Atmospheric Aerosol Clearing by Femtosecond Filaments

A. Goffin, J. Griff-McMahon, I. Larkin, and H.M. Milchberg

Phys. Rev. Applied 18, 014017 – Published 8 July 2022

Abstract

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 the transmission of subsequent pulses. However, the detailed fog-clearing mechanism had yet to be determined. Here, we directly measure and simulate the dynamics of water droplets with a radius of about $5 μ m$ , typical of fog, under the influence of optical and acoustic interactions that are characteristic of femtosecond filaments. We find that, for filaments generated by the collapse of collimated near-infrared 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 nonfilamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.

Received 28 February 2022
Revised 26 April 2022
Accepted 12 May 2022
Corrected 1 August 2022

DOI:https://doi.org/10.1103/PhysRevApplied.18.014017

Physics Subject Headings (PhySH)

Light-matter interaction Nonlinear optics Self-focusing & filamentation in plasmas Ultrafast optics

Aerosols Wireless communication networks

Femtosecond laser irradiation Infrared techniques

Atomic, Molecular & OpticalPlasma PhysicsCondensed Matter, Materials & Applied Physics

Corrections

1 August 2022

Correction: The verb tense in the second sentence of the abstract has been restored to match that present in the original manuscript.

Authors & Affiliations

A. Goffin, J. Griff-McMahon , I. Larkin, and H.M. Milchberg ^*

Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA

^*milch@umd.edu

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Vol. 18, Iss. 1 — July 2022

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Images

Figure 1
(a) Diagram of Experiment 1. Beam is down-collimated by a reflective telescope to a $w_{0} = 1 mm$ waist ( $e^{- 2}$ intensity radius) to generate a filament with peak intensity $I_{0} \sim 80 TW/c m^{2}$ [estimated from panel (a)(i) and simulations], radiating a single-cycle acoustic wave. Filament is terminated mid-flight by a helium cell, enabling linear imaging of the filament cross section [29, 30]. Piezodropper places a 5-µm-radius droplet a controlled distance from the center of the filament core; droplet interaction is imaged from the side using a $λ_{0} = 532 - nm$ 7-ns transverse probe. (i) End-on image of filament core-intensity profile, using the $He$ cell, with the y line out plotted. (ii) End-on image of the droplet in a low-power beam. (iii) Side image of an unperturbed 5-µm-radius droplet. All color maps are in arbitrary units. (b) Diagram of Experiment 2. Dielectric mirror (M1) is used to copropagate the pump and on-axis probe, and a second dielectric mirror (M2) filters out the pump to image the probe. Peak intensity for this $f / 120$ focusing geometry is $I_{0} \sim 160 TW/c m^{2}$ at the highest pump energy. Inset, end-on image showing the density hole, radial acoustic wave, and water droplet. Color map is in arbitrary units.
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Figure 2
Shadowgrams of filament-induced dynamics for droplets with a radius of about $5 μ m$ placed at varying radial distances (labels at left) from the center of the filament core. Filament propagates from left to right (white arrows). Two columns show droplet images at 0.1 and 500 µs after filament arrival. Black arrows mark the initial droplet axial location, while red arrows mark the droplet location (if a droplet is still present) 500 µs after the filament interaction. As the droplet is moved closer to the beam axis, the images at 0.1 µs show droplet distortion and far-side cavitation, while the 500-µs images show axial droplet displacement toward the laser and the complete disintegration of droplets placed closer than about 200 µm from the core center.
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Figure 3
(a) Energy deposition per unit length, $\partial_{z} ε_{dep}$ , versus pump energy, $ε_{pump}$ . (b) Droplet displacement, $Δ R_{drop}$ , as a function of pump energy for initial position $R_{init} = 100 - 110 μ m$ . Blue trace is the mean displacement, ${\bar{Δ R}}_{drop}$ , and error bars represent variance over 100 shots at each point. Red region is $Δ R_{drop}$ from simulations (see Sec. 3). (c) $Δ R_{drop}$ versus $R_{init}$ for $ε_{pump} = 450 μ J$ . Each value of $R_{init}$ plotted corresponds to the average at that point, with standard deviation <4.5 µm.
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Figure 4
(a) Peak temperature rise versus laser energy deposited per unit length, $\partial_{z} ε_{dep}$ . (b) Droplet displacement, $Δ R_{drop}$ , at 500-µs delay for droplet radius $a = 5 μ m$ ; $\partial_{z} ε_{dep} = 4.8 μ J/cm$ for various $r_{h}$ (heated-region radius) labeled in the legend. (c) $Δ R_{drop}$ at 500-µs delay versus droplet radius a for $\partial_{z} ε_{dep} = 4.8 μ J/cm$ , initial droplet position $R_{init} = 50 μ m$ , and $r_{h} = 60 μ m$ . (d) $Δ R_{drop}$ at 500-µs delay versus $R_{init}$ for $r_{h} = 60 μ m$ and droplet radii $a = 0.125$ , 0.25, 0.5, and 0.75 µm and $\partial_{z} ε_{dep} = 4.8 μ J/cm$ . $Δ R_{drop}$ decreases for $R_{init} <\sim 50 μ m$ because the acoustic wave reaches its maximum amplitude just inside the heated region.
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Figure 5
(a) Peak energy deposition per unit length for each pump-pulse energy. For ε_pump= 0.3 mJ, L ∼ 1 mm, and for ε_pump= 2.3 mJ, L ∼ 5 mm [31]. (b) Droplet displacement for each pump energy after 105-µs delay. (c) Droplet displacement after 500 µs simulated by the hydrocode for r_h = 60 µm and droplet diameter a ∼ 10 µm, for energy depositions labeled in the legend.
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