Ultrafast spectroscopy and role of interlayer coupling in high harmonic generation from layered solids

TL;DR

This paper develops a theoretical framework for high harmonic generation in layered solids considering interlayer coupling, revealing how it influences HHG characteristics and proposing it as a probe for interlayer interactions.

Contribution

The authors introduce a perturbation theory for HHG in layered materials that accounts for interlayer coupling effects, validated through numerical simulations.

Findings

Interlayer coupling can significantly modify HHG emission patterns.

HHG yields follow a 4th order polynomial dependence on interlayer coupling.

The theory enables probing interlayer interactions via HHG spectroscopy.

Abstract

High harmonic generation (HHG) in solids has recently emerged as a powerful all-optical approach for probing material properties and ultrafast electron dynamics in quantum systems. It has been widely applied for studying two-dimensional and layered solids of various kinds. In these studies, the laser is usually polarized within the layered planes, where most electron dynamics occurs, while out-of-plane hopping is commonly neglected. This is despite of interlayer hopping being ubiquitous in nano-systems. Here we develop theory for HHG in layered solids in presence of interlayer coupling and employ it for studying strong-field driven hexagonal BN, graphite, and the transition metal dichalcogenide WS$_2$. We show that sufficiently intense couplings can alter typical HHG emission characteristics such as angular or ellipticity dependence even when the driving laser is polarized in-plane. We develop an analytic perturbation theory for the laser-driven current expanded in the interlayer coupling parameter and explicitly show that HHG yields follow a 4'th order polynomial form, which is validated numerically. Our work should motivate experiments for probing interlayer coupling via HHG spectroscopy, as well as exploring its modulation as a control parameter for ultrafast dynamics and attopulse generation via laser driving and mechanical strain.