Gigantic second-harmonic generation and bulk photovoltaic effect in two-dimensional selenium and tellurium

Few-layer selenium and tellurium films have been recently prepared, and they provide a new platform to explore novel properties of two-dimensional (2D) elemental materials. In this work, we have performed a systematic first-principles study on the electronic, linear and nonlinear optical (NLO) properties of atomically thin selenium and tellurium films within the density-functional theory with the generalized gradient approximation. Interestingly, we find that few-layer Se and Te possess large second-harmonic generation (SHG), linear electro-optic (LEO) effect and bulk photovoltaic effect. In particular, trilayer (TL) Te possesses large SHG coefficient, being more than 60 times larger than that of GaN, a widely used NLO material. Bilayer (BL) Te has huge static SHG coefficient, being more than 100 times larger than that of GaN. Furthermore, monolayer (ML) Se possesses large SHG coefficient being six times larger than that of GaN. Both ML Se and BL Te possess large linear electro-optic coefficients rxyy(0) and ryzx(0), which is about 6 times and 5 times larger than that of bulk GaN polytypes, this http URL addition, we predict that TL Te exhibits huge shift photocurrent with NLO conductivity of about 220 uA/V2, being greater than that of GeS, a polar system with the largest bulk photovoltaic effect (BPVE) found so far, and hence its maximal Glass coefficient exceeds 10-6cm/V, being the largest Glass coefficient reported for 2D noncentrosymmetric materials. Finally, an analysis of the calculated electronic band structures indicates that the strong NLO responses of 2D Se and Te materials are primarily derived from their quasi-one-dimensional structures with high anisotropy, directional covalent bonding, lone-pair electrons and band gap. These findings provide a practical strategy to search for excellent NLO and BPVE materials .