HHG in solids

When matter is irradiated by intense laser pulses (with typical powers of ~ 1012 W/cm2 and amplitudes of ~0.1 V/Å), electrons inside the medium are excited and start accelerating as a response to the external field. The electronic motion within the bands (intraband), and transitions in-between the bands (interband), cause coherent light emission from the material in high frequencies and a broadband spectral range. This emission is called high harmonic generation (HHG), and is a non-perturbative nonlinear optical process; that is, it is a process that inherently cannot be described with perturbation theory. HHG is especially appealing due to its potential to develop new coherent light sources, attosecond pulses, and ultrafast spectroscopy techniques (which won the 2023 Physics Nobel Prize).

The first measurement of non-perturbative HHG in condensed phases was in 2011, and since then the field has been growing with focus both on applied and fundamental aspects. In terms of theory, the physical mechanism that generates harmonics from solids is still under debate, and this is a very active research area with multiple open questions.

We are interested in studying fundamental aspects of solid-state HHG by employing a combination of ab-initio techniques (based on time-dependent density functional theory (TDDFT)), and numerical models. For instance, we study the role of light-dressed Floquet phases of matter in HHG, and are interested in the role of spin-orbit coupling in HHG. We are attempting to develop ultrafast spectroscopy techniques in solids based on HHG – i.e. inferring information about the material (e.g. its symmetry, band structure, topology, etc.) and the ultrafast dynamics (e.g. phase transitions, phonons, currents, etc.) from measured/calculated spectra. We are especially interested in HHG in novel quantum materials such as systems with valley degrees of freedom (TMDs), topological character (topological insulators, Weyl semimetals, Dirac semimetals), and two dimensional materials.