Optimization of terahertz radiation generation in air by adjusting time delay between the pump pulses

Terahertz (THz) frequency radiation has recently drawn special attention due to its broad applications and a unique nature [1] . One of the sources generating a strong signal of such waves is an air plasma induced by focused bichromatic femtosecond laser pulses. Since in this case conversion efficiency of THz radiation generation is quite low, but the risk of damaging active medium practically does not exist, usually during the generation pump pulse energy is increased to as high as possible. This brings a variety of nonlinear effects including a temporal pulse reshaping into the action. However a good spatial and temporal overlap of bichromatic pump pulses is crucial for the efficient THz radiation generation. Note that in the common experimental setups a single focused bichromatic beam is used for the plasma creation, which eliminates the possibility to vary initial time delay between the fundamental and the second laser harmonics. Therefore here we present the results of an experiment where we demonstrate a strong interrelation between generation efficiency of THz radiation and both the time delay between the bichromatic pump pulses and their energies.