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.