Conventional pump–probe spectroscopy often relies solely on either transmission (ΔT/T0) or reflection (ΔR/R0) measurements and frequently attributes the transient optical response to changes in the extinction coefficient (k) while neglecting the concomitant variations in the real part of the refractive index (n). This simplification could sometimes introduce significant uncertainty or irrationality in explaining ultrafast dynamics. Here, we introduce a generalized methodology that integrates transmission/reflection dual-channel (DC) detection with optical transfer matrix (OTM) analysis, enabling complete and independent extraction of both Δn(t) and Δk(t) dynamics. We validate this DC-OTM approach through systematic studies of gallium arsenide (GaAs) and silicon (Si). In the direct-bandgap GaAs, the traditional analysis holds primarily in the quasi-equilibrium regime but exhibits significant deviations during the initial nonequilibrium stage. More strikingly, in the indirect-bandgap Si, we observe a carrier-density-driven sign reversal in Δk at an 820 nm probe wavelength, a phenomenon invisible to single-channel methods, which we attribute to the competition between band-filling and free-carrier absorption. Furthermore, we demonstrate the existence of scenarios where the pump–probe response is dominated by the changes in the real part of the refractive index (Δn). Our work not only challenges the conventional interpretation framework but also establishes a robust strategy for uncovering complex photophysical processes, particularly in regimes where Δn and Δk are both significant or competing mechanisms coexist.
Wei et al. (Thu,) studied this question.