Strong in-plane anisotropic optical properties of monolayer, few-layer and bulk ReSe$_{2}$*

Recently, there has been growing interest in the anisotropic properties of certain two-dimensional (2D) materials with reduced lattice symmetry, such as black phosphorus, for developing novel applications in nanoelectronics and infrared optoelectronics. In this work, we report the strong anisotropic optical and electronic properties of monolayer, few-layer and bulk ReSe$_{2}$, an emerging member of the 2D transition metal dichalcogenides (TMDCs) family. With its bulk bandgap around 1.1 eV and potentially tunable with layer number and strain, ReSe$_{2}$ may complement black phosphorus for optoelectronic applications utilizing its anisotropic properties in the near-infrared and visible range. Through careful investigations of the polarization-resolved Raman spectroscopy, photoluminescence (PL), polarization-resolved optical extinction spectrum, angle-resolved DC conductance and first principles calculations, we observed and explained the consistent dependence of phonon, optical and electrical properties of ReSe$_{2}$ on its in-plane crystal orientation. Our results reveal the interesting anisotropic properties of 2D ReSe$_{2}$ and shed light on its potential applications in electronics and optoelectronics.