arXiv subjects

Arnaud Couairon

Publications and source records attributed to Arnaud Couairon.

18 recordsLinked to original sources

arXiv2023

Femtosecond laser-induced sub-wavelength plasma inside dielectrics: III. Terahertz radiation emission

Electromagnetic radiation within the terahertz (THz) frequency range is of great interest for applications in remote sensing and time-domain spectroscopy. The laser-induced plasmas are promising mediums for generating THz radiation. It has been recently reported that focusing femtosecond Bessel pulses inside dielectrics induces a high aspect ratio over-critical plasmas. Here we show that the intense resonantly driven electrostatic fields at the so-called critical surface lead to THz radiation emission. Through three-dimensional particle-in-cell simulation and analytical derivation, we have investigated the emission of THz radiation. We show that the THz radiation is associated with a hot population of electrons trapped in ambipolar electric fields of the double layers.

physics.plasm-ph↗

arXiv2022

Femtosecond laser-induced sub-wavelength plasma inside dielectrics: I. Field enhancement

The creation of high energy density ($\gtrsim10^6$ joules per cm$^3$) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with sub-wavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two { limiting} cases are considered: one is a homogeneous step-like profile, that can sustain plasmon formation, the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region.

physics.plasm-ph↗

arXiv2022

Femtosecond laser-induced sub-wavelength plasma inside dielectrics: II. Second-harmonic generation

Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser-plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons.

physics.plasm-ph↗

arXiv2022

High energy density plasma mediated by collisionless resonance absorption inside dielectrics

We demonstrate for the first time to our knowledge the generation of overcritical plasma densities inside transparent solids over long distances using femtosecond laser pulses. This opens new avenues for high energy density physics in confined geometry such as warm dense matter study or the synthesis of new material phases. We show both with experiments and first-principles simulations, that femtosecond conical interference via a Bessel beam creates a dense plasma rod with typically 100 nm diameter in sapphire. The interaction is in ideal conditions to trigger collisionless resonance absorption. This mechanism plays a primary role in the energy deposition process, yielding a plasma with an energy density on the order of MJ/cm3 and a length that can reach several cm using only tabletop femtosecond lasers.

physics.plasm-ph↗

arXiv2020

Efficient second-harmonic generation of a high-energy, femtosecond laser pulse in a lithium triborate (LBO) crystal

We demonstrate the highest efficiency ($\sim$80%) second harmonic generation (SHG) of Joule level, 27 femtosecond, high contrast pulses in a type-I lithium triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate (KDP) gives a maximum efficiency of 26%. LBO thus offers high intensity ($>$10$^{19}$ W/cm$^{2}$), ultra-high contrast femtosecond pulses, which have great potential for high energy density science particularly with nanostructured targets as well as technological applications.

physics.optics↗

arXiv2020

Post-compression of picosecond pulses into the few-cycle regime

In this work, we demonstrate post-compression of 1.2 picosecond laser pulses to 13 fs via gas-based multi-pass spectral broadening. Our results yield a single-stage compression factor of about 40 at 200 W in-burst average power and a total compression factor >90 at reduced power. The employed scheme represents a route towards compact few-cycle sources driven by industrial-grade Yb:YAG lasers at high average power.

physics.optics↗

arXiv2018

Ab-initio calculations of the linear and nonlinear susceptibilities of N$_2$, O$_2$, and air in the mid-infrared

We present first-principles calculations of the linear and nonlinear susceptibilities of N$_2$, O$_2$, and air in the mid-infrared wavelength regime, from $1-4$ $μ$m. We extract the frequency-dependent susceptibilities from the full time-dependent dipole moment that is calculated using time-dependent density functional theory. We find good agreement with curves derived from experimental results for the linear susceptibility, and with measurements for the nonlinear susceptibility up to 2.4 $μ$m. We also find that the susceptibilities are insensitive to the laser intensity even in the strong field regime up to $5\times 10^{13}$ W/cm$^2$. Our results will allow accurate calculations of the long-distance propagation of intense MIR laser pulses in air.

physics.optics↗

arXiv2017

Ultrafast supercontinuum generation in bulk condensed media (Invited Review)

Nonlinear propagation of intense femtosecond laser pulses in bulk transparent media leads to a specific propagation regime, termed femtosecond filamentation, which in turn produces dramatic spectral broadening, or superbroadening, termed supercontinuum generation. Femtosecond supercontinuum generation in transparent solids represents a compact, efficient and alignment-insensitive technique for generation of coherent broadband radiation at various parts of the optical spectrum, which finds numerous applications in diverse fields of modern ultrafast science. During recent years, this research field has reached a high level of maturity, both in understanding of the underlying physics and in achievement of exciting practical results. In this paper we overview the state of the art of femtosecond supercontinuum generation in various transparent solid-state media, ranging from wide-bandgap dielectrics to semiconductor materials and in various parts of the optical spectrum, from the ultraviolet to the mid-infrared. A particular emphasis is given to the most recent experimental developments: multioctave supercontinuum generation with pumping in the mid-infrared spectral range, spectral control, power and energy scaling of broadband radiation and the development of simple, flexible and robust pulse compression techniques, which deliver few optical cycle pulses and which could be readily implemented in a variety of modern ultrafast laser systems.

physics.optics↗

arXiv2016

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

Acoustic signals generated by filamentation of ultrashort TW laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.

physics.optics↗

arXiv2015

Laser beam self-symmetrization in air in the multifilamentation regime

We show experimental and numerical evidence of spontaneous self-symmetrization of focused laser beams experiencing multi-filamentation in air. The symmetrization effect is observed as the multiple filaments generated prior to focus approach the focal volume. This phenomenon is attributed to the nonlinear interactions amongst the different parts of the beam mediated by the optical Kerr effect, which leads to a symmetric redistribution of the wave vectors even when the beam consists of a bundle of many filaments.

physics.optics↗

arXiv2014

Generation of long-lived underdense channels using femtosecond filamentation in air

Using femtosecond laser pulses at 800 and 400 nm, we characterize the formation of underdense channels in air generated by laser filamentation at the millijoule energy level by means of transverse interferometry. We find that using tight focusing conditions, filamentation generates a shock wave and that the resulting low-density channel lasts for more than 90 ms. Comparison of these results with hydrodynamic simulations using an Eulerian hydrodynamic code gives an good agreement and allows us to estimate the initial gas peak temperature at $\sim$ 1000 K. The influence of experimental parameters such as the focusing conditions for the ultrashort laser pulse, its polarization or the wavelength is studied and linked to previous characterizations of filamentation-generated plasma columns.

physics.optics↗

arXiv2014

Energy deposition dynamics of femtosecond pulses in water

We exploit inverse Raman scattering and solvated electron absorption to perform a quantitative characterization of the energy loss and ionization dynamics in water with tightly focused near-infrared femtosecond pulses. A comparison between experimental data and numerical simulations suggests that the ionization energy of water is 8 eV, rather than the commonly used value of 6.5 eV. We also introduce an equation for the Raman gain valid for ultra-short pulses that validates our experimental procedure.

physics.optics↗

arXiv2014

Superfilamentation in air

The interaction between a large number of laser filaments brought together using weak external focusing leads to the emergence of few filamentary structures reminiscent of standard filaments, but carrying a higher intensity. The resulting plasma is measured to be one order of magnitude denser than for short-scale filaments. This new propagation regime is dubbed superfilamentation. Numerical simulations of a nonlinear envelope equation provide good agreement with experiments.

physics.optics↗

arXiv2014

Externally seeded backward lasing radiation from femtosecond laser filament in nitrogen gas

Recently, S. Mitryukovskiy et al. presented experimental evidence showing that backward stimulated radiation at 337 nm can be obtained from plasma filaments in nitrogen gas pumped by circularly polarized 800 nm femtosecond pulses (Opt. Express, 22, 12750 (2014)). Here, we report that this backward stimulated radiation is enhanced by a factor of ~ 16 in the presence of a seed pulse. This enhanced stimulated radiation can be either linearly or circularly polarized, dictated by the seeding pulse, which is distinct from the non-polarized nature of the ASE without seeding pulse. We also measured the spatial profile and estimated the energy of the radiation. This seeding effect confirms unambiguously the existence of population inversion between the C3Πu and B3Πg state of nitrogen molecules inside plasma filament and provides a possible solution to control the properties of this backward stimulated radiation.

physics.optics↗

arXiv2013

Self-compression to sub-3-cycle duration of mid-infrared optical pulses in bulk

The generation of few-cycle pulses with controlled waveforms in the mid-infrared spectral region is a long-standing challenge but is expected to enable a new generation of high-field physics experiments, uncovering intricate physical phenomena. Successful generation of such optical pulses is limited by the tremendous spectral width required to withstand few-cycle pulses in the mid-IR correlated with the need to tightly control the spectral phase over such a broad bandwidth. Here, we present the first demonstration of sub-three cycle optical pulses at 3.1 μm central wavelength using for the first time self-compression in the anomalous dispersion regime in bulk material. The pulses emerging from this compact and efficient self-compression setup could be focused to intensities exceeding 100 TW/cm^2, a suitable range for high field physics experiments. Our experimental findings are corroborated by numerical simulations using a 3D nonlinear propagation code, therefore providing theoretical insight on the processes involved.

physics.optics↗

arXiv2012

Scaling mechanism for efficient wavelength conversion in laser plasmas

Laser-induced ionization is a fundamental tool for the frequency conversion of lasers into spectral regions so far inaccessible, including both extreme ultraviolet and terahertz. The low-frequency currents induced by laser-driven ionization generate extremely broadband, single-cycle terahertz pulses, with applications ranging from remote sensing to optical pulse diagnostic, yet strong limitations arise from the low conversion efficiencies of this mechanism. We show a remarkable increase of the radiated terahertz energy with the laser wavelength and we relate this observation to the stronger action of long-wavelength fields on ionization-induced free-carriers. Ultimately, the use of mid-infrared pulses instead of the near-infrared ones employed so far enables the unprecedented table-top generation of the extremely high terahertz fields (>4 MV/cm) required for, e.g. the optical manipulation of quantum states, attosecond pulse synthesis and time-resolved studies of ultrafast electron dynamics. Furthermore, such high fields allowed us to perform space-time resolved terahertz diagnostics exploiting standard optical components.

physics.optics↗

arXiv2006

Collapse-driven spatiotemporal dynamics of filament formation

The transition from spatial to spatiotemporal dynamics in Kerr-driven beam collapse is modelled as the instability of the Townes profile. Coupled axial and conical radiation, temporal splitting and X waves appear as the effect of Y-shaped unstable modes, whose growth is experimentally detected.

physics.optics↗

arXiv2005

Conical emission, pulse splitting and X-wave parametric amplification in nonlinear dynamics of ultrashort light pulses

The precise observation of the angle-frequency spectrum of light filaments in water reveals a scenario incompatible with current models of conical emission (CE). Its description in terms of linear X-wave modes leads us to understand filamentation dynamics requiring a phase- and group-matched, Kerr-driven four-wave-mixing process that involves two highly localized pumps and two X-waves. CE and temporal splitting arise naturally as two manifestations of this process.

physics.optics↗