Third harmonic generation in plasma channels in air induced by intense femtosecond laser pulses

Strong third-order harmonic (TH) emission is observed from long plasma channels formed when intense femtosecond laser pulses propagate in air. The conversion efficiency between the fundamental wave and the TH emission is roughly constant over the length of the plasma channels. The divergence angle of the TH emission in the conical emission of the plasma channels is also investigated, which is in good agreement with calculations