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Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments have been shown to clear foggy regions, improving transmission of subsequent pulses. However, the detailed fog clearing mechanism had yet to be determined. Here we directly measure and simulate the dynamics of ~5 μm radius water droplets, typical of fog, under the influence of optical and acoustic interactions characteristic of femtosecond filaments. We find that for filaments generated by the collapse of collimated nearinfrared femtosecond pulses, the main droplet clearing mechanism is optical shattering by laser light. For such filaments, the single cycle acoustic wave launched by filament energy deposition in air leaves droplets intact and drives negligible transverse displacement, and therefore negligible fog clearing. Only for tightly focused non-filamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.

Through scattering and absorption, aerosols can interfere with long-distance laser propagation through the atmosphere. Here we directly measure the dynamics of aerosol particles acoustically cleared by an optical pulse.

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. This technique enables single-shot 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.The propagation of high peak power laser pulses through gases has applications spanning sub-millimeter scales for laser-driven relativistic electron acceleration [1] in thin gas jets to hundreds of meters in the atmosphere for applications in light detection and ranging (LIDAR) [2] and laser-induced breakdown spectroscopy (LIBS) [3]. In many cases, it is important to have a visualization of the full propagation path of the pulse in the gas. For long propagation ranges in the atmosphere, shot-to-shot variations from jitter in laser parameters and atmospheric fluctuations will lead to significant variations in the beam's transverse profile, axial energy deposition, and collapse location [4,5].In prior work, records of long (> few cm) propagation profiles have been experimentally determined in several ways. One method is intercepting the beam along the propagation path and then, via propagation simulations, inferring aspects of the pulse propagation history to the point of interception [6,7]. Each shot, however, is sensitive to fluctuations and has a different propagation evolution. For femtosecond filaments, one approach for single-shot imaging is to use the recombination radiation from plasma generation [6]. However, the huge field of view needed to capture the full filament path precludes resolving axial detail. Another method is shot-by-shot scanning of a miniature microphone along the propagation path [8] to pick up the single-cycle cylindrical acoustic wave launched locally [9]. The acoustic signal is an excellent proxy for the local energy absorbed by the air, allowing a reconstruction of the laser pulse's axial energy deposition profile [8]. However, owing to unavoidable laser and air fluctuations, the

We report the generation of coherent radiation from terahertz to ultraviolet via two- color mid-infrared laser mixing in air. We achieve laser-to-terahertz conversion efficiency of ~1%, 10~100 times greater than those obtained with 800 nm lasers.