Length Dependence on Broadband Microwave Emission From Laser-Generated Plasmas

A high-power ultrashort laser pulse focused in air generates a plasma that radiates broadband electromagnetic waves. The transient current source responsible for the radiation remains an open area of study. To better understand how the laser drives currents in the plasma, we investigate the dependence of radiation at microwave frequencies on the length of the plasma column. Radiation patterns of the microwaves are consistent with currents that flow longitudinally (in the direction of laser propagation). We measure the angular dependence of the microwave frequency spectrum to determine the microwave radiation source characteristics. We also model the plasma evolution using a PIC code to provide initial conditions for a finite-difference time domain (FDTD) simulation of the plasma currents responsible for the microwaves. The experiments and simulations show that the plasma length determines the shape of the radiation pattern, and also weakly influences the microwave frequency content. Our research demonstrates that the radiation mechanism is coupled with variation in the current along the entire plasma length.