization is shown to catastrophically enhance the plasma wakefield, causing a net downshift in the optical spectrum and exciting THz fields with tens of GV/m amplitude in the laser direction. This emission is accompanied by coherent transition radiation (CTR) of comparable amplitude due to wakefield-driven electron acceleration. We analytically evaluate the fraction of CTR energy up to 30% of the total radiated emission including the particle self-field and numerically calibrate the efficiency of the matched blowout regime for electron densities varied over three orders of magnitude.

Terahertz Pulse Generation in Underdense Relativistic Plasmas: From Photoionization-Induced Radiation to Coherent Transition Radiation.

Terahertz to far-infrared emission by two-color, ultrashort optical pulses interacting with underdense helium gases at ultrahigh intensities (>10^{19} W/cm^{2}) is investigated by means of 3D particle-in-cell simulations. The terahertz field is shown to be produced by two mechanisms occurring sequentially, namely, photoionization-induced radiation (PIR) by the two-color pulse, and coherent transition radiation (CTR) by the wakefield-accelerated electrons escaping the plasma. We exhibit laser-plasma parameters for which CTR proves to be the dominant process, providing terahertz bursts with field strength as high as 100 GV/m and energy in excess of 10 mJ. Analytical models are developed for both the PIR and CTR processes, which correctly reproduce the simulation data.

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

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.