Michele Puppin

Two-photon absorption laser-induced fluorescence (TALIF) is a well-established diagnostic technique for measuring the density of neutral atoms in plasmas. We demonstrate a new setup for a femtosecond-TALIF (fs-TALIF) diagnostic in fusion-relevant hydrogen plasmas in the Resonant Antenna Ion Device (RAID). The system is designed to produce high-energy laser pulses with a broad spectral width, suitable for exciting both atomic hydrogen and krypton in known TALIF schemes without the need to retune the laser wavelength. The setup consists of a commercial fs laser, an in-house-developed optical harmonic generator, and the fluorescence detection system on the plasma device. The system produces laser pulses at the atomic hydrogen two-photon absorption line at 205 nm, with pulse energies up to 145 μJ, spectral widths of 1.2 nm (1/e2 width), and pulse durations of 250 fs (FWHM). The pulse parameters are stable on a pulse-to-pulse basis and over longer time periods. We characterize the effect of the transport of the pulses to the plasma device through 19 m of atmospheric air and a thin MgF2 window. We show a one-dimensional fs-TALIF profile in a krypton gas in RAID, in which the laser is collimated (3 mm diameter) and tuned to the hydrogen resonance. This result shows the potential of the system to be used as a fs-TALIF diagnostic for calibrated atomic hydrogen density measurements.

High-intensity coherent light can realize hybrid phases of matter that are not accessible in equilibrium. The coherent interaction between Bloch states and a periodic electric field, which leads to Floquet-engineered band structures, has been extensively discussed in the literature and shown to enable modified electronic states, and it potentially induces topological phases in semiconductors. However, a Floquet topological insulator has not yet been realized experimentally in a semiconductor. Here we show that femtosecond light pulses can create a short-lived topological state in SnTe that is absent from the equilibrium ground state. This occurs when the photo-excitation energy is close to the bandgap of this polar semiconductor. We observe a concomitant renormalization of the band dispersions that reveals the generation of Floquet states connected to the topological state. These results provide a direct experimental observation of a Floquet topological state and indicate that it is driven by a light-induced band inversion in SnTe, although further theoretical work will provide a more detailed characterization of the state. Our work also indicates that optical control of the topological properties of semiconductors on demand is possible.

Ternary semiconductors with chalcopyrite structure are of growing interest for electronic, energy-conversion, and spintronic applications. Using angle-resolved photoemission spectroscopy, we directly map the band structure of CdGeAs 2 , the prototypical chalcopyrite described by a modified Kane model that predicts the coexistence of flat bands and highly dispersive linear states. The combined effect of tetragonal distortion and broken inversion symmetry reduces the band degeneracy, allowing us to resolve three distinct states at the top of the valence band, with band velocities comparable to those reported in Dirac semimetals. We further probe the spin-to-charge conversion properties of CdGeAs 2 by optically injecting spin-polarized carriers and detecting the resulting inverse spin Hall effect (ISHE). The photon-energy dependence of the ISHE signal and theoretical calculations of the optically injected spin polarization allow us to identify the specific valence-to-conduction transitions responsible for net spin accumulation and to estimate the spin diffusion length. These results demonstrate that chalcopyrite semiconductors provide a versatile platform for optospintronics.

Jahn–Teller polarons are quasiparticles that stem from symmetry breaking and strong local electron–phonon coupling. They originate from an excess charge carrier being dressed by a local lattice distortion, caused by the Jahn–Teller effect, and they critically impact electrical, structural, and magnetic properties in transition metal oxides. The observation of the microscopic steps involved in their formation is essential for enabling control over material properties through the targeted activation of local, site-specific modifications with light pulses. While Jahn–Teller distortion associated with polaron formation was predicted to contribute significantly to changes in electronic band gap and optical properties in Co₃O₄, its experimental observation remains elusive, requiring signatures of local symmetry reduction. In this work, we demonstrate Jahn–Teller polaron formation in spinel Co₃O₄. By exciting electronic transitions at 3.10 eV and 1.55 eV, we target either the O_h Cobalt(III) or the T_d Cobalt(II) ions, and drive the subsequent coherent responses of the system through two different pathways. For the former, we demonstrate that ligand-to-metal charge transfer leads to Jahn–Teller polaron formation, which is linked to the deformation potential and magnetoelastic coupling. For the latter, we identify the coherent excitation of a T_2g phonon mode launched by on-site d-d electronic transitions. Key to our observations is the ability to target site-specific electronic excitations in spinel Co₃O₄ using ultrafast optical pulses, while monitoring the ensuing low-energy collective modes through the coherent time-domain response of the material. Our approach, which combines the comparative analysis of experimental fingerprints with the support from density functional theory calculations, is broadly applicable to systems in which Jahn–Teller polaron physics has been theoretically predicted but remains experimentally unverified, and it underscores the potential of electronic-state targeting as a route to selectively excite and probe quasiparticle dynamics in solids.

We demonstrate a novel shot-to-shot acquisition method for optical pump—keV electron energy probe in ultrafast scattering experiments. We integrate a phase-locked acquisition scheme at a repetition rate of 20 kHz in a conventional ultrafast electron diffraction setup. We proceed to a full characterization of the noise level in different configurations and for realistic scenarios. The shot-to-shot acquisition improves the signal-to-noise ratio by one order of magnitude and can be readily implemented in other high-repetition-rate electron diffraction and spectroscopy setups.

Scattering between charges and collective modes in materials governs phenomena such as electrical resistance, energy dissipation, and phase switching. Studying such scattering requires simultaneous access to ultrafast and momentum-resolved dynamics of single-particle and collective excitations, which remains an experimental challenge. Here, we present time- and momentum-resolved electron energy loss spectroscopy, and we apply it to graphite, demonstrating that large (Δ q ≃1.2 Å ⁻¹ ) photoexcited electron-hole pockets induce a renormalization of in-plane and bulk plasmons. This effect is explained by intra- and intervalley scattering processes mediated by E _2g and A ′ 1 phonon modes, which we directly observe via ultrafast electron diffraction and identify via ab initio calculations. Conversely, smaller electron-hole pockets (Δ q ≃0.7 Å ⁻¹ ) result in the renormalization of in-plane plasmons, which can only be partially explained by phonon-mediated scattering and thermal expansion. Our results highlight the importance of combining momentum- and time-resolved information to elucidate electronic scattering processes.

We present a new setup for resonant inelastic hard x-ray scattering at the Bernina beamline of SwissFEL with energy, momentum, and temporal resolution. The compact R = 0.5 m Johann-type spectrometer can be equipped with up to three crystal analyzers and allows efficient collection of RIXS spectra. Optical pumping for time-resolved studies can be realized with a broad span of optical wavelengths. We demonstrate the performance of the setup at an overall ∼180 meV resolution in a study of ground-state and photoexcited (at 400 nm) honeycomb 5d iridate α-Li2IrO3. Steady-state RIXS spectra at the iridium L3-edge (11.214 keV) have been collected and are in very good agreement with data collected at synchrotrons. The time-resolved RIXS transients exhibit changes in the energy loss region <2 eV, whose features mostly result from the hopping nature of 5d electrons in the honeycomb lattice. These changes are ascribed to modulations of the Ir-to-Ir inter-site transition scattering efficiency, which we associate to a transient screening of the on-site Coulomb interaction.