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

Optics