High-harmonic generation from artificially stacked 2D crystals

TL;DR

This paper demonstrates layer-dependent high-harmonic generation in artificially stacked 2D TMDCs, revealing how stacking configuration influences harmonic intensity and symmetry, with potential applications in ultrafast probing of layered materials.

Contribution

It introduces the first experimental observation of coherent HHG build-up in artificially stacked 2D crystals with different stacking orders, highlighting the effects of stacking on harmonic generation.

Findings

Quadratic increase of harmonic intensity with layer number in AAAA stacking.

Inversion symmetry suppresses even-order harmonics in AB and ABAB stacking.

HHG can probe high-momentum states and atomic-scale features in layered materials.

Abstract

We report a coherent layer-by-layer high-order harmonic generation (HHG) build-up in artificially stacked transition metal dichalcogenides (TMDC) crystals in their various stacking configurations. In the experiments, millimeter-sized single crystalline monolayers are synthesized using the gold foil-exfoliation method, followed by artificially stacking on a transparent substrate. High-order harmonics up to the 19th order are generated by the interaction with an ultrafast mid-infrared (MIR) driving laser. We find that the generation is sensitive to the number of layers and their relative orientation. For AAAA stacking configuration, both odd- and even-orders exhibit a quadratic increase in intensity as a function of the number of layers, which is a signature of constructive interference of high-harmonic emission from successive layers. Particularly, we observe some deviations from this scaling at photon energies above the bandgap, which is explained by self-absorption effects. For AB and ABAB stacking, even-order harmonics remain below the detection level, consistent with the presence of inversion symmetry. Our study confirms the capability of producing non-perturbative high-order harmonics from stacked layered materials subjected to intense MIR fields without damaging samples. It has implications for optimizing solid-state HHG sources at the nano-scale and developing high-harmonics as an ultrafast probe of artificially stacked layered materials. Because the HHG process is a strong-field driven process, it has the potential to probe high-momentum and energy states in the bandstructure combined with atomic-scale sensitivity in real space, making it an attractive probe of novel material structures such as the Moir\'e pattern.