Time-resolved Raman spectroscopy of optical phonons in graphite: Phonon anharmonic coupling and anomalous stiffening

Abstract

Time-resolved Raman spectroscopy has been applied to probe the anharmonic coupling and electron-phonon interaction of optical phonons in graphite. From the decay of the transient anti-Stokes scattering of the G-mode following ultrafast excitation, we measured a lifetime of 2.2 ± 0.1ps for zone-center optical phonons. We also observed a transient stiffening of G-mode phonons, an effect attributed to the reduction of the electron-phonon coupling for high electronic temperatures.Because of its intimate relation with nanotubes and graphene, graphite has recently attracted renewed attention. The interactions between its fundamental excitations --electron-electron, electron-phonon and phonon-phonon --play crucial roles in determining the basic physical properties of graphite. Ultrafast pump-probe spectroscopy, by permitting us to achieve non-equilibrium conditions, provides a powerful probe of these interactions [1][2][3][4][5]. Analysis of the dynamics indicates that optical phonons play an important or even dominant role in the relaxation of the excited system [3]. Further, an understanding of phonon dynamics and phonon-phonon interactions is crucial in defining high-field transport properties of graphitic materials [6,7].In this Letter, we present measurements of the ultrafast dynamics of phonons in graphite.Through application of femtosecond time-resolved Raman scattering [8,9], we trace the generation of non-equilibrium optical phonons by carrier cooling, their subsequent interaction with electronic excitations, and their decay through anharmonic coupling to lower-energy phonons. The experimental approach permits a direct determination of the absolute phonon mode population and its temporal evolution following femtosecond laser excitation. It complements a recent independent study by Ishioka et al. [10] in which ultrafast reflectivity measurements were used to trace the dynamics of coherent phonons in graphite. In our investigation we have established a broad understanding of the role of phonons in the ultrafast dynamics in graphite. We find that: (1) photo-excited carriers transfer most of their energy to a set of strongly-coupled optical phonons (SCOPs), including the zone-center (G-mode) phonons, 3 within a few hundred femtoseconds. This produces a significant non-equilibrium phonon population. The electronic excitations retain only a minor fraction of the initial excitation energy.(2) The optical phonons cool with a time constant of 2.2 ± 0.1 ps. Energy flows from the SCOPs to lower-energy phonons by anharmonic coupling. This process also cools the coupled electronic excitations in graphite. (3) In the transient regime, the non-equilibrium G-mode

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…They concluded that more than 90% of the initially deposited excitation energy is transferred to a few SCOPs immediately after thermalization; the carrier scattering rate increases from 10 THz at equilibrium to 14 THz shortly after photoexcitation resulted from scattering from hot SCOPs. The slow decay of T e with a time constant of 7 ps was interpreted as the SCOP lifetime, which is compatible with the value of 2 ps from a direct measurement based on the time-resolved Raman spectroscopy (Yan et al, 2009).…”

Cooling of photoexcited carriers in graphene by internal and substrate phonons

“…The acoustic and optical phonon baths can each establish a different temperature upon interactions with the electrons, but processes such as anharmonic phonon-phonon scattering serve to thermalize them on a picosecond time scale 3,[63][64][65] . In this work, we shall assume a common lattice temperature T L , but acknowledge that in experiments with ultrafast pump-probe, this will not hold true.…”

Ultrafast momentum imaging of pseudospin-flip excitations in graphene

“…Hence, we expect to measure a delayed rise and a lower peak signal for excitation with x-polarized light compared to excitation with y-polarized light. We also expect the two curves to overlap once the distribution becomes isotropic, before further cooling by optical and acoustic phonon emission occurs at longer time delays [17][18][19][20][21][22][23][24][25].…”