Nonlinear Optical Activities in Two-Dimensional Gallium Sulfide: A Comprehensive Study

TLDR

Nonlinear optical properties of two‑dimensional materials hold great promise for physics and device applications, yet few studies have comprehensively mapped the combined NLO activities of a single 2D material. The study aims to characterize multiple NLO properties of 2D gallium sulfide, including second‑harmonic generation, two‑photon excited fluorescence, and NLO absorption. The authors probe SHG, TPEF, and saturable absorption in 2D GaS across varying layer numbers and wavelengths to assess its nonlinear behavior. Layer‑dependent SHG reveals noncentrosymmetric odd layers with a χ^(2) of 47.98 pm/V for trilayers, while structural deformation and liquid‑phase exfoliation enhance SHG, and saturable absorption measurements show large nonlinear coefficients (β ≈ −9.3 × 10^3 to −6.05 × 10^3 cm/GW) and modulation depths up to 35.3 % at 1066 nm, indicating 2D GaS’s promise for future nonlinear optoelectronic devices.

Abstract

The nonlinear optical (NLO) properties of two-dimensional (2D) materials are fascinating for fundamental physics and optoelectronic device development. However, relatively few investigations have been conducted to establish the combined NLO activities of a 2D material. Herein, a study of numerous NLO properties of 2D gallium sulfide (GaS), including second-harmonic generation (SHG), two-photon excited fluorescence (TPEF), and NLO absorption are presented. The layer-dependent SHG response of 2D GaS identifies the noncentrosymmetric nature of the odd layers, and the second-order susceptibility (χ2) value of 47.98 pm/V (three-layers of GaS) indicates the superior efficiency of the SHG signal. In addition, structural deformation induces the symmetry breaking and facilitates the SHG in the bulk samples, whereas a possible efficient symmetry breaking in the liquid-phase exfoliated samples results in an enhancement of the SHG signal, providing prospective fields of investigation for researchers. The generation of TPEF from 800 to 860 nm depicts the two-photon absorption characteristics of 2D GaS material. Moreover, the saturable absorption characteristics of 2D GaS are realized from the largest nonlinear absorption coefficient (β) of -9.3 × 103, -91.0 × 103, and -6.05 × 103 cm/GW and giant modulation depths (Ts) of 24.4%, 35.3%, and 29.1% at three different wavelengths of 800, 1066, and 1560 nm, respectively. Hence, such NLO activities indicate that 2D GaS material can facilitate in the technical advancements of future nonlinear optoelectronic devices.