arXiv subjects

D. M. Villeneuve

Publications and source records attributed to D. M. Villeneuve.

15 recordsLinked to original sources

arXiv2025

Giant enhancement of attosecond tunnel ionization competes with disorder-driven decoherence in silicon

High-harmonic generation (HHG) is a strong-field phenomenon that is sensitive to the attosecond dynamics of tunnel ionization and coherent transport of electron-hole pairs in solids. While the foundations of solid HHG have been established, a deep understanding into the nature of decoherence on sub-cycle timescales remains elusive. Furthermore, there is a growing need for tools to control ionization at the nanoscale. Here, we study HHG in silicon along a crystalline-to-amorphous (c-Si to a-Si) structural phase transition and observe a dramatic reshaping of the spectrum, with enhanced lower-order harmonic yield accompanied by quenching of the higher-order harmonics. Modelling the real-space quantum dynamics links our observations to a giant enhancement (>250 times) of tunnel ionization yield in the amorphous phase and a disorder-induced decoherence that damps the electron-hole polarization over approximately six lattice sites. HHG spectroscopy also reveals remnant order that was not apparent with conventional probes. Finally, we observe a rapid and targeted non-resonant laser annealing of amorphous silicon islands. Our results offer a unique insight into attosecond decoherence in strong-field phenomena, establish HHG spectroscopy as a probe of structural disorder, and pave the way for new opportunities in lightwave nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2023

Orbital perspective on high-harmonic generation from solids

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics. Here, we report a strong intensity-dependent anisotropy in the high-harmonic generation from ReS$_2$ that we attribute to angle-dependent interference of currents from the different atoms in the unit cell. Furthermore, we demonstrate how the laser parameters control the relative contribution of these atoms to the high-harmonic emission. Our findings provide an unprecedented atomic perspective on strong-field dynamics in crystals and suggest that crystals with a large number of atoms in the unit cell are not necessarily more efficient harmonic emitters than those with fewer atoms.

physics.optics↗

arXiv2022

Observation and dynamic control of a new pathway of H$_3^+$ formation

We propose and experimentally demonstrate that the trihydrogen cation (H$_3^+$) can be produced via single photoionization of the molecular hydrogen dimer (H$_4$). Using near-infrared, femtosecond laser pulses and coincidence momentum imaging, we find that the dominant channel after single ionization of the dimer is the ejection of a hydrogen atom within a few hundred femtoseconds, leaving an H3+ cation behind. The formation mechanism is supported and well reproduced by an ab-initio molecular dynamics simulation. This is a new pathway of H$_3^+$ formation from ultracold hydrogen gas that may help explain the unexpected high abundance of H$_3^+$ in the interstellar medium in the universe.

physics.atm-clus↗

arXiv2021

Single-shot Dispersion Sampling for Optical Pulse Reconstruction

We present a novel approach to single-shot characterization of the spectral phase of broadband laser pulses. Our method is inexpensive, insensitive to alignment and combines the simplicity and robustness of the dispersion scan technique, that does not require spatio-temporal pulse overlap, with the advantages of single-shot pulse characterization methods such as single-shot frequency-resolved optical gating at a real-time reconstruction rate of several Hz.

physics.optics↗

arXiv2020

Control of $\text{N}_2^+$ Air Lasing

A near-infrared laser generates gain on transitions between the $\text{B}^{\text{2}} Σ_{\text{u}}^{\text{+}}$ and $\text{X}^{\text{2}} Σ_{\text{g}}^{\text{+}}$ states of the nitrogen molecular cation in part by coupling the $\text{X}^{\text{2}} Σ_{\text{g}}^{\text{+}}$ and $\text{A}^{\text{2}} Π_{\text{u}}$ states in the V-system. Traditional time resolved pump-probe measurements rely on post-ionization coupling by the pump pulse to initialize dynamics in the $\text{A}^{\text{2}} Π_{\text{u}}$ state. Here we show that a weak second excitation pulse reduces ambiguity because it acts only on the ion independent of ionization. The additional control pulse can increase gain by moving population to the $\text{A}^{\text{2}} Π_{\text{u}}$ state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on $\text{X}^{\text{2}} Σ_{\text{g}}^{\text{+}}$ to $\text{A}^{\text{2}} Π_{\text{u}}$ transitions may prevent the formation of a coherent rotational wave packet in the ground state in our experiment, but the control pulse can reverse impulsive alignment by the pump pulse to remove rotational wave packets in the $\text{B}^{\text{2}} Σ_{\text{u}}^{\text{+}}$ state.

physics.optics↗

arXiv2020

Femtosecond streaking in ambient air

We demonstrate a novel method to measure the temporal evolution of electric fields with optical frequencies. Our technique is based on the detection of transient currents in air plasma. These directional currents result from sub-cycle ionization of air with a short pump pulse, and the steering of the released electrons with the pulse to be sampled. We assess the validity of our approach by comparing it with different state-of-the-art laser-pulse characterization techniques. Notably, our method works in ambient air and facilitates a direct measurement of the field waveform, which can be viewed in real time on an oscilloscope in the exact same way as a radio frequency signal.

physics.optics↗

arXiv2017

Time Delay in Molecular Photoionization

Time-delays in the photoionization of molecules are investigated. As compared to atomic ionization, the time-delays expected from molecular ionization present a much richer phenomenon, with a strong spatial dependence due to the anisotropic nature of the molecular scattering potential. We investigate this from a scattering theory perspective, and make use of molecular photoionization calculations to examine this effect in representative homonuclear and hetronuclear diatomic molecules, nitrogen and carbon monoxide. We present energy and angle-resolved maps of the Wigner delay time for single-photon valence ionization, and discuss the possibilities for experimental measurements.

physics.atom-ph↗

arXiv2017

Bootstrapping to the Molecular Frame with Time-domain Photoionization Interferometry

Photoionization of molecular species is, essentially, a multi-path interferometer with both experimentally controllable and intrinsic molecular characteristics. In this work, XUV photoionization of impulsively aligned molecular targets ($N_2$) is used to provide a time-domain route to "complete" photoionization experiments, in which the rotational wavepacket controls the geometric part of the photoionization interferometer. The data obtained is sufficient to determine the magnitudes and phases of the ionization matrix elements for all observed channels, and to reconstruct molecular frame interferograms from lab frame measurements. In principle this methodology provides a time-domain route to complete photoionization experiments, and the molecular frame, which is generally applicable to any molecule (no prerequisites), for all energies and ionization channels.

physics.chem-ph↗

arXiv2012

High-harmonic transient grating spectroscopy of NO2 electronic relaxation

We study theoretically and experimentally the electronic relaxation of NO2 molecules excited by absorption of one ~400 nm pump photon. Semi-classical simulations based on trajectory surface hopping calculations are performed. They predict fast oscillations of the electronic character around the intersection of the ground and first excited diabatic states. An experiment based on high-order harmonic transient grating spectroscopy reveals dynamics occuring on the same timescale. A systematic study of the detected transient is conducted to investigate the possible influence of the pump intensity, pump wavelength, and rotational temperature of the molecules. The quantitative agreement between measured and predicted dynamics shows that, in NO2, high harmonic transient grating spectroscopy encodes vibrational dynamics underlying the electronic relaxation.

physics.chem-ph↗

arXiv2012

Probing Asymmetric Molecules with High Harmonic Generation. [Original manuscript as prepared on 22/05/2011]

Asymmetric molecules look different when viewed from one side or the other. This difference influences the electronic structure of the valence electrons, thereby giving stereo sensitivity to chemistry and biology. We show that attosecond and re-collision science provides a detailed and sensitive probe of electronic asymmetry. On each 1/2 cycle of an intense light pulse, laser-induced tunnelling extracts an electron wave packet from the molecule. When the electron wave packet recombines, alternately from one side of the molecule or the other, its amplitude and phase asymmetry determines the even and odd harmonics radiation that it generates. This harmonic spectrum encodes three manifestations of asymmetry; an amplitude and phase asymmetry in electron tunneling; an asymmetry in the phase that the electron wave packet accumulates relative to the ion between the moment of ionization and recombination; and an asymmetry in the amplitude and phase of the transition moment. We report the first measurement of high harmonics from oriented gas samples. We determine the phase asymmetry of the attosecond XUV pulses emitted when an electron recollides from opposite sides of the CO molecule, and the phase asymmetry of the recollision electron just before recombination. We discuss how the various contributions to asymmetry can be isolated in future experiments.

physics.atom-ph↗

arXiv2012

Oriented rotational wave-packet dynamics studies via high harmonic generation

We produce oriented rotational wave packets in CO and measure their characteristics via high harmonic generation. The wavepacket is created using an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm pulse probes the wave packet, generating even-order high harmonics that arise from the broken symmetry induced by the orientation dynamics. The even-order harmonic radiation that we measure appears on a zero background, enabling us to accurately follow the temporal evolution of the wave packet. Our measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation.

physics.atom-ph↗

arXiv2011

Order-dependent structure of High Harmonic Wavefronts

The physics of high harmonics has led to the generation of attosecond pulses and to trains of attosecond pulses. Measurements that confirm the pulse duration are all performed in the far field. All pulse duration measurements tacitly assume that both the beam's wavefront and intensity profile are independent of frequency. However, if one or both are frequency dependent, then the retrieved pulse duration depends on the location where the measurement is made. We measure that each harmonic is very close to a Gaussian, but we also find that both the intensity profile and the beam wavefront depend significantly on the harmonic order. Thus, our findings mean that the pulse duration will depend on where the pulse is observed. Measurement of spectrally resolved wavefronts along with temporal characterization at one single point in the beam would enable complete space-time reconstruction of attosecond pulses. Future attosecond science experiments need not be restricted to spatially averaged observables.

physics.optics↗

arXiv2011

Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N$_{2}$ molecules}

We report measurements and theoretical simulations of high-order harmonic generation (HHG) in aligned N$_2$ molecules using a 1200-nm intense laser field when the generating pulse is perpendicular to the aligning one. With increasing laser intensity, the minimum in the HHG spectra first shifts its position and then disappears. Theoretical simulations including the macroscopic propagation effects in the medium reproduce these observations and the disappearance of the minimum is attributed to the additional contribution of HHG from inner orbitals. We also predict that the well-known shape resonance in the photoionization spectra of N$_2$ should exist in the HHG spectra. It is most clearly seen when the generating laser is parallel to the aligning one, and disappears gradually as the angle between the two lasers increases. No clear evidence of this shape resonance has been reported so far when using lasers with different wavelengths. Further experimentation is needed to draw conclusions.

physics.optics↗

arXiv2011

Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers

We present experimental results showing the appearance of a near-continuum in the high-order harmonic generation (HHG) spectra of atomic and molecular species as the driving laser intensity of an infrared pulse increases. Detailed macroscopic simulations reveal that these near-continuum spectra are capable of producing IAPs in the far field if a proper spatial filter is applied. Further, our simulations show that the near-continuum spectra and the IAPs are a product of strong temporal and spatial reshaping (blue shift and defocusing) of the driving field. This offers a possibility of producing IAPs with a broad range of photon energy, including plateau harmonics, by mid-IR laser pulses even without carrier-envelope phase stabilization.

physics.atom-ph↗

arXiv2010

Separation of Target Structure and Medium Propagation Effects in High-Harmonic Generation

We calculate high-harmonic generation (HHG) by intense infrared lasers in atoms and molecules with the inclusion of macroscopic propagation of the harmonics in the gas medium. We show that the observed experimental spectra can be accurately reproduced theoretically despite that HHG spectra are sensitive to the experimental conditions. We further demonstrate that the simulated (or experimental) HHG spectra can be factored out as a product of a \macroscopic wave packet" and the photo-recombination transition dipole moment where the former depends on the laser properties and the experimental conditions, while the latter is the property of the target only. The factorization makes it possible to extract target structure from experimental HHG spectra, and for ultrafast dynamic imaging of transient molecules.

physics.atom-ph↗