Direction of THz emission by ionizing two-color pulses

This work addresses the directional dependence of THz emission, in particular the possibility of backward emission. To this end, we perform direct numerical simulations of Maxwell´s equations using a 2D Finite Difference Time Domain algorithm, accounting for the plasma current due to ionization. The paper also shows the exemplary results for a focused two-color 40 fs-pulse (800 nm and 400 nm) generating a ~ 30 μm long plasma spot. The amplitude of backward propagating THz fields is about one order of magnitude smaller than their forward counterparts.