AbstractAn experimental investigation into the nonlinear optical properties of rhenium diselenide (ReSe₂) was conducted at a wavelength of 1.9 μm using the open-aperture and closed-aperture Z-scan techniques for the nonlinear optical coefficient (β) and nonlinear refractive index (n₂) of ReSe₂, respectively. β and n₂ measured at 1.9 μm were ~ − 11.3 × 10³ cm/GW and ~ − 6.2 × 10^–2 cm²/GW, respectively, which to the best of our knowledge, are the first reported measurements for ReSe₂ in the 1.9-μm spectral region. The electronic band structures of both ReSe₂ and its defective structures were also calculated via the Perdew–Becke–Erzenhof functional to better understand their absorption properties. A saturable absorber (SA) was subsequently fabricated to demonstrate the usefulness of ReSe₂ for implementing a practical nonlinear optical device at 1.9 μm. The 1.9-μm SA exhibited a modulation depth of ~ 8% and saturation intensity of ~ 11.4 MW/cm². The successful use of the ReSe₂-based SA for mode-locking of a thulium–holmium (Tm–Ho) co-doped fiber ring cavity was achieved with output pulses of ~ 840 fs at 1927 nm. We believe that the mode-locking was achieved through a hybrid mechanism of saturable absorption and nonlinear polarization rotation.