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.

Thickness Identification of Thin InSe by Optical Microscopy Methods

“…[9,14,15] Therefore, the isolation of a new family of 2D materials by mechanical exfoliation is usually followed by experimental efforts to provide appropriate methods to determine the thickness of the exfoliated flakes. Until now, many optical techniques, such as transmittance/ reflectance spectroscopy, [16,17] photoluminescence (PL), [10,18] optical absorption, [19] Raman spectroscopy, [20] second harmonic generation, as well as optical contrast, [21][22][23][24][25] have been used as fast and nondestructive methods to determine the number of layers of different 2D materials. [7] Indium selenide (InSe), as a novel 2D semiconductor, recently has drawn great research interests because of its interesting nonlinear optical properties, [26,27] superior electrical and optoelectronic device performance, [28][29][30][31] as well as the distinct mechanical features.…”

Investigation of Second- and Third-Harmonic Generation in Few-Layer Gallium Selenide by Multiphoton Microscopy

“…In recent years, the second- and third-order optical nonlinearities in layered materials (including graphene, MoS 2 , WS 2 and h-BN) have gained great interest 22 23 24 25 26 27 28 . After the submission of our work, a study of layer-dependent second-harmonic generation in few-layer gallium selenide at excitation wavelengths from 800 nm to 1080 nm was also reported 29 .…”

2D Layered Materials of Rare‐Earth Er‐Doped MoS2 with NIR‐to‐NIR Down‐ and Up‐Conversion Photoluminescence

“…Formation of 2D forms is promised by the in‐plane stability of monolayers supplied by strong covalent bonds. The study of photoluminescence (PL) properties in the single‐ and few‐layer 2D layers has become an important research focus . For instance, in 2D transition metal dichalcogenides (TMDs) such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 , PL measurements have provided experimental evidences to prove the exotic nature of excitons in TMDs .…”