High-order harmonic generation (HHG) driven by optical vortices is a powerful route to produce structured light in different spectral regions. The nonlinear process transfers orbital angular momentum (OAM) from the driving field to the emitted harmonics according to the scaling law $l_q = q\times l$, a consequence of the rotational invariance and angular momentum conservation. Here, we show that, in the regime of few-cycle pulses, the topological charge (TC) of the harmonic radiation detected within a finite spectral window is no longer fixed by this scaling law alone, but is governed by the interplay between broken crystal inversion symmetry and carrier-envelope phase (CEP)-sensitive sub-cycle electron dynamics. By driving HHG in a ZnO crystal with few-cycle ($\approx 1.5$ cycles) vortex beams centered at 3.2~$μ$m, we observed that the measured TC becomes strongly CEP-dependent, switching between adjacent integer values, but only when the inversion symmetry is broken and the harmonic emission is CEP-sensitive. The TC switching vanishes when either condition is removed. Numerical analysis reveals that the TC switching originates from a CEP-controlled redistribution of spectral weight among spectrally overlapping harmonic orders, which changes the dominant OAM channel within the detection window. These results identify the CEP as a degree of freedom for tailoring the topological structure of high-harmonic radiation, pointing toward waveform-controlled structured attosecond light sources.

We demonstrate a 3.2 µm mid-infrared source delivering 30 µJ, single-cycle pulses via two-stage bulk post-compression of a 10 W four-cycle OPCPA. By optimizing a tandem configuration of BaF2 and Si, we achieved a compressed pulse duration of 11 fs with a CEP stability of 145 mrad RMS. The output exhibited high spatio-spectral uniformity with small residual spatial and angular chirp. The reliability and long-term stability of the system are tested. Additionally, high-order harmonics were generated in ZnO with locked CEP. The demonstrated performance is ideal for strong-field and attosecond-science applications.

We present a gigahertz (GHz) repetition rate fiber opticalparametric oscillator (FOPO) pumped by a wavelength-tunable electro-optic (EO) frequency comb operatingnear 1030 nm. This FOPO is based on a 24 cm long pho-tonic crystal fiber and operates in burst mode, drivenby picosecond pulses with a repetition rate of 16 GHz.It generates picosecond signal and idler pulses tunablefrom 866 nm to 901 nm and 1225 nm to 1300 nm, respec-tively. The maximum overall conversion efficiency frompump to signal and idler reaches 41%. We also report thebuild-up and relaxation dynamics of the signal pulse.The experimental measurements of the FOPO tuning,and parametric fluorescence spectra show good agree-ment with our numerical simulations.

We demonstrate the generation of femtosecond pulse trains at 1030 nm with repetition rates tunable from 330 GHz to 1 THz in a fully polarization-maintaining (PM) fiber system. Two detuned single-frequency lasers create a dual-frequency optical beat signal that undergoes strong nonlinear evolution in a PM photonic crystal fiber (PM-PCF) pumped in the anomalous dispersion regime, producing pulses with durations ranging from about 120 fs down to 60 fs without external post-compression. To deliver the required peak power, the dual-frequency signal is carved into 1-ns-long bursts at a repetition rate of 1 MHz. The nonlinear propagation is well described by breather theory. At higher input powers, the breathers evolve into Raman-shifted fundamental solitons, whose self-frequency shift and noise-driven dynamics are confirmed experimentally and numerically. These results highlight the versatility of the dual-pump scheme and the rich nonlinear dynamics of PM-PCFs, establishing a route toward compact sources of tunable, ultra-high-repetition-rate short pulses.

Although solid-state platforms underpin modern electronics, little is known about how intense ultrashort light pulses carrying orbital angular momentum (OAM) interact with solids. This gap persists even though, for more conventional light-matter interactions, the complex underlying electron dynamics can often be confined to a single Brillouin zone and described well within the dipole approximation. Previous studies were restricted to nonlinear, perturbative regimes, largely because the generation of intense ultrashort vortex pulses, particularly in the mid-infrared spectral regime, has remained a long-standing challenge. Consequently, the role of structured light in driving nonlinear, non-perturbative processes in solids, and the associated transfer of angular momentum during these interactions, has not been systematically explored. Here, we investigate solid-state high-harmonic generation (HHG) driven by intense ultrashort structured light using a versatile experimental approach applicable to different materials and geometries. We demonstrate that the OAM of the driving field is coherently transferred to the emitted harmonics. In particular, we show that the OAM is conserved independently of the crystal symmetry, the range of electronic interactions, and the presence of strong spin-orbit coupling. These results establish OAM-resolved HHG as a robust framework for characterizing and controlling angular momentum transfer in solid-state HHG and open new avenues for structured-light-driven quantum technologies and topological materials investigations.

Uncovering the dipole phase of gas-phase high harmonic generation was instrumental to understanding the recollision physics underlying attosecond pulse generation. Corresponding measurements in the condensed phase have not yet yielded a consistent picture. Here, we present a compact and inherently stable approach to high-harmonic interferometry in thin-film solids. We employ it to reveal the dipole phase of high-harmonic generation in polycrystalline ZnO, driven by broadband mid-IR laser pulses. We demonstrate that, under the conditions of our experiments, recollisions facilitated by Bloch oscillations represent the dominant contribution to high-harmonic generation just above the bandgap.

The ability to sculpt light in space, time, and polarization has revolutionized studies of light-matter interaction and enabled breakthroughs in optical communication, imaging, and ultrafast science. Among the many degrees of freedom of light, orbital angular momentum (OAM) further expands these capabilities by unlocking new regimes of control in information encoding, particle manipulation, and symmetry-driven selection rules. However, exploiting OAM to drive nonlinear, non-perturbative effects in solids remains challenging, especially in the mid-infrared (MIR) spectral regime, a key region for accessing these effects in ambient air, where spatial light modulators do not operate. Here, we circumvent this limitation by generating femtosecond, few-cycle MIR Bessel-Gauss vortex (BGV) and Perfect optical vortices (POVs), using a robust, static spatial-shaping strategy. Using those beams to drive the high-harmonic generation (HHG) process in solids, we show that the resulting harmonic beams faithfully inherit the structural properties of the drivers: the constant-intensity ring of the POV is preserved across harmonic orders, while the harmonic BGVs retain their intrinsic TC-dependent profiles. Furthermore, by verifying the OAM up-scaling law, we confirm OAM conservation during HHG in solids. These results establish strong-field HHG in solids as a robust platform for synthesizing ultrafast structured harmonic light with controllable, high-value OAM.

Four-cycle pulses from a 3.2 µm OPCPA system were characterized by spatially resolved Fourier transform spectroscopy, for the first time. Combining the spatio-spectral information with temporal characterization yielded the spatio-temporal structure of the pulses revealing up to now inaccessible details in the mid-IR spectral region. The proposed technique offers efficient and simple characterization for the optimization of MIR OPCPA systems in terms of spatio-spectral couplings.

The generation of high-repetition-rate short pulse trains has been extensively studied since the 1980s. One common approach is to nonlinearly compress an initial beat signal through multiple four-wave mixing processes in the anomalous chromatic dispersion region of an optical fiber [1]. Recent studies have linked this compression process to Akhmediev Breathers, a periodic analytical solution to the nonlinear Schrödinger equation [2], which is of significant interest due to its connection with modulation instability, supercontinuum generation and rogue waves [3], [4]. So far, the experimental studies have been limited to pulses with a duration of few hundred of femtosecond at wavelengths over 1300 nm where standard optical fibers have anomalous chromatic dispersion.

ThomX is a compact x-ray source based on Compton scattering, installed at IJCLab (Laboratoire de physique des 2 infinis-Irène Joliot-Curie) in Orsay. The machine uses a small electron storage ring and an intense laser pulse stored in a high-finesse optical cavity. This article describes the various subsystems of the machine and their initial results of the commissioning, which began in mid-2021. This first commissioning phase led to the production of 1010 x-rays/s with an on-axis energy of 45 keV. The main steps to be taken to reach the nominal flux are outlined at the end. Published by the American Physical Society 2025

We present the comprehensive characterization of a series of harmonic fields generated in a ZnO crystal by a few-cycle MIR driving pulse that spans the visible to mid-infrared (MIR) spectral region. The characterization is conducted using the recently developed Plasma-Induced Frequency Resolved Optical Switching (PI-FROSt) technique. We demonstrate the ability of this method to accurately characterize the MIR driving field (lambda = 3.2 mu m), as well as both odd and even harmonics up to the fifth order. The total spectral bandwidth extends over an exceptionally wide range of 2.6 octaves. All assessments validate the high precision of the field reconstructions and confirm the suitability of the PI-FROSt method for the metrology of over-octave-spanning waveforms. The results offer valuable insights into the fundamental mechanisms governing harmonic generation and emphasize the crucial influence of propagation and cascading.

Laser pulses are now available via optical parametric chirped pulse amplification (OPCPA) in the visible, near-infrared (NIR), mid-infrared (MIR) and even in the far-infrared spectral regions [1], [2]. However, diagnostics for pulses with long central wavelengths, especially for those containing only a few field oscillations, remain limited. Techniques already available in the visible and NIR spectral regions are challenging to implement at longer wavelengths due to the lack of high-resolution detectors, appropriate nonlinear crystals, and low-loss optical components.

ThomX is a compact x-ray source based on Compton scattering, installed at IJCLab (Laboratoire de physique des 2 infinis-Irène Joliot-Curie) in Orsay. The machine uses a small electron storage ring and an intense laser pulse stored in a high-finesse optical cavity. This article describes the various subsystems of the machine and their initial results of the commissioning, which began in mid-2021. This first commissioning phase led to the production of 10^{10} x-rays/s with an on-axis energy of 45 keV. The main steps to be taken to reach the nominal flux are outlined at the end.

Ultrafast lasers are widely used in microfabrication due to their precision and minimal thermal damage, but their throughput is often limited. Processing with bursts of pulses with GHz repetition rates aims to enhance ablation efficiency. However, recent results have shown strong dependencies on the number of pulses within the burst. The present work focuses on the pulse repetition rate. We report ultrafast laser ablation in copper and silicon at unexplored intra-burst repetition rates, from 1 up to 15 GHz, in both single and multi-burst processing. High-resolution scanning electron microscopy is used to characterize ablation volumes and crater morphology. Our results demonstrate a benefit in the ablation efficiency at very high repetition rates in the case of copper. It also shows that optimal processing for silicon and copper is observed with very different parameter configurations. This difference is attributed to the different thermal diffusivities. In addition, the crater morphologies across the different studied configurations strongly suggest a dependency on the viscosity of the liquid phase, providing insight into the varying temperatures reached during ablation.

Strong-field ionization can induce electron motion in both the continuum and valence shell of the parent ion. Here we report on a joint theoretical and experimental investigation of laser-induced electron diffraction in xenon. We explore the interplay of electron recollision with spin-orbit dynamics in the valence shell of the xenon cation. On the theory side, the electron-hole potentials for two different states are constructed, and the quantitative rescattering model is used to calculate the photoelectron momentum distributions (PMDs) for high-order above-threshold ionization of xenon. Measurements were carried out using 40-fs laser pulses with a central wavelength of 3100 nm and a peak laser intensity of 6 x 1013 W/cm2. The simulated PMDs describe well the features of the measured angular distributions of photoelectrons. Our study reveals a theoretical distinction between the electron signals resulting from rescattering off the m = 0 and m = 1 hole states, particularly noting a distinct change along the backward scattering angles. However, to fully identify the contributions of the hole states, a more accurate agreement between theory and experiment will be needed.

Fully configurable picosecond pulses at 1 to 7.5 GHz with tens to thousands pulses per burst are demonstrated at the kW average power level and several mJ energy per burst.

We report on a laser producing bursts of GHz picosecond pulses with pulse repetition rate between 1 and 7.5 GHz, tens to thousands pulses per burst, 1 kW average power and inJ energy per burst. © 2024 The Author(s).

With the increase in laser power and finesse of optical cavities over the last decade, laboratory-size Compton sources are very promising. These sources produce X-rays through interactions between relativistic electrons and laser photons and, in term of brightness, fall between large synchrotron facilities and classical laboratory X-ray sources. The ThomX source is the French project in this field. This article first presents a state of the art of high-intensity Compton sources, then the ThomX source is briefly described, and the first results are detailed, in particular the production of the first X-rays, the acquisition of the first spectrum and the first image of the beam. Finally, the next objectives are discussed.

Monitoring the carrier-envelope phase (CEP) is of paramount importance for experiments involving few cycle intense laser fields. Common measurement techniques include f-2f interferometry or stereo-ATI setups. These approaches are adequate, but are challenging to implement on demand, at different locations as additional metrology tools, in intense few cycle laser-matter interaction experiments, such as those prevalent in sophisticated user beamlines. In addition there are inherent difficulties for CEP measured at non-conventional laser wavelengths (like e.g. mid infrared) and measurements above 10 kHz laser repetition rates, on single shot basis. Here we demonstrate both by simulations and by experiments a machine learning (ML) driven method for CEP estimation in the mid infrared, which is readily generalizable for any laser wavelength and possibly up to MHz repetition rates. The concept relies on the observation of the spectrum of high harmonic generation (HHG) in bulk material and the use of ML techniques to estimate the CEP of the laser. Once the ML model is trained, the method provides a way for cheap and compact real-time CEP tagging. This technique can complement the otherwise sophisticated monitoring of CEP, and is able to capture the complex correlation between the CEP and the observable HHG spectra.