Efficient high-harmonic generation in van der Waals ferroelectric NbOI$_2$ crystals

TL;DR

This study demonstrates efficient high-harmonic generation in NbOI$_2$ crystals driven by mid-infrared lasers, revealing strong anisotropic harmonic emission and potential for compact quantum light sources.

Contribution

It systematically investigates HHG in NbOI$_2$, showing high efficiency and anisotropic harmonic responses, and links electronic structure to HHG enhancement.

Findings

High-order harmonics up to the 16th are generated at low intensity.

Harmonics extend from near-infrared to deep-ultraviolet.

Even-order harmonics align with the crystal axis regardless of laser orientation.

Abstract

Layered NbOX$_2$ ($X=\mathrm{Cl,\,Br,\,I}$), a member of the van der Waals ferroelectric family, exhibits intrinsic ferroelectricity and pronounced nonlinear optical responses, making it a promising candidate for integrated nanophotonics applications. While previous studies have emphasized the material's strong second-order nonlinear responses, higher-order nonlinear responses are still mostly unexplored. This work systematically investigates NbOI$_2$ using high harmonic generation (HHG) spectroscopy. Driven by an intense mid-infrared laser field centered at $\sim4~\mu\mathrm{m}$ wavelength, highly anisotropic odd- and even-order harmonics up to the 16th order are generated at a low peak intensity of $0.4~\mathrm{TW\,cm^{-2}}$, extending beyond the material's bandgap. Both bulk and flake forms of NbOI$_2$ display pronounced harmonic emission from the near-infrared to the deep-ultraviolet spectral region, with a notably high overall conversion efficiency compared to other known materials. Polarization-resolved measurements reveal that even-order harmonics remain aligned with the crystal polar axis regardless of the driving-field orientation, whereas odd-order harmonics are dynamically affected. First-principles calculations suggest that the flat valence band associated with Peierls dimerization enhances HHG efficiency through electron correlation. These findings provide fresh perspectives on HHG in van der Waals ferroelectric materials and facilitate the development of compact and tunable quantum light sources.