Directionality of tera-hertz emission from photoinduced gas plasmas

Forward and backward tera-hertz emission by ionizing two-color laser pulses in gas is investigated by means of a simple semi analytical model based on Jefimenko's equations and rigorous Maxwell simulations in one and two dimensions. We find the emission in the backward direction has a much smaller spectral bandwidth than in the forward direction and explain this by interference effects. Forward tera-hertz radiation is generated predominantly at the ionization front and is thus almost not affected by the opacity of the plasma, in excellent agreement with results obtained from a unidirectional pulse propagation model. (authors)