Two-dimensional ReS₂ for nonlinear photonics

The layered transition metal dichalcogenides (TMDs) have been the focus of increasing research in photonics, in part due to their large nonlinear optical (NLO) susceptibilities. Amongst the TMD family, the semiconducting rhenium dichalcogenides (ReX₂, where X = S or Se) are unusual due to their highly-anisotropic in-plane crystal structure. A Peierls distortion of the crystal lattice results in the formation of chains of Re atoms along one of the crystallographic directions. This structure leads to anisotropic optical, electrical and mechanical properties. Here, we demonstrate that this anisotropic crystal structure is highly useful for incorporating such 2D flakes within waveguides. It is shown that ReS₂ flakes preferentially cleave along the a axis, producing flakes with long and narrow dimensions. This allows for waveguide integration with a long interaction length between the propagating light and the NLO material. ReS₂ flakes were exfoliated and characterised using Raman and atomic force microscopy (AFM). Waveguide devices incorporating these flakes were fabricated.