Carrier Dynamics Characterization in Saturable Absorbers via Cascaded Nonlinear Optical Spectroscopy

We demonstrate a novel, to the best of our knowledge, two-stage experimental approach that cascades Z-scan and transient absorption (TA) spectroscopy for comprehensive carrier dynamics characterization in semiconductor saturable absorber mirrors (SESAMs). This approach exploits the complementary nature of steady-state and dynamic measurements to extract complete sets of nonlinear optical parameters across multiple temporal regimes. When applied to a 2-period In 0.25 Ga 0.75 As/GaAs 0.9 P {0.1} quantum wells SESAM sample, our method systematically extracts saturation intensity ( I sat ), modulation depth (Δ R ), and temporal constants ( τ fast , τ slow ) with high precision. The derived absorption cross-sections ( σ gs , σ es ) enable accurate carrier dynamics simulations, revealing bi-exponential recovery behavior with fast intraband thermalization and slow recombination processes. We argue that the cascaded characterization strategy provides a tool for understanding ultra-fast carrier dynamics in saturable absorbers (SAs), bridging steady-state and transient optical properties for comprehensive material characterization.