Techniques

Our group makes use of laboratory-based techniques, such as time-resolved optical spectroscopy, and time- and momentum-resolved electron energy loss spectroscopy (trEELS), as well as time-resolved X-ray scattering at free electron laser facilities. Below we offer a brief overview of our main experimental methods.

Time-Resolved Optical Spectroscopy

Optical spectroscopy is a versatile experimental tool for studying the long-wavelength behavior of condensed matter systems. The relevant measured quantity is the current-current correlation function, encoded in the complex dielectric function. When perfomed with ultrafast lasers, optical spectroscopy can reveal key features of the emergent electronic dynamics following a strong laser excitation. We are particularly interested in the low-energy dynamics of materials, roughly located in the midinfrared/THz region of the electromagnetic spectrum, and in the posssibility of using strong THz pulses to manipulate electronic degress of freedom in quantum materials. Our group has a vast expertise in using ultrafast light pulses to tailor material properties and we are strongly interested in developing THz-pump/THz probe experiments, as well as performing 2D THz spectroscopy. on quantum materials.

Time-Resolved EELS

EELS is a scattering technique commonly used to study the charge response of a material. An electron beam impinges on a solid and it gets scattered via Coulomb interaction off the target electron clouds. By analyzing both the energy spectrum and the exchanged momentum, one can deduce the structure of the so-called "density-density correlation function" \(\chi''({\bf q},\omega)\) along both the energy and momentum axis. While being an established technique at equilibrium, recent developments in ultrafast electron sources enabled the possibility of performing trEELS experiments, i.e. to probe the transient electronic response function with full momentum and energy resolution. In the past, trEELS has been successfully performed in pioneering electron microscope experiments with eV energy resolution.

Our group is developing a custom trEELS spectrometer synchronized to a femtosecond laser amplifier following a different concept. A key element of our approach is the use of microwave cavities to manipulate the electron pulses and yield sub-ps pulses with an energy resolution down to 10 meV. At this resolution, trEELS becomes a useful tool for the investigation of emergent quantum material behavior. By coupling this setup to a multi-axis, cryogenic sample goniometer, this instrument will be able to perform scattering studies of driven quantum materials, and thus unraveling for the first time their finite-momentum electrodynamic response at ultrafast timescales.

Time-Resolved RIXS

Resonant Inelastic X-ray Scattering (RIXS) is a powerful spectroscopic method for the investigation of valence-band excitations in solids. X-ray photons are tuned resonantly to transitions between core and valence states in order to enhance the scattering cross section. The excitation process leaves a core hole, which is then filled by electrons from the valence shell through the emission of a second photon. The detection of the scattered photon as a function of energy and momentum provides valuable information about the valence-band elementary excitations (phonons, spin waves, orbital transitions, plasmons, etc…). Furthermore, since X-rays have wavelengths of the order of the interatomic spacing, this technique allows accessing their dispersion in momentum space. The availability of fs X-ray pulses at free electron lasers enables performing time-resolved RIXS (trRIXS) experiments. We are interested in using trRIXS both as a spectroscopy tool for the study of extremely low-energy dynamics under weak perturbation, as well as a diagnostic tool for novel light-induced states of matter. Our group performed pioneering soft X-ray trRIXS experiments on driven high-Tc superconductors and will continue developing this spectroscopy at X-ray free electron laser sources.