Anisotropic Saturable and Excited-State Absorption in Bulk ReS₂

The intensity-scan (I-scan) technique to study the polarization-dependent, nonlinear processes in exfoliated bulk ReS₂ is utilized. The polarization-dependent reflection and transmission of ReS₂, from which the absorption coefficients are extracted using the transfer matrix method, are measured. Absorption coefficients under high laser peak power show a transition from saturable absorption (SA) to reverse saturable absorption when rotating the laser polarization with respect to the b-axis. It is found that SA and excited-state absorption (ESA) contribute to the nonlinear optical processes. Both the SA and ESA show strong dependence on the polarization angle, which is attributed to the anisotropic optical transition probability and electronic band structure in ReS₂. The anisotropic nonlinear optical properties of ReS₂ may find applications as saturable absorbers in lasers and optical modulators.