Layer- and Frequency-Dependent Second Harmonic Generation in Reflection from GaSe Atomic Crystals

TL;DR

This study investigates how the second harmonic generation (SHG) signal in GaSe atomic crystals varies with layer thickness and frequency, revealing optimal SHG at around 20 layers and a reduction in susceptibility in very thin layers due to bandgap changes.

Contribution

It provides the first detailed analysis of layer- and frequency-dependent SHG in GaSe, linking optical response to layer thickness and electronic structure.

Findings

Maximal SHG at ~20 layers, decreasing in thicker samples due to interference.

Second-order susceptibility in layers >7 matches bulk values.

Reduced susceptibility in ≤7 layers due to increased bandgap.

Abstract

We report optical second-harmonic generation (SHG) in reflection from GaSe crystals of 1 to more than 100 layers using a fundamental picosecond pulsed pump at 1.58 eV and a supercontinuum white light pulsed laser with energies ranging from 0.85 to 1.4 eV. The measured reflected SHG signal is maximal in samples of $\sim$20 layers, decreasing in thicker samples as a result of interference. The thickness- and frequency-dependence of the SHG response of samples thicker than $\sim$7 layers can be reproduced by a second-order optical susceptibility that is the same as in bulk samples. For samples $\lesssim$7 layers, the second-order optical susceptibility is reduced compared to that in thicker samples, which is attributed to the expected bandgap increase in mono- and few-layer GaSe.