Improvements in ultrabroadband microwave generation by USPL filaments by using far IR wavelength laser

When focused to form a plasma, ultrashort pulse lasers (USPLs) can generate several decades of spectral content that range from above to far below the frequency of the driving laser. The ultra broadband microwave generation is of particular interest, however when originally studied with 800nm, femtosecond laser, resulted in low signals on the order of 10 mV peak to peak. Improvements were previously seen moving to longer pulse durations (ps) with higher energies, which allowed for more effective laser heating of the plasma. Due to wavelength scaling of the ponderomotive force of the laser, it was predicted that increasing the wavelength to far IR would dramatically improve the amplitude of the resulting microwaves. This was investigated at using the 9.2-micron CO₂ laser at Brookhaven National Laboratory. Microwaves were measured from 1-70 GHz using multiple broadband horns and a Tektronix high frequency oscilloscope. Plasma florescence was measured using a 2 ns gated ICCD camera. Experiments demonstrated an order magnitude increase in the signal of the microwave fields measured from a filament driven with a 9.2 micron laser compared to a 1.053 micron laser of comparable energy and pulse duration. Experimental results will be compared to computational model of the microwave generation.

Approved for public release; distribution is unlimited. Public Affairs release approval #AFRL-2023-3069.