We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite interacting with a photoinduced nonequilibrium electron-hole plasma. Unlike conventional materials, upon photoexcitation the phonon frequency of graphite upshifts, and within a few picoseconds relaxes to the stationary value. Our first-principles density functional calculations demonstrate that the phonon stiffening stems from the light-induced decoupling of the nonadiabatic electron-phonon interaction by creating a nonequilibrium electron-hole plasma. Timeresolved vibrational spectroscopy provides a window on the ultrafast nonquilibrium electron dynamics. Graphite possesses highly anisotropic crystal structure, with strong covalent bonding of atoms within and weak van der Waals bonding between the hexagonal symmetry graphene sheets. The layered lattice structure translates to a quasi-two-dimensional ͑2D͒ electronic structure, in which the electronic bands disperse linearly near the Fermi level ͑E F ͒ and form pointlike Fermi surfaces. The discovery of massless relativistic behavior of quasiparticles at E F of graphene and graphite has aroused great interest in the nature of carrier transport in these materials.1-3 Because of the linear dispersion of the electronic bands in graphene, the quasiparticle mass associated with the charge carrier interaction with the periodic crystalline lattice nearly vanishes, leading to extremely high electron mobilities and unusual halfinteger quantum Hall effect.1,2 Since graphite has a quasi-2D band structure very similar to that of graphene, these electronic properties may be expressed also in graphite.The electron-phonon ͑e-p͒ interaction contributes to the carrier mass near E F and limits the high-field transport through the carrier scattering. The strong e-p interaction in graphite is a distinctive characteristic of ineffective screening of the Coulomb interaction in semimetals. 4,5 It is expressed in the phonon frequency shift by carrier doping 6 and the strong electronic renormalization of the phonon bands ͑Kohn anomalies͒.7 Time-resolved measurements on the optically generated nonthermal electron-hole ͑e-h͒ plasma in graphite provide evidence for the carrier thermalization within 0.5 ps both through electron-electron ͑e-e͒ scattering and optical phonon emission. 8 The nonthermal carriers decay nonuniformly in phase space because of the anisotropic band structure of graphite. 5,9 Quasiparticle correlations in nonthermal plasmas can also be probed from the perspective of the coherent optical phonons. In the present work, through the time-dependent complex self-energy ͑frequency and lifetime͒ of the 47 THz E 2g2 phonon of graphite, we study the transient changes in the e-p coupling induced by the optical perturbation of the nonadiabatic Kohn anomaly.To probe the ultrafast response of the coherent phonons, we perform transient anisotropic reflectivity measurements on a natural single crystal and highly oriented pyrolytic graphite ͑HOPG͒ samples. Because the phonon properties were identical, we repor...

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We have studied the A(1g) coherent phonons in bismuth generated by high fluence ultrashort laser pulses. We observed that the nonlinear regime, where the phonons' oscillation parameters depend on fluence, consists of subregimes with distinct dynamics. Just after entering the nonlinear regime, the phonons become chirped. Increasing the fluence further leads to the emergence of a collapse and revival, which next turns into multiple collapses and revivals. This is explained by the dynamics of a wave packet in an anharmonic potential, where the packet periodically breaks up and reconstitutes in its original form, giving convincing evidence that the phonons are in a quantum state, with no classical analog.

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