Self-Guiding of Long-Wave Infrared Laser Pulses Mediated by Avalanche Ionization

D. Woodbury, A. Goffin, R. M. Schwartz, J. Isaacs, and H. M. Milchberg

Phys. Rev. Lett. 125, 133201 – Published 21 September 2020

Abstract

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest, as it promises high power transmission without beam breakup and multifilamentation. Central to self-guiding is the mechanism for the arrest of self-focusing collapse. Here, we show that discrete avalanche sites centered on submicron aerosols can arrest self-focusing, providing a new mechanism for self-guided propagation of moderate intensity LWIR pulses in outdoor environments. Our conclusions are supported by simulations of LWIR pulse propagation using an effective index approach that incorporates the time-resolved plasma dynamics of discrete avalanche breakdown sites.

Received 16 March 2020
Revised 18 August 2020
Accepted 24 August 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.133201

Physics Subject Headings (PhySH)

Light propagation, transmission & absorption Light-matter interaction Mie scattering Nonlinear optics Ultrafast optics

Atomic, Molecular & Optical

Authors & Affiliations

D. Woodbury ¹, A. Goffin ¹, R. M. Schwartz ¹, J. Isaacs ², and H. M. Milchberg ^1,*

¹Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA
²Plasma Physics Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA

^*Corresponding author. milch@umd.edu

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Vol. 125, Iss. 13 — 25 September 2020

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Images

Figure 1
Comparative propagation simulations [15] for initial $1 TW / {cm}^{2}$ , $w_{0} = 4 mm$ ( $1 / e^{2}$ intensity radius), 3.5 ps pulses. (a) $λ = 10.2 μ m$ LWIR pulse. Aerosols (yellow balls) seed rapid discrete avalanche breakdowns of characteristic size $r_{d}$ , which arrest the collapse of a high power pulse, leading to a wide self-guided channel. (b) $λ = 10.2 μ m$ LWIR pulse. Here, in aerosol free air, avalanche breakdown initiated by single electrons from tunneling ionization proceeds too slowly to arrest collapse; collapse is arrested by continuous plasma formation and harmonic walk-off, leading to a smaller, high-intensity channel. This is not what is observed in experiments [12]. (c) $λ = 0.8 μ m$ near-IR pulse. Beam slices show filamentation and beam breakup.
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Figure 2
(a) Temperature-dependent heating and loss rates ${(d k_{B} T / d t)}_{heating} = 2 U_{p} (2 ν_{e n} + ν_{i}) / 3$ (for $λ = 10.2 μ m$ , $1 TW / {cm}^{2}$ ) and $| {(d k_{B} T / d t)}_{loss} | = 2 (Σ_{l} ν_{l} χ_{l}) / 3 + ν_{i} T$ (left scale, log) in air. The corresponding ionization growth rate $ν_{i}$ (right scale) is shown as a function of electron energy $E$ and temperature $T$ . (b) Generated electron density vs peak intensity from ionization of air and a $χ_{p} \sim 6 eV$ contaminant [14] by a $λ = 10.2 μ m$ , 1 ps FWHM Gaussian pulse. Dashed lines indicate bounds of approximate uncertainty in the number density of contaminant species [14].
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Figure 3
(a),(b) Pulse parameters over 3 m of propagation. (a) Left axis, pulse peak intensity ( $TW / {cm}^{2}$ ). Right axis, density at the intensity peak of the pulse (dashed) and after the pulse has completely passed (solid). (b) Beam FWHM and temporal FWHM. (c) On-axis intensity and plasma density and temperature after 2.25 m of propagation, showing a rapid increase in the density $N_{sc}$ of breakdown sites due to seed generation by tunneling (dashed blue line), followed by slower increase in volume average density $N_{e} = N_{sc} V \bar{N_{e}}$ due to avalanche. Temperature (solid, right scale) roughly follows the pulse intensity profile (dashed).
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Figure 4
(a),(b) Simulation parameters including aerosol-enhanced avalanche for initial aerosol density of $2 \times 10^{4} {cm}^{- 3}$ . (c) On-axis intensity and plasma density and temperature after 6 m of propagation, showing a rapid increase in density due to breakdowns in aerosols, followed by slower avalanche in air, with volume average density reaching $\sim 10^{13} {cm}^{- 3}$ near the intensity peak of the pulse, broadly consistent with self-guiding as shown above. The horizontal dashed line indicates the limit of full ionization of the aerosol-initiated breakdown sites. Right scale, distorted intensity profile (dashed) and corresponding plasma temperature (solid).
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