Optical second harmonic generation in encapsulated single-layer InSe

TL;DR

This paper demonstrates optical second harmonic generation (SHG) in single-layer InSe, using polarization-resolved SHG to accurately determine crystal axes and nonlinear properties, even in encapsulated samples, aiding the fabrication of van der Waals heterostructures.

Contribution

It introduces SHG as a fast, non-invasive method to determine crystal orientation and nonlinear properties in single-layer InSe, including encapsulated samples, with high precision.

Findings

SHG signal >10^3 counts/sec under pulsed excitation.

Polarization-resolved SHG determines crystal axes with ±0.2° accuracy.

Measured second-order nonlinear sheet polarizability |χ^(2)_sheet| = (17.9 ± 11.0)×10^(-20) m^2 V^(-1).

Abstract

We report the observation of optical second harmonic generation (SHG) in single-layer indium selenide (InSe). We measure a second harmonic signal of $>10^3$ $\textrm{cts/s}$ under nonresonant excitation using a home-built confocal microscope and a standard pulsed pico-second laser. We demonstrate that polarization-resolved SHG serves as a fast, non-invasive tool to determine the crystal axes in single-layer InSe and to relate the sharp edges of the flake to the armchair and zigzag edges of the crystal structure. Our experiment determines these angles to an accuracy better than $\pm$ $0.2^{\circ}$. Treating the two-dimensional material as a nonlinear polarizable sheet, we determine a second-order nonlinear sheet polarizability $| \chi_{\textrm{sheet}}^{(2)}|=(17.9 \pm 11.0)\times 10^{-20}$ $\textrm{m}^2 \textrm{V}^{-1}$ for single-layer InSe, corresponding to an effective nonlinear susceptibility value of $| \chi_\textrm{eff}^{(2)}| \approx (223 \pm 138) \times 10^{-12}$ $\textrm{m} \textrm{V}^{-1}$ accounting for the sheet thickness ($\textrm{d} \approx 0.8$ $\textrm{nm}$). We demonstrate that the SHG technique can also be applied to encapsulated samples to probe their crystal orientations. The method is therefore suitable for creating high quality van der Waals heterostructures with control over the crystal directions.