Ultrafast Tunable Broadband Optical Anisotropy in Two-Dimensional Re S2

Anisotropic two-dimensional materials provide promising platforms for polarization-driven optoelectronic and photonic devices. In particular, layered rhenium disulfide (ReS2) has recently attracted intensive attention due to its unique linear dichroism. However, control over the optical properties of ReS2 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 ReS2. 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.