arXiv subjects

A. Goffin

Publications and source records attributed to A. Goffin.

14 recordsLinked to original sources

arXiv2026

Optical air waveguides in strong turbulence

Filament-generated air waveguides offer a route to low loss long-distance optical transport, but their viability in atmospheric turbulence has remained uncertain. Here, we demonstrate with experiments and simulations that guided beams in air waveguides can overcome prior estimated turbulence limits by orders of magnitude in the refractive index structure parameter C_n^2; turbulent index fluctuations much larger than the waveguide index contrast average out along the propagation path. Instead, turbulence imposes two distinct limits: scattering of the guided field beyond the waveguide acceptance aperture and, more importantly at atmospheric strengths, scintillation of the waveguide-forming beam. Scaling of experimentally validated models indicates that filament-generated air waveguides remain viable over kilometer-scale paths under strong near-ground turbulence. Our results establish turbulence limits for long-range optical guiding in the atmosphere.

physics.optics↗

arXiv2026

Topologically constrained high intensity light propagation in air

We experimentally demonstrate how spatiotemporal optical vortices (STOVs) control long-range atmospheric filamentation of intense laser pulses. High-power pulses long enough to overlap with the delayed rotational nonlinearity of air molecules undergo periodic collapse arrest events, each of which generates toroidal STOV pairs with +/- topological charge that separate and accumulate into increasingly squeezed arrays of +1 charges at the front of the pulse and -1 charges at the back. These dynamics manifest as periodic energy deposition peaks along the propagation path and a pulse envelope modulated into a temporal intensity comb. Filamentation in this regime can be understood in terms of self-organized, topologically constrained defect dynamics embedded within nonlinear wave propagation.

physics.optics↗

arXiv2024

Transverse orbital angular momentum of amplitude-perturbed fields

We measure the change in transverse orbital angular momentum (tOAM) per photon, delta L_y, applied to an optical pulse by a pure amplitude perturbation. The results are in excellent agreement with calculations and simulations of the spatiotemporal torque based on our tOAM theory [Phys. Rev. Lett. 127, 193901 (2021)]. The crucial factor in determining delta L_y is the spatiotemporal distribution of tOAM density in the pulse. We show that even Gaussian pulses with zero total tOAM can have net tOAM induced by an amplitude perturbation stationary in the lab frame. As a prelude to the paper, we review and clarify several recent theoretical approaches to tOAM and reemphasize several fundamental principles needed for the correct analysis of experiments and simulations.

physics.optics↗

arXiv2024

Self-focused pulse propagation is mediated by spatiotemporal optical vortices

We show that the dynamics of high-intensity laser pulses undergoing self-focused propagation in a nonlinear medium can be understood in terms of the topological constraints imposed by the formation and evolution of spatiotemporal optical vortices (STOVs). STOVs are born from point phase defects on the sides of the pulse nucleated by spatiotemporal phase shear. These defects grow into closed loops of spatiotemporal vorticity that initially exclude the pulse propagation axis, but then reconnect to form a pair of toroidal vortex rings that wrap around it. STOVs constrain the intrapulse flow of electromagnetic energy, controlling the focusing-defocusing cycles and pulse splitting inherent to nonlinear pulse propagation. We illustrate this in two widely studied but very different regimes, relativistic self-focusing in plasma and non-relativistic self-focusing in gas, demonstrating that STOVs mediate nonlinear propagation irrespective of the detailed physics.

physics.optics↗

arXiv2023

Spatiotemporal torquing of light

We demonstrate the controlled spatiotemporal transfer of transverse orbital angular momentum (OAM) to electromagnetic waves: the spatiotemporal torquing of light. This is a radically different situation than OAM transfer to longitudinal, spatially-defined OAM light by stationary or slowly varying refractive index structures such as phase plates or air turbulence. We show that transverse OAM can be imparted to a short light pulse only for (1) sufficiently fast transient phase perturbations overlapped with the pulse in spacetime, or (2) energy removal from a pulse that already has transverse OAM. Our OAM theory for spatiotemporal optical vortex (STOV) pulses [Phys. Rev. Lett. 127, 193901 (2021)] correctly quantifies the light-matter interaction of this experiment, and provides a torque-based explanation for the first measurement of STOVs [Phys. Rev. X 6, 031037 (2016)].

physics.optics↗

arXiv2023

Quasi-steady-state air waveguide

We report the first generation of quasi-steady-state air waveguides capable of guiding high average power laser beams. The guides are produced by high-repetition rate patterned filamentation of femtosecond laser pulses. We demonstrate near-continuous guiding of a CW probe beam with significantly higher efficiency than transient guides at lower repetition rates.

physics.optics↗

arXiv2022

Optical guiding in 50-meter-scale air waveguides

The distant projection of high peak and average power laser beams in the atmosphere is a longstanding goal with a wide range of applications. Our early proof-of-principle experiments [Phys. Rev. X 4, 011027 (2014)] presented one solution to this problem, employing the energy deposition of femtosecond filaments in air to sculpt millisecond lifetime sub-meter length air waveguides. Here, we demonstrate air waveguiding at the 50 meter scale, 60X longer, making many practical applications now possible. We employ a new method for filament energy deposition: multi-filamentation of Laguerre-Gaussian LG01 "donut" modes. We first investigate the detailed physics of this scheme over a shorter 8 m in-lab propagation range corresponding to ~13 Rayleigh lengths of the guided pulse. We then use these results to demonstrate optical guiding over ~45 m in the hallway adjacent to the lab, corresponding to ~70 Rayleigh lengths. Injection of a continuous wave probe beam into these waveguides demonstrates very long lifetimes of tens of milliseconds.

physics.optics↗

arXiv2022

Atmospheric aerosol clearing by femtosecond filaments

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 micron 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 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 non-filamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.

physics.optics↗

arXiv2020

Self-waveguiding of relativistic laser pulses in neutral gas channel

We demonstrate that an ultrashort high intensity laser pulse can propagate for hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by generating its own plasma waveguide as it propagates; the front of the pulse generates a waveguide that confines the rest of the pulse. A wide range of suitable initial index structures will support this "self-waveguiding" process; the necessary feature is that the gas density on axis is a minimum. Here, we demonstrate self-waveguiding of pulses of at least $1.5\times10^{17} W/cm^2$ (normalized vector potential $a_0\sim0.3)$ over 10 cm, or $\sim100$ Rayleigh ranges, limited only by our laser energy and length of our gas jet. We predict and observe characteristic oscillations corresponding to mode-beating during self-waveguiding. The self-waveguiding pulse leaves in its wake a fully ionized low density plasma waveguide which can guide another pulse injected immediately following; we demonstrate optical guiding of such a follow-on probe pulse

physics.plasm-ph↗

arXiv2020

Optical guiding in meter-scale plasma waveguides

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J_0 beam, generates the core of the waveguide, while the delayed second pulse, here a J_8 or J_16 beam, generates the waveguide cladding. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on axis plasma densities as low as N_e0=5x10^16 cm^-3.

physics.optics↗

arXiv2020

Air hydrodynamics of the ultrafast laser-triggered spark gap

We present space and time resolved measurements of the air hydrodynamics induced by ultrafast laser pulse excitation of the air gap between two electrodes at high potential difference. We explore both plasma-based and plasma-free gap excitation. The former uses the plasma left in the wake of femtosecond filamentation, while the latter exploits air heating by multiple-pulse resonant excitation of quantum molecular wavepackets. We find that the cumulative electrode-driven air density depression channel initiated by the laser plays the dominant role in the gap evolution leading to breakdown.

physics.plasm-ph↗

arXiv2020

Self-guiding of long-wave infrared laser pulses mediated by avalanche ionization

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest owing to the high critical power for self-focusing collapse at long wavelengths. This promises transmission of very high power in a single filament as opposed to beam breakup and multi-filamentation. Here, using the most current picture of LWIR ionization processes in air, we present extensive simulations showing that isolated avalanche sites centered on aerosols can arrest self-focusing, providing a route to self-guided propagation of moderate intensity LWIR pulses in outdoor environments.

physics.optics↗

arXiv2020

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. 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.

physics.optics↗

arXiv2020

Free-space propagation of spatio-temporal optical vortices (STOVs)

Spatio-temporal optical vortices (STOVs) are a new type of optical orbital angular momentum (OAM) with optical phase circulation in space-time. In prior work [N. Jhajj et al., Phys. Rev X 6, 031037 (2016)], we demonstrated that a STOV is a universal structure emerging from the arrest of self-focusing collapse leading to nonlinear self-guiding in material media. Here, we demonstrate linear generation and propagation in free space of STOV-carrying pulses. Our measurements and simulations demonstrate STOV mediation of space-time energy flow within the pulse and conservation of OAM in space-time. Single-shot amplitude and phase images of STOVs are taken using a new diagnostic, transient grating single-shot supercontinuum spectral interferometry (TG-SSSI).

physics.optics↗