Quentin Courtade

Nonequilibrium manipulation of quantum materials via electromagnetic dressing provides an on-demand route to tailoring electronic band structures through Floquet engineering. Time- and angle-resolved photoemission spectroscopy offers a direct means to probe these light-dressed electronic states. In such photoemission experiments, electromagnetic dressing can also occur for quasifree electrons outside the material, giving rise to so-called Volkov states. In certain cases, most notably in metallic systems, strong surface screening reduces the penetration of the driving electromagnetic field into the solid, resulting in Volkov contributions that dominate over Floquet ones. This underscores the importance of carefully accounting for the material's dielectric response when analyzing the origin of light-induced band replicas using time- and angle-resolved photoemission spectroscopy. In this work, we systematically investigate the influence of materials' dielectric properties on Floquet-Volkov dressing of semiconductors, focusing on bulk layered van der Waals materials GeS, SnS, and 2 H − WSe 2 . First, by combining a simple model based on Fresnel equations with an electron-scattering description of Volkov amplitudes, we use polarization-dependent Volkov sideband intensities to extract a lower bound for the real part of the materials' dielectric function. The extracted values typically lie between the reported dielectric constants for monolayer and bulk crystals. Furthermore, we demonstrate that increasing the pump fluence of the infrared pump laser enables the generation of high-order Volkov sidebands, which exhibit clear signatures of nonlinear light-matter interactions in their temporal dynamics, polarization dependence, and angular distributions. Finally, we show that for our experimental geometry, the quasitransparent nature of semiconductors in the below-band-gap driving regime allows the optical pump to propagate within the sample and undergo multiple total internal reflections. This process produces a series of temporally delayed Volkov replicas in pump-probe measurements via electromagnetic dressing of photoelectrons by the evanescent field associated with total internal reflections. Together, these systematic studies uncover several previously unexplored aspects of Floquet-Volkov dressing in solids, highlighting the central role of dielectric screening of the driving field.

The complex phase diagram of 1T-TiSe ₂ consists of a charge density wave (CDW) below 200 K, and CDW fluctuations of still unknown origin at higher temperatures. Here, we use time-resolved extreme ultraviolet momentum microscopy and density functional perturbation theory to uncover the formation mechanism of CDW fluctuations and their spectral features at 295 K. We investigated the transient dynamics of fluctuations upon nonresonant ultrafast photoexcitation, and directly correlate it with the CDW soft-phonon hardening. Surprisingly, our results show that the coherent amplitude mode modulating ultrafast CDW recovery persists above T _CDW , and reveal that CDW fluctuations are dominated by the electron-phonon interaction rather than excitonic correlations as commonly believed. Our findings on these microscopic CDW fluctuations clarify the complex interplay between electronic and lattice degrees of freedom at elevated temperatures and, therefore, could be useful in understanding the nature of the CDW phase transition in 1T-TiSe ₂ and similar quantum materials.

We report the development of an instrument combining an ultrafast, high-repetition-rate, polarization-tunable monochromatic extreme ultraviolet (XUV, 21.6 eV) beamline and a next-generation momentum microscope end station. This setup enables time- and angle-resolved photoemission spectroscopy of quantum materials, offering multimodal photoemission dichroism capabilities. The momentum microscope simultaneously detects the full surface Brillouin zone over an extended binding energy range. It is equipped with advanced electron optics, including a new type of front lens that supports multiple operational modes. Enhanced spatial resolution is achieved by combining the small XUV beam footprint (33 × 45 μm2) with the selection of small regions of interest using apertures positioned in the Gaussian plane of the momentum microscope. This instrument achieves an energy resolution of 44 meV and a temporal resolution of 144 fs. We demonstrate the capability to perform linear, Fourier, and circular dichroism in photoelectron angular distributions from photoexcited 2D materials. This functionality paves the way for time-, energy-, and momentum-resolved investigations of orbital and quantum geometrical properties underlying the electronic structures of quantum materials driven out of equilibrium.

The study of magnetic frustration in classical spin systems is motivated by the prediction and discovery of classical spin liquid states. These uncommon magnetic phases are characterized by a massive degeneracy of their ground state implying a finite magnetic entropy at zero temperature. While the classical spin liquid state is originally predicted in the Ising triangular lattice antiferromagnet in 1950, this state has never been experimentally observed in any triangular magnets. The discovery of an electric analogue of classical spin liquids on a triangular lattice of uniaxial electric dipoles in EuAl ₁₂ O ₁₉ is reported here. This new type of frustrated antipolar phase is characterized by a highly‐degenerate state at low temperature implying an absence of long‐range antiferroelectric order, despite short‐range antipolar correlations. Its dynamics are governed by a thermally activated process, slowing down upon cooling toward a complete freezing at zero temperature.