Frequency-resolved measurement of two-color air plasma terahertz emission

We investigated the far-field THz beam profile generated from two-color air plasma induced by bichromatic femtosecond laser pulses. Under our experimental conditions, using electro-optic sampling in both ZnTe and GaP crystals, along with ultra-broadband ABCD technique (0.2-30 THz), we were able to show that the THz beam exhibited an unimodal beam pattern below 4 THz and a conical beam pattern above 6 THz. This experimental result agrees with other theoretical studies based on the unidirectional pulse propagation equation, which have shown the transition of the THz emission from flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament.