title: Novel Electron-Phonon Relaxation Pathway in Graphite Revealed by Time-Resolved Raman Scattering and Angle-Resolved Photoemission Spectroscopy.
abstract: Time dynamics of photoexcited electron-hole pairs is important for a number of technologies, in particular solar cells. We combined ultrafast pump-probe Raman scattering and photoemission to directly follow electron-hole excitations as well as the G-phonon in graphite after an excitation by an intense laser pulse. This phonon is known to couple relatively strongly to electrons. Cross-correlating effective electronic and phonon temperatures places new constraints on model-based fits. The accepted two-temperature model predicts that G-phonon population should start to increase as soon as excited electron-hole pairs are created and that the rate of increase should not depend strongly on the pump fluence. Instead we found that the increase of the G-phonon population occurs with a delay of ~65 fs. This time-delay is also evidenced by the absence of the so-called self-pumping for G phonons. It decreases with increased pump fluence. We show that these observations imply a new relaxation pathway: Instead of hot carriers transferring energy to G-phonons directly, the energy is first transferred to optical phonons near the zone boundary K-points, which then decay into G-phonons via phonon-phonon scattering. Our work demonstrates that phonon-phonon interactions must be included in any calculations of hot carrier relaxation in optical absorbers even when only short timescales are considered.
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Jhih-An
authors.initials: JA
authors.lastname: Yang
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Stephen
authors.initials: S
authors.lastname: Parham
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, CO 80309.|Center For Experiments on Quantum Materials, University of Colorado, Boulder, CO 80309.
authors.forename: Daniel
authors.initials: D
authors.lastname: Dessau
authors.affiliation: Department of Physics, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Dmitry
authors.initials: D
authors.lastname: Reznik