Large Second Harmonic Generation in Elemental α-Sb and α-Bi Monolayers

Different from the extensively studied polynary compounds for second-order nonlinear optics (NLO), for the first time, we demonstrate that large intrinsic second harmonic generation (SHG) can also exist in two-dimensional (2D) elemental materials such as monolayers α-Sb and α-Bi that have been recently synthesized with large scale and high quality where the Jahn–Teller-like distortions induced by the insufficient sp3 hybridization break the centrosymmetry, thus meeting the requirements for NLO response and leading to huge values of static SHG susceptibilities, which are two orders of magnitude larger than those of hotly studied MoS2 and h-BN. By analyzing the evolution of density matrix, we further show that the major intraband contributions to SHG susceptibilities are from the significant linear dispersions near the band gaps in the two studied systems. Moreover, different from the alternating even–odd behaviors observed in other nonelemental 2D sheets, the SHG response remains in the multilayered α-Sb and α-Bi regardless of their thickness. These findings are of significance in promoting the applications of 2D α-Sb and α-Bi sheets for future NLO devices.