Thickness-dependent second-harmonic generation and electric-field modulation in layered GaSe and InSe

Two-dimensional van der Waals layered materials combining strong second-order nonlinearity with electrical tunability offer attractive opportunities for integrated nonlinear photonics. Among them, layered III-VI semiconductors such as GaSe and InSe exhibit robust second-order nonlinear responses independent of odd-even layer-number parity. Here, we investigate the thickness-dependent second-harmonic generation (SHG) and electric-field modulation in layered GaSe and InSe. The SHG intensity shows a nonlinear scaling with thickness, enabling quantitative extraction of the effective second-order nonlinear susceptibility, reaching approximately 28.1 pm V^-1 for GaSe and 31.1 pm V^-1 for InSe. Over a thickness range of 10-120 nm, phase-matching effects are minimal while the susceptibility shows a slight decrease with increasing thickness. Electrically tunable SHG with a modulation depth exceeding 20% is demonstrated in field-effect transistor devices, and reversible electro-optic modulation of evanescent-field-coupled optical transmission is achieved in fiber-integrated structures. These results highlight the potential of layered III-VI semiconductors for electrically tunable nonlinear and electro-optic photonic technologies.