Gas pressure dependence of microwave pulses generated by laser-produced filament plasmas.

The plasma arising due to the propagation of a filamenting ultrafast laser pulse in air contains currents driven by the pulse that generate radiated electromagnetic fields. We report absolutely calibrated measurements of the frequency spectrum of microwaves radiated by the filament plasma from 2-40 GHz. The emission pattern of the electric field spectrum is mapped as a function of air pressure from atmosphere to 0.