High-harmonic spectroscopy of the nonadiabatic coupling via Floquet-Bloch states

TL;DR

This paper demonstrates how high-harmonic generation spectroscopy can be used to observe and analyze laser-dressed Floquet-Bloch states at the Brillouin zone edge in MgO, revealing nonadiabatic band couplings.

Contribution

It introduces a novel method to probe nonadiabatic couplings in solid-state systems via HHG spectroscopy of Floquet-Bloch states at the BZ edge.

Findings

HHG spectrum encodes nonadiabatic band couplings

Observation of laser-dressed Floquet-Bloch states at BZ edge

Extension of HHG spectroscopy to BZ tomography

Abstract

Strong laser light drives complex quantum electron dynamics in solids, which is captured by the high-harmonic generation (HHG) process. Here we report the observation of laser-dressed Floquet-Bloch states (FBSs) at the edge of the Brillouin zone (BZ) in HHG, by driving a large-bandgap MgO crystal in the strong-field regime where the laser pulse is intense enough both to accelerate electrons to the BZ edge as well as to dress the bands. We experimentally show and numerically confirm that the HHG spectrum encodes the nonadiabatic coupling between the conduction bands and FBSs, induced by the high-intensity driver, when some high harmonics are resonant with the dressed band structure. This enables us to trace the bandgap morphology - expanding HHG-spectroscopy techniques to cover the HHG tomography of the BZ edge.