We are excited to share our latest pre-print on “Signatures of multi-band effects in high-harmonic generation in monolayer molybdenum disulfide”. In this joint experimental and theoretical work on solid-state high-harmonic generation (HHG), we illustrate that the polarization properties of the harmonics encode not only the dynamical symmetry properties of the crystal and laser field system, but also material-specific properties such as the vectorial character of the transition dipole moments from different valence-conduction-band pairs. Importantly, we conduct experiments on monolayer materials which enables us to show that angular spectral shifts of the harmonic emission stem from multiband contributions instead of bulk propagation effects.

This work was done in collaboration with researchers from the Louisiana State University, Friedrich Schiller University Jena, and Yale University.

Pre-print available here:
https://arxiv.org/abs/2112.13032

We are excited to share our latest pre-print on “Saturable absorption of free-electron laser radiation by graphite near the carbon K-edge”. In this experiment, we study in a single experiment supported by state-of-the-art numerical simulations how intense femtosecond X-ray free-electron laser pulses induce two different nonlinear responses, saturable absorption (SA) and two-photon absorption (TPA), depending on intensity. Our data suggests that SA and TPA are competing processes in the X-ray regime and the relative transition strengths determine at which intensities TPA becomes dominant over SA which is ultimately a material-specific property. Our results reveal the competing contributions of distinct nonlinear material responses to spectroscopic signals measured in the X-ray regime, demonstrating an approach of general utility for interpreting FEL spectroscopies. This opens new routes for dynamically probing properties of matter.

Congrats to Lars for his first submitted manuscript in the group.

This work was done in collaboration with researchers from the Saykally group at UCB, LBNL, U of Texas Rio Grande Valley, UC San Diego, and FERMI. The experiments were conducted at the free-electron laser facility FERMI in Italy.

Pre-print available here:
https://arxiv.org/abs/2112.12585

We are excited to see our work on polarization-resolved second harmonic spectroscopy enabling element-resolved angular anisotropy investigations published. In LiNbO3 we directly resolve the Li ion displacement and its correlated action on the Nb-O bonds. This study constitutes the first observation of polarization-resolved SHG in the extreme ultraviolet (XUV) and we show that dipole-based SHG models used regularly in the optical regime allow predicting the SHG polarization in the in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.
This work as performed at the SACLA free-electron laser at Sping8/RIKEN in primary collaboration with UC San Diego, Argonne National Lab, LBNL and U Tokyo.
Our paper was now published in Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.237402