Strong Second-Harmonic Generation in Atomic Layered GaSe

Abstract

Nonlinear effects in two-dimensional (2D) atomic layered materials have recently attracted increasing interest. Phenomena such as nonlinear optical edge response, chiral electroluminescence, and valley and spin currents beyond linear orders have opened up a great opportunity to expand the functionalities and potential applications of 2D materials. Here we report the first observation of strong optical second-harmonic generation (SHG) in monolayer GaSe under nonresonant excitation and emission condition. Our experiments show that the nonresonant SHG intensity of GaSe is the strongest among all the 2D atomic crystals measured up to day. At the excitation wavelength of 1600 nm, the SHG signal from monolayer GaSe is around 1-2 orders of magnitude larger than that from monolayer MoS2 under the same excitation power. Such a strong nonlinear signal facilitates the use of polarization-dependent SHG intensity and SHG mapping to investigate the symmetry properties of this material: the monolayer GaSe shows 3-fold lattice symmetry with an intrinsic correspondence to its geometric triangular shape in our growth condition; whereas the bilayer GaSe exhibits two dominant stacking orders: AA and AB stacking. The correlation between the stacking orders and the interlayer twist angles in GaSe bilayer indicates that different triangular GaSe atomic layers have the same dominant edge configuration. Our results provide a route toward exploring the structural information and the possibility to observe other nonlinear effects in GaSe atomic layers.

Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion

“…Bulk GaSe has a direct band-gap of~2.1 eV and has been widely investigated for its superior second harmonic generation and photoresponse properties. 4,5 In the monolayer limit, GaSe exhibits a direct-to-indirect band gap transition, and therefore shows an opposite trend to the thickness dependence in most transition metal dichalcogenides. 6,7 As the number of layers is decreased below seven layers, the shape of the valence band close to its maximum at the Γ point undergoes a change and develops a Mexican hat or caldera-like inverted plateau 6,8,9 as shown in Fig.…”

Mechanical properties of monolayer GaS and GaSe crystals

“…Li et al studied the interlayer orientations and stackings of 2D bilayer GaSe crystals and reported that the rotational energy barriers increase as the size of the system increases [21]. Moreover, Zhou et al studied the second-harmonic generation in layered GaSe crystals and found that the strongest second-harmonic generation intensity is observed for GaSe among all 2D crystals [22].…”

Oxidation dynamics of ultrathin GaSe probed through Raman spectroscopy

“…Its bulk band gap of 2.12 eV 2,11 is predicted to increase to more than 3.5 eV for the monolayer 12,13 . Atomically thin flakes and devices have shown a high photoresponse 9,14 , a composition tunable band gap 15 , near-unity optical polarization 16 , the strongest second-harmonic generation observed in a monolayer 2D material 17,18 , and transistors with high ON/OFF ratios 19 . Like most 2D materials, the properties of GaSe are expected to sensitively depend on the interaction of its surface with its chemical environment.…”