Observation of entangled electron-zone boundary phonon states with transient spectroscopic ellipsometry

Kurt Hingerl

Silicon has three optical phonons. These phonons remain inaccessible with linear optics. Here we demonstrate that time-resolved pump-probe spectroscopic ellipsometry enables the detection of optical phonon responses at both the BZ center and -edge. Using pump pulses with photon energies below the indirect bandgap of silicon, we leverage two-photon absorption to induce sub-bandgap excitation. Transient optical effects were probed in the 1.9-3.6 eV spectral range with pump-probe time delays from 50 fs to 4.5 ns. We observed distinct features: a structure at the E1 critical point persisting for 4.5 ns; longitudinal optical phonons with an energy spacing of 57±9 meV, lasting approximately 300 fs; and two-phonon replicas, exhibiting a spacing of 81±7 meV.