François Légaré

Achieving high piezoelectric performance in lead‐free halide perovskites remains challenging because substituting lead typically compromises piezoelectricity through reduced lattice distortion and introduces stability concerns. Here, a lead‐free two‐dimensional Ruddlesden‐Popper hybrid perovskite, (ATHP) ₂ SnCl ₄ (ATHP = 4‐aminotetrahydropyran), is developed with strongly distorted [SnCl6] ^{4 −} octahedra and enhanced lattice asymmetry relative to its lead‐based analogue. First‐principles calculations reveal pronounced Sn 5s‐Cl 3p hybridization and an anisotropic Sn ²⁺ lone‐pair lobe, producing stereochemical activity and structural polarization. Cooperative interactions between the lone pair and uniformly oriented [ATHP] ⁺ cations stabilize the layered structure, yielding an experimental spontaneous polarization of 8.3 µC.cm ⁻² , and a d ₃₃ of 65.7 pC.N ⁻¹ in the optimized composite. The piezoelectric nanogenerator (PENG) delivers a milliwatt‐level power density of 2.34 mW.cm ⁻² , among the highest reported for lead‐free hybrid‐perovskite PENGs and competitive with lead‐based systems. The device maintains 65% of its voltage and 25% of its current density at −90°C, corresponding to ∼3.5‐ and ∼12‐fold higher retention than pure poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)), while retaining 76.8% and 71.3%, respectively, at 90% relative humidity. Cryogenic operation is demonstrated by continuously powering an electronic watch for over 15 min at −50°C. These findings establish a lone‐pair‐engineering route to stable, flexible, high‐efficiency lead‐free perovskite PENGs for extreme environments.

We demonstrate blue-shifted few-cycle pulse generation in a short Krypton-filled capillary fiber operated near the self-focusing threshold. Using a 25 cm capillary, the propagation remains sufficiently short to limit the buildup of accumulated nonlinear effects and to access the early self-focusing-driven dynamics. Experimentally, the onset of weak self-focusing is accompanied by stable sideband generation, spatial confinement of the blue-shifted frequencies, and progressive temporal self-compression as the spectrum broadens. Multimode simulations support a short-distance self-action picture in which energy is redistributed from the fundamental mode into low-order radially symmetric modes, followed by rapid, blue-shifted spectral broadening. These results demonstrate that near-threshold self-focusing in weakly confined gas-filled capillary fibers can be used as a controllable route to energetic few-cycle visible/ultraviolet pulses and as a platform for studying multimode nonlinear dynamics.

Angle-resolved photoemission spectroscopy (ARPES) provides a direct access to the electronic band structure of solid and molecular systems. The momentum range accessible by this technique depends directly on the photon energy used, and low-photon-energy sources are insufficient to photoemit electrons over the full Brillouin zone of most quantum materials. In addition, while electrons are emitted over a 2π solid angle, conventional hemispherical analyzers only collect a small subset of those electrons. A previous work [Gauthier et al., Rev. Sci. Instrum. 92, 123907 (2021)] demonstrated that electrons emitted over a larger field-of-view can be acquired in one fixed configuration by accelerating them toward the analyzer with a bias voltage. Here, we extend this work by leveraging the deflector technology of novel ARPES hemispherical analyzers. We demonstrate the ability to detect all 2π photoemitted electrons in a fixed configuration for various materials, such as gold, cuprates, and transition-metal dichalcogenides. This approach is especially advantageous for time-resolved ARPES, as electron dynamics over a large momentum range can be accessed with identical measurement conditions.

Recent studies have demonstrated the possibility of accelerating electrons to MeV energies in ambient air using tightly focused laser configurations [Vallières et al., “High dose-rate MeV electron beam from a tightly-focused femtosecond IR laser in ambient air,” Laser Photonics Rev. 18, 2300078 (2024); Lytova et al., “Electron acceleration in ambient air using tightly focused ultrashort infrared laser beams,” Phys. Rev. E 111, 035210 (2025)]. 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 λ0≈1.5μm and a0≥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.

We present an environmentally stable, all-polarization-maintaining (PM) dispersion-based swept source system optimized for narrow linewidth and high-speed wavelength scanning, specifically tailored for fiber Bragg grating (FBG) sensor interrogation. The system integrates a chirped fiber Bragg grating (CFBG) as a highly dispersive element to balance scan speed and spectral resolution, enabling a compact cavity design and stable lasing. The laser achieves precise wavelength tuning over a 40 nm range in the telecom C-band, linewidth around 12 MHz, and an impressive scanning speed exceeding 3 MHz with stable output power of over 20 mW, outperforming mechanically tuned lasers. The all-PM design ensures robustness against environmental perturbations. We validate the performance through static and dynamic characterization, including linewidth narrowing via the spatial hole burning effect. Furthermore, we demonstrate its application in the interrogation of FBG sensor arrays in wavelength division multiplexing (WDM), time division multiplexing (TDM), and hybrid modes, as well as real-time dynamic strain sensing up to 1kHz. These results position the system as a scalable and robust solution for applications requiring rapid spectral acquisition and fine resolution, such as border security, biomedical imaging, and structural health monitoring.

A transmission parabola is a tight-focusing on-axis parabolic reflector that potentially enables ultra-high intensities when combined with high-power laser pulses. We propose a method to characterize the peak intensity generated with this optic by measuring the laser beam wavefront after reflection. The focused electromagnetic field and the corresponding peak intensity at the focal plane are calculated using the Stratton-Chu formulation. Without wavefront corrections, the measured intensity is strongly degraded due to alignment constraints and the quality of the manufactured optic. The intensity reached is at most 6% of the ideal case, where the beam wavefront is perfectly flat and aberration-free. However, we demonstrate that the focused peak intensity can be improved substantially by correcting the laser beam wavefront with a deformable mirror. An intensity approaching 70% of the ideal case is obtained, thus showing the relevance of the transmission parabola for high-intensity applications.

We present an in-depth study on the impact of spatiotemporal Raman enhancement in molecular gas-filled hollow-core fibers (HCFs), demonstrating the efficient generation and post-compression of multidimensional solitary states (MDSS). Through different experimental scenarios—employing large-core HCFs filled with molecular gases (N2 and N2O) and driven by high energy, sub-picosecond and picosecond Fourier transform-limited ytterbium laser pulses—this work leverages multimode propagation and enhanced spatiotemporal interactions to achieve significant spectral broadening and asymmetric redshift, contrasting sharply with self-phase modulation. Our findings reveal that, beyond the regime of maximum nonadiabatic molecular alignment, spatiotemporal nonlinear enhancement primarily governs spectral broadening for input pulse durations up to 1 ps. The process shows limited sensitivity to input pulse duration and the two investigated molecular gases (N2 and N2O), with only subtle differences in broadening arising from their distinct Raman spectroscopic properties. Furthermore, post-compression of MDSS was achieved in various cases. Notably, using 7 mJ, 1 ps laser pulses, we generated 22 fs pulses with a 47% energy conversion efficiency of the input pulse energy. These results position MDSS as a powerful platform for generating high-energy, ultrashort pulses with tunable wavelengths, offering a robust solution for applications such as high harmonic generation.

Hollow-core fiber (HCF) based temporal pulse compression is widely used to generate few-cycle pulses. Here, we demonstrate the strength of methane (CH₄) as a nonlinear medium for HCF-based pulse compressors to achieve high average power few-cycle pulses of sub-16 fs duration with > 70% transmission and up to 250 kHz repetition rate in a compact setup and present it as a cost-effective alternative to highly expensive and scarcely available noble gases.

Frequency-resolved optical switching (FROSt) is a phase-matching-free characterization technique for ultrashort pulses based on transient absorption in semiconductors. So far, this technique has been limited to characterizing pulses with photon energies smaller than the bandgap of the semiconductors used. In this work, we extend the method to characterize pulses of photon energy greater than the bandgap of the semiconductor used for characterization. We demonstrate this by characterizing ultrashort visible pulses and supercontinuum using silicon (Si) thin films deposited on a sapphire substrate. We also demonstrate that visible light sources up to a repetition rate of 250 kHz can be characterized using these samples. Therefore, this study highlights the potential of FROSt as a suitable technique for the temporal characterization of weak visible to infrared pulses, including high harmonics generated in solids.