title: Layer-dependent nonlinear optical properties and stability of non-centrosymmetric modification in few-layer GaSe sheets.
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.
authors.affiliation: Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong (China).
authors.forename: Wenjing
authors.initials: W
authors.lastname: Jie
authors.forename: Xi
authors.initials: X
authors.lastname: Chen
authors.forename: Dian
authors.initials: D
authors.lastname: Li
authors.forename: Lu
authors.initials: L
authors.lastname: Xie
authors.forename: Yeung Yu
authors.initials: YY
authors.lastname: Hui
authors.forename: Shu Ping
authors.initials: SP
authors.lastname: Lau
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Cui
authors.forename: Jianhua
authors.initials: J
authors.lastname: Hao
keywords: fluorescence; gallium selenide; photoluminescence; second harmonic generation; two-dimensional materials