Resolving Transient Electron–Phonon Coupling with Time-Resolved Spontaneous Raman Spectroscopy

Abstract Understanding the interaction of charge carriers with lattice vibrations in the quasi-equilibrium regime is crucial for semiconductor functionality. However, the structural signatures of these interactions are often too subtle for conventional ultrafast techniques to detect. We developed a time-resolved spontaneous Raman technique based on time-correlated single-photon counting to track the spectral response following photoexcitation, providing subwavenumber spectral resolution and a subnanosecond temporal resolution. Unlike traditional pump–probe schemes, our method utilizes a modulated continuous-wave probe to maintain high spectral resolution, enabling detection of low-frequency Raman shifts down to 10 cm–1. Applied to lightly boron-doped silicon, we resolve intra-valence band and inter-valence band electronic transitions. A coupled-mode analysis of transient phonon asymmetry, resulting from interference with the intervalence band transitions, reveals electron–phonon coupling parameters that directly relate to carrier recombination. By capturing these subtle dynamical shifts, we demonstrate that this platform offers a powerful probe for investigating electron–phonon interactions in long-lived excited states.