Structural Symmetry-Breaking-Driven Giant Optical Second Harmonic Generation in Janus Monolayer ReSSe

Structural symmetry-breaking plays a crucial role indeterminingthe second harmonic generation (SHG) intensity and pattern for two-dimensional(2D) materials. Herein, we report that the giant optical SHG can beachieved by designing a Janus structure for ReS2 and ReSe2. The designing of a Janus ReSSe monolayer structure can breakboth the in-plane and out-of-plane symmetry of monolayer ReS2 and ReSe2, resulting in the occurrence of larger second-ordernonlinear coefficients in the in-plane (d (11), d (16), d (21), and d (22)) and out-of-plane (d (15) and d (31)) components.The second-order nonlinear coefficient dispersion properties for theasymmetric Janus ReSSe monolayer give rise to a multifaceted dependenceof the SHG on the azimuthal and polarization angles at different incidentwavelengths including double and quadruple symmetries, rotation ofpolar axis, and variations in intensity. These results highlight thepotential to deterministically engineer novel nonlinear optical propertiesin the designing of the Janus structure based on layered materials.