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_0.25Ga_0.75As/GaAs_0.9P_{0.1} quantum wells SESAM sample, our method systematically extracts saturation intensity (I_sat), modulation depth (ΔR), 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.