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

Filamentation in low pressure conditions

Filamentation is favorable for many long-range outdoor laser applications, some of which require propagation to or at high altitudes. Understanding how the filamentation process and filament properties are impacted by the low pressure conditions present at high altitudes is essential in designing effective applications. The scaling of filament preconditions with pressure is considered. An increase in critical power and decrease in transition numerical aperture (NA) is predicted to occur with a drop in pressure, indicating that nonlinear pulse propagation and filamentation at high altitudes requires higher energy and a longer assisted focal length than sea level filamentation. A summary of pressure-scaled filament properties is also presented. New simulations demonstrate filamentation at pressures as low as 0.0035 atm (38.5 km altitude) is possible.

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.

Laser filaments as pulsed antennas

Secondary radiation emission of laser-induced filaments is revisited from a perspective of transient antenna radiation. Solutions for transient-antenna radiation fields are shown to provide an accurate description of the spectral and polarization properties, radiation patterns, and the angular dispersion of terahertz and microwave radiation emitted by laser filaments. Time-domain pulsed-antenna analysis offers a physically clear explanation for the bandwidth of this radiation, relating the low-frequency cutoff in its spectrum to the filament length, thus explaining efficient microwave generation in laser filamentation experiments.

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.

Coherently enhanced microwave pulses from midinfrared-driven laser plasmas

Ultrafast ionization of a gas medium driven by ultrashort midinfrared laser pulses provides a source of bright ultrabroadband radiation whose spectrum spans across the entire microwave band, reaching for the sub-gigahertz range. We combine multiple, mutually complementary detection techniques to provide an accurate polarization-resolved characterization of this broadband output as a function of the gas pressure. At low gas pressures, the lowest-frequency part of this output is found to exhibit a drastic enhancement as this field builds up its coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. This behavior of the intensity, coherence, and polarization of the microwave output is shown to be consistent with Cherenkov-type radiation by ponderomotively driven plasma currents.

Full path single-shot imaging of femtosecond pulse collapse in air turbulence

In a single shot, we measure the full propagation path, including the evolution to pulse collapse, of a high power femtosecond laser pulse propagating in air. Our technique enables examination of the effect of parameters that fluctuate on a shot-to-shot basis, such as pulse energy, pulse duration, and air turbulence-induced refractive index perturbations. We find that even in lab air over relatively short propagation distances, turbulence plays a significant role in determining the location of pulse collapse.

Saturation regime of THz generation in nonlinear crystals by pumps with arbitrary spectral modulations

A self-consistent analytic formalism of the description of saturation effects in optical rectification is provided. It is shown that a nonlinear absorption term arises from this process that is dominant over two-photon absorption, deriving instead from the nonlinear susceptibility of the third order. An analytical expression for the saturation intensity is provided and compared to experiments in literature. Moreover, it is shown how the saturation effects modify the transfer of the pump spectral phase and amplitude into the terahertz domain.