Optimization of microwave emission from laser filamentation with a machine learning algorithm.

We demonstrate that is it possible to optimize the yield of microwave radiation from plasmas generated by laser filamentation in atmosphere through manipulation of the laser wavefront. A genetic algorithm controls a deformable mirror that reconfigures the wavefront using the microwave waveform amplitude as feedback. Optimization runs performed as a function of air pressure show that the genetic algorithm can double the microwave field strength relative to when the mirror surface is flat. An increase in the volume and brightness of the plasma fluorescence accompanies the increase in microwave radiation, implying an improvement in the laser beam intensity profile through the filamentation region due to the optimized wavefront.

Generation of radio frequency radiation by femtosecond filaments.

Recent experiments have shown that femtosecond filamentation plasmas generate ultrabroadband radio frequency radiation (RF). We show that a combination of plasma dynamics is responsible for the RF: A plasma wake field develops behind the laser pulse, and this wake excites (and copropagates with) a surface wave on the plasma column. The surface wave proceeds to detach from the end of the plasma and propagates forward as the RF pulse. We have developed a four-stage model of these plasma wake surface waves and find that it accurately predicts the RF from a wide range of experiments, including both 800-nm and 3.9-µm laser systems.

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 0.5 Torr. For fixed laser pulse energy, duration, and focal geometry, we observe that decreasing the air pressure by a factor of approximately 103 increases the amplitude of the electric field waveform by a factor of about 40. As the air pressure decreases, the lower frequency components (<10 GHz) increase in amplitude faster than those at higher frequencies (>20 GHz). To the best of our knowledge, this behavior has not been observed before, is not predicted by existing theory, and implies the existence of a radiation mechanism in the plasma distinct from that which emits at terahertz frequencies.

Control of the configuration of multiple femtosecond filaments in air by adaptive wavefront manipulation.

We demonstrate the ability to position single and multiple filaments arbitrarily within the energy reservoir of a high power femtosecond laser pulse. A deformable mirror controlled by a genetic algorithm finds the optimal phase profile for producing filaments at user-defined locations within the energy reservoir to within a quarter of the nominal filament size, on average. This proof-of-principle experiment demonstrates a potential technique for fast control of the configuration of the filaments.