Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton-Enhanced Second Harmonic Generation

TL;DR

This study demonstrates nanoscale optical imaging of 2D semiconductor stacking orders using enhanced second harmonic generation, revealing local symmetry and excitonic effects at 20 nm resolution.

Contribution

The paper introduces near-field SHG imaging with 20 nm resolution for 2D materials, enabling local symmetry and stacking order analysis beyond conventional far-field methods.

Findings

Near-field SHG efficiency is enhanced by excitons in 2D semiconductors.

Nanoscale variations in stacking order are resolved in bilayer WSe2.

Stacking order influences excitonic light-matter interactions.

Abstract

Second harmonic generation (SHG) is a nonlinear optical response arising exclusively from broken inversion symmetry in the electric-dipole limit. Recently, SHG has attracted widespread interest as a versatile and noninvasive tool for characterization of crystal symmetry and emerging ferroic or topological orders in quantum materials. However, conventional far-field optics is unable to probe local symmetry at the deep subwavelength scale. Here, we demonstrate near-field SHG imaging of 2D semiconductors and heterostructures with the spatial resolution down to 20 nm using a scattering-type nano-optical apparatus. We show that near-field SHG efficiency is greatly enhanced by excitons in atomically thin transition metal dichalcogenides. Furthermore, by correlating nonlinear and linear scattering-type nano-imaging, we resolve nanoscale variations of interlayer stacking order in bilayer WSe2, and reveal the stacking-tuned excitonic light-matter-interactions. Our work demonstrates nonlinear optical interrogation of crystal symmetry and structure-property relationships at the nanometer length scales relevant to emerging properties in quantum materials.