Giant Nonlinear Optical Response in a Chiral 2D Semiconductor

Nonlinear optical responses enable energy conversion between frequencies, which is vital to optics and photonics. Recently, 2D materials have shown promise for more tunable and miniature frequency converting devices. However, due to their intrinsically small size, efficiency remains an issue. Here, we report exceptionally large second harmonic generation (SHG) in a novel van der Waals semiconductor, a member of the emerging transition metal phosphorus chalcogenide family. The SH intensity in thin films of this material reaches more than fifty times that of GaAs (a traditional SHG material) in a reflection experiment. We combine experiment and first-principles calculations to investigate the reason for this large response and its enhancement in thin films. We also explore its tunability via strain. Our work advances a new candidate for 2D frequency converting devices and contributes to the understanding of the enhancement of nonlinear optical properties in 2D materials.