Dynamical resonance quench and Fano interference in spontaneous Raman scattering from quasiparticle and collective excitations

Time-resolved spontaneous Raman spectroscopy serves as a probe for incoherent quasiparticle and collective excitation dynamics, and allows to distinguish symmetry changes across a photoinduced phase transition through the inelastic light scattering selection rules. Largely unexplored is the role of the Raman resonance enhancement in the time domain, and the transient interaction between scattering from quasiparticles and collective excitations, with the latter interaction leading to a Fano interference. In this work we report on the observation of a phonon Raman tensor quench and Fano interference after strong photoexcitation of an intrinsic semiconductor. We observed a dynamic phonon scattering rate asymmetry and spectral asymmetry through simultaneous detection of both the anti-Stokes and Stokes response. The asymmetric phonon scattering rate is ascribed to the combined effect of the transient phonon population and the reduction of the phonon Raman tensor resulting from the photoexcited hole population. This same hole population results in a strong enhancement of the Fano spectral asymmetry.