Layer‐Dependent Nonlinear Optical Properties and Stability of Non‐Centrosymmetric Modification in Few‐Layer GaSe Sheets

TLDR

Gallium selenide is a key second‑order nonlinear semiconductor, yet its nonlinear properties in two‑dimensional form remain largely unknown. The differing SHG responses arise from reduced stability of non‑centrosymmetric GaSe in thin flakes, a trend confirmed by first‑principles free‑energy calculations. Strong second‑harmonic generation was observed in bilayer and multilayer GaSe sheets, with a quadratic thickness dependence above five layers and a cubic dependence below, marking the first SHG detection in 2D GaSe and also revealing two‑photon excited fluorescence.

Abstract

Abstract Gallium selenide, an important second‐order nonlinear semiconductor, has received much scientific interest. However, the nonlinear properties in its two‐dimensional (2D) form are still unknown. A strong second harmonic generation (SHG) in bilayer and multilayer GaSe sheets is reported. This is also the first observation of SHG on 2D GaSe thin layers. The SHG of multilayer GaSe above five layers shows a quadratic dependence on the thickness; while that of a sheet thinner than five layers shows a cubic dependence. The discrepancy between the two SHG responses is attributed to the weakened stability of non‐centrosymmetric GaSe in the atomically thin flakes where a layer–layer stacking order tends to favor centrosymmetric modification. Importantly, two‐photon excited fluorescence has also been observed in the GaSe sheets. Our free‐energy calculations based on first‐principles methods support the observed nonlinear optical phenomena of the atomically thin layers.