Saturable absorber mirrors based on layered platinum diselenide

Saturable absorber mirrors (SAMs) are key devices in passive mode-locked lasers. Two-dimensional (2D) materials, with outstanding nonlinear optical (NLO) properties such as saturable absorption, show great promise for next-generation SAMs. Here, we deposited layered platinum diselenide (PtSe₂) thin films of various thicknesses directly onto gold mirrors to form 2D SAMs and study their thickness-dependent ultrafast carrier dynamics, as well as NLO properties. Time-resolved differential reflectance spectra, with a 1.55 eV excitation pulse, revealed that the recombination of photoinduced electron-hole pairs in PtSe₂-SAMs occurred on the order of hundreds of picoseconds following excitation. Tunable NLO modulation depth was demonstrated, ranging from ∼3.40% to ∼13.38% for the SAMs with initially deposited Pt film thicknesses of 0.5 nm, 1 nm, 2 nm, and 3 nm, respectively. A reflective geometry pump-probe technique was further adapted to perform a nondestructive study of the uniformity of the PtSe₂-SAMs, confirming a low standard deviation of NLO response across the SAM-mirror surface. These results offer not only profound insights into ultrafast and NLO response in PtSe₂-SAMs, but also promising applications for 2D material-based photonic devices in mode-locked lasers.