arXiv subjects

François Légaré

Publications and source records attributed to François Légaré.

At least 19 recordsLinked to original sources

arXiv2026

Phonon-driven decoherence of high-harmonic generation in the solid-state

High-harmonic generation in solids has emerged as a powerful probe of ultrafast electron dynamics and lattice motion, and recent theoretical work has suggested that thermally driven lattice fluctuations can act as an effective source of decoherence in the harmonic-generation process. However, a direct experimental link between high-harmonic emission and temperature-driven incoherent phonons has remained unclear. Here, we investigate the temperature dependence of high-harmonic generation in ultrapure silicon using reflection-geometry measurements over a wide temperature range. We observe that the harmonic yield increases significantly with decreasing temperature. To interpret these results, we introduce a one-dimensional atomic-chain model in which finite temperature is represented by random lattice displacements that mimic incoherent phonon fluctuations. The simulations reproduce the magnitude of temperature-dependent change of the harmonic signal and support a picture in which thermally induced lattice disorder enhances electron-hole decoherence, thereby reducing high-harmonic emission. Our results establish incoherent phonons as an important source of decoherence in solid-state high-harmonic generation.

cond-mat.mes-hall↗

arXiv2025

Comparison of wave-mixing processes in rarefied gas and QED vacuum using numerical simulations

We study the conditions required to distinguish laser-induced nonlinear quantum electrodynamics (QED) effects in vacuum from competing signals due to interactions of laser pulses with ionized residual gas. The latter is inevitably present in vacuum chambers where experiments are performed because the vacuum is never perfect and there is always some residual pressure. The rarefied gas contribution is modeled statistically using the 1D-1V Vlasov-Maxwell system, while vacuum nonlinearities are described within the weak-field expansion of the Heisenberg-Euler effective action. In both cases, photon spectra from wave-mixing processes are evaluated by solving numerically the resulting partial differential equations using a semi-Lagrangian scheme. We consider short pulses in co- and counter-propagating configurations, allowing us to identify the laser intensities and vacuum pressures for which the vacuum signal dominates. These results provide quantitative guidance for future all-optical experiments aiming to detect light-by-light scattering in vacuum.

physics.optics↗

arXiv2025

Generation of directed electron beams by tight focusing of an ultrashort IR laser in a near-critical plasma

Recent studies have demonstrated the possibility of accelerating electrons to MeV energies in ambient air using tightly focused laser configurations. In this article, we explore possible strategies to control and optimize the resulting electron beams using laser and gas parameters. Our theoretical analysis shows that in near-critical plasmas, linearly and circularly polarized pulses are more efficient than radially polarized pulses for electron acceleration. In addition, electron beams obtained from linearly polarized pulses have lower divergence angles. By studying the efficiency of the acceleration process - characterized by the maximum kinetic energy of electrons and their total number - we identify optimal conditions in ambient air at wavelength lambda_0 approximately 1.5 micrometers and a_0 >= 15. We also scale our results to lower-density air and demonstrate that some noble gases (Ne, Ar) are suitable media for accelerating electrons. Our investigations show that this acceleration scheme enables multi-MeV electrons with low divergence using millijoule-class high-repetition rate lasers, making it a promising candidate for applications in medical sciences and ultrafast imaging.

physics.plasm-ph↗

arXiv2024

Near- and mid-infrared excitation of ultrafast demagnetization in a cobalt multilayer system

In the last few decades, ultrafast demagnetization elicited by ultrashort laser pulses has been the subject of a large body of work that aims to better understand and control this phenomenon. Although specific magnetic materials' properties play a key role in defining ultrafast demagnetization dynamics, features of the driving laser pulse such as its duration and photon energy might also contribute. Here, we report ultrafast demagnetization of a cobalt/platinum multilayer in a broad spectral range spanning from the near-infrared (near-IR) to the mid-infrared (mid-IR), with wavelengths between 0.8 and 8.7 $μ$m. The ultrafast dynamics of the macroscopic magnetization is tracked via the time-resolved magneto-optical Kerr effect. We show that the ultrafast demagnetization of the sample can be efficiently induced over that entire excitation spectrum with minimal dependence on the excitation wavelength. Instead, we confirm that the temporal profile of the pump excitation pulse is an important factor influencing ultrafast demagnetization dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Ambient air plasma acceleration in tightly-focused ultrashort infrared laser beams

Recent experimental and theoretical results have demonstrated the possibility of accelerating electrons in the MeV range by focusing tightly a few-cycle laser beam in ambient air. Using Particle-In-Cell (PIC) simulations, this configuration is revisited within a more accurate modelling approach to analyze and optimize the mechanism responsible for electron acceleration. In particular, an analytical model for a linearly polarized tightly-focused ultrashort laser field is derived and coupled to a PIC code, allowing us to model the interaction of laser beams reflected by high-numerical aperture mirrors with laser-induced plasmas. A set of 3D PIC simulations is performed where the laser wavelength is varied from 800 nm to 7.0 $μ$m while the normalized amplitude of the electric field is varied from $a_{0} = 3.6$ to $a_{0} = 7.0$. The preferential forward acceleration of electrons, as well as the analysis of the laser intensity evolution in the plasma and data on electron number density, confirm that the relativistic ponderomotive force is responsible for the acceleration. We also demonstrate that the electron kinetic energy reaches a maximum of $\approx1.6$ MeV when the central wavelength is of 2.5 $μ$m.

physics.plasm-ph↗

arXiv2024

Self-dispersion-managed adiabatic frequency conversion

Dispersion management of few-cycle pulses is crucial for ultrafast optics and photonics. Often, nontrivial dispersion is compensated using complex optical systems or minimized through careful design of waveguides. Here, we present dispersion-managed adiabatic frequency conversion enabling efficient downconversion of an 11.1-fs near-IR pulse to an 11.6-fs mid-IR pulse spanning an octave of bandwidth from 2-4 $μ$m. The adiabatic frequency converter is designed to impart a constant group delay over the entire bandwidth, eliminating the need for complex dispersion management and opening a new avenue for dispersion engineering in ultrafast optics and photonics. Notably, dispersion engineering through the position-dependent conversion position of an adiabatic conversion device constitutes an additional mechanism for dispersion control in ultrafast optics and photonics while maintaining high conversion efficiency for broadband pulses.

physics.optics↗

arXiv2024

Relativistic electrons from vacuum laser acceleration using tightly focused radially polarized beams

We generate a tabletop pulsed relativistic electron beam at 100 Hz repetition rate from vacuum laser acceleration (VLA) by tightly focusing a radially polarized beam into a low-density gas. We demonstrate that strong longitudinal electric fields at the focus can accelerate electrons up to 1.43 MeV by using only 98 GW of peak laser power. The electron energy is measured as a function of laser intensity and gas species, revealing a strong dependence on the atomic ionization dynamics. These experimental results are supported by numerical simulations of particle dynamics in a tightly focused configuration that take ionization into consideration. For the range of intensities considered, it is demonstrated that atoms with higher atomic numbers like krypton can optimally inject electrons at the peak of the laser field, resulting in higher energies and an efficient acceleration mechanism that reaches a significant fraction of the theoretical energy gain limit.

physics.optics↗

arXiv2021

Attosecond correlated electron dynamics at C$_{60}$ giant plasmon resonance

Fullerenes have unique physical and chemical properties that are associated with their delocalized conjugated electronic structure. Among them, there is a giant ultra-broadband - and therefore ultrafast - plasmon resonance, which for C$_{60}$ is in the extreme-ultraviolet energy range. While this peculiar resonance has attracted considerable interest for the potential downscaling of nanoplasmonic applications such as sensing, drug delivery and photocatalysis at the atomic level, its electronic character has remained elusive. The ultrafast decay time of this collective excitation demands attosecond techniques for real-time access to the photoinduced dynamics. Here, we uncover the role of electron correlations in the giant plasmon resonance of C$_{60}$ by employing attosecond photoemission chronoscopy. We find a characteristic photoemission delay of up to 200 attoseconds pertaining to the plasmon that is purely induced by coherent large-scale correlations. This result provides novel insight into the quantum nature of plasmonic resonances, and sets a benchmark for advancing nanoplasmonic applications.

physics.atom-ph↗

arXiv2021

10.4 kW coherently combined ultrafast fiber laser

An ultrafast laser delivering 10.4 kW average output power based on coherent combination of twelve stepindex fiber amplifiers is presented. The system emits close-to-transform-limited 254 fs pulses at 80 MHz repetition rate, has a high beam quality (M2<=1.2), and a low relative intensity noise of 0.56% in the frequency range of from 1 Hz to 1 MHz. Automated spatiotemporal alignment allows for hands-off operation.

physics.optics↗

arXiv2020

Discrete dispersion scan setup for measuring few-cycle laser pulses in the mid-infrared

In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 micrometer wavelength. This minimal measuring scheme using only five optical components without the need for high-precision positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

physics.optics↗

arXiv2020

Attosecond-resolved photoionization of chiral molecules

Chiral light-matter interactions have been investigated for two centuries, leading to the discovery of many chiroptical processes used for discrimination of enantiomers. Whereas most chiroptical effects result from a response of bound electrons, photoionization can produce much stronger chiral signals that manifest as asymmetries in the angular distribution of the photoelectrons along the light propagation axis. Here we implement a self-referenced attosecond photoelectron interferometry to measure the temporal profile of the forward and backward electron wavepackets emitted upon photoionization of camphor by circularly polarized laser pulses. We found a delay between electrons ejected forward and backward, which depends on the ejection angle and reaches 24 attoseconds. The asymmetric temporal shape of electron wavepackets emitted through an autoionizing state further reveals the chiral character of strongly-correlated electronic dynamics.

physics.chem-ph↗

arXiv2020

Tracking ultrafast solid-state dynamics using high harmonic spectroscopy

We establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy for photoinduced dynamics in strongly correlated materials through a detailed investigation of the insulator-to-metal transitions in vanadium dioxide. We benchmark our technique by comparing our measurements to established momentum-resolved ultrafast electron diffraction, and theoretical density functional calculations. Tr-HHG allows distinguishing of individual dynamic channels, including a transition to a thermodynamically hidden phase. In addition, the HHG yield is shown to be modulated at a frequency characteristic of a coherent phonon in the equilibrium monoclinic phase over a wide range of excitation fluences. These results demonstrate that tr-HHG is capable of tracking complex dynamics in solids through its sensitivity to the band structure.

physics.optics↗

arXiv2018

Multiphoton Photoelectron Circular Dichroism of Limonene with Independent Polarization State Control of the Bound-Bound and Bound-Continuum Transitions

Photoionization of randomly oriented chiral molecules with circularly polarized light leads to a strong forward/backward asymmetry in the photoelectron angular distribution. This chiroptical effect, referred to as Photoelectron Circular Dichroism (PECD), was shown to take place in all ionization regimes, from single photon to tunnel ionization. In the Resonance Enhanced Multiphoton Ionisation (REMPI) regime, where most of the table-top PECD experiments have been performed, understanding the role of the intermediate resonances is currently the subject of experimental and theoretical investigations. In an attempt to decouple the role of bound-bound and bound-continuum transitions in REMPI-PECD, we photoionized the (+)-limonene enantiomer using two-color laser fields in [1+1'] and [2+2'] ionization schemes, where the polarization state of each color can be controlled independently. We demonstrate that the main effect of the bound-bound transition is to break the sample isotropy by orientation-dependent photoexcitation, in agreement with recent theoretical pre- dictions. We show that the angular distribution of PECD strongly depends on the anisotropy of photoexcitation to the intermediate state, which is different for circularly and linearly polarized laser pulses. On the contrary, the helicity of the pulse that drives the bound-bound transition is shown to have a negligible effect on the PECD.

physics.chem-ph↗

arXiv2018

Laser wakefield acceleration with high-power, few-cycle mid-IR lasers

The study of laser wakefield electron acceleration (LWFA) using mid-IR laser drivers is a promising path for future laser driven electronaccelerators, when compared to traditional near-IR laser drivers uperating at 0.8-1 μm central wavelength (λlaser), as the necessary vector potential a_0 for electron injection can be achieved with smaller laser powers due to the linear dependence on λlaser. In this work, we perform 2D PIC simulations on LWFA using few-cycle high power (5-15 TW) laser systems with λlaser ranging from 0.88-10 μm. Such few-cycle systems are currently under development, aiming at Gas High Harmonics Generation applications where the favourable λlaser^2 scaling extends the range of XUV photon energies. We keep a_0 and n_e/n_cr (n_e being the plasma density and n_cr being the critical density for each λlaser) as common denominators in outr simulations, allowing for comparisons between drivers of different λlaser, with respect to the accelerated electron beam energy, charge, and conversion efficiency. While the electron energies are mainly dominated by the plasma dynamics, the laser to electron beam energy conversion efficiency shows significant enhancement with longer wavelength laser drivers.

physics.plasm-ph↗

arXiv2017

Measurement of ultrashort optical pulses via time lens imaging in CMOS compatible waveguides

We demonstrate temporal measurements of subpicosecond optical pulses via time-to-frequency conversion in a 45cm long CMOS compatible high index glass spiral waveguide. The measurements are based on efficient four wave mixing in the C-band, using around 1W of peak pump power. We achieve a resolution of 400fs over a time window of 100ps, representing a time-bandwidth product > 250.

physics.optics↗

arXiv2016

Role of Excited States In High-order Harmonic Generation

We investigate the role of excited states in High-order Harmonic Generation by studying the spectral, spatial and temporal characteristics of the radiation produced near the ionization threshold of argon by few-cycle laser pulses. We show that the population of excited states can lead either to direct XUV emission through Free Induction Decay or to the generation of high-order harmonics through ionization from these states and recombination to the ground state. By using the attosecond lighthouse technique, we demonstrate that the high-harmonic emission from excited states is temporally delayed by a few femtoseconds compared to the usual harmonics, leading to a strong nonadiabatic spectral redshift.

physics.atom-ph↗

arXiv2014

Tabletop imaging of structural evolutions in chemical reactions

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using VUV light from a Free Electron Laser [Phys. Rev. Lett. 105, 263002 (2010)]. Here we show that 266 nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond-breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and definitively quantitative ab initio trajectory simulations.

physics.chem-ph↗

arXiv2013

Nonlinear imaging and 3D-mapping of terahertz fields with Kerr media

We investigate the spatially and temporally resolved four-wave mixing of terahertz fields and optical pulses in large band-gap dielectrics, such as diamond. We show that it is possible to perform beam profiling and space-time resolved mapping of terahertz fields with sub-wavelength THz resolution by encoding the spatial information into an optical signal, which can then be recorded by a standard CCD camera.

physics.optics↗