Ultrafast Tunable Broadband Optical Anisotropy in Two-Dimensional ReS2

Anisotropic two-dimensional materials provide promising platforms for polarization-driven optoelectronic and photonic devices. In particular, layered rhenium disulfide (${\mathrm{Re}\mathrm{S}}_{2}$) has recently attracted intensive attention due to its unique linear dichroism. However, control over the optical properties of ${\mathrm{Re}\mathrm{S}}_{2}$ is mostly limited to the band-edge excitons, which significantly limits the application wavelength range. Here, we utilize ultrafast transient absorption microscopy to tune the broadband optical anisotropy of few-layer ${\mathrm{Re}\mathrm{S}}_{2}$. We observe a broad nonexcitonic photoinduced-absorption feature, which exhibits weak anisotropy immediately after pump excitation, but, surprisingly, shows a strong polarization dependence after a few picoseconds, leading to a broadband enhancement in the optical anisotropy. We attribute this to carrier cooling and subsequent anisotropic free-carrier absorption due to anisotropic carrier effective masses. This work provides not only a principle for the ultrafast active control of broadband polarization-sensitive photonic devices but also insights into anisotropic carrier dynamics in two-dimensional materials.