Kunie Ishioka

Ultrafast Electron-Phonon Decoupling in Graphite

We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite interacting with a photoinduced non-equilibrium 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 non-adiabatic electron-phonon interaction by creating the non-equilibrium electron-hole plasma. Time-resolved vibrational spectroscopy provides a window on the ultrafast non-equilibrium electron dynamics.

Coherent optical phonons in diamond

The authors report femtosecond dynamics of the coherent optical phonon of single crystal diamond. Sub-10fs, 395nm laser pulses excite 40THz coherent phonons with an extremely small damping rate (0.15ps−1). Linear power dependence of the phonon amplitude under off-resonant excitation condition gives a direct evidence for an eletric-field-driven generation mechanism. The coherent phonon generation is noticeably suppressed by doping with nitrogen impurities, in spite of their absorption in the near ultraviolet. The present study demonstrates that a simple pump-probe technique can be a powerful tool for evaluating the ultrafast coherent electronic and lattice dynamics of diamond materials.

Dephasing of coherent phonons by lattice defects in bismuth films

We have studied the effect of point defects on coherent optical phonons in ion-implanted bismuth polycrystalline films. Ultrafast dynamics of coherent phonons and photogenerated carriers in the femtosecond time domain have been investigated by means of pump-probe reflectivity measurements. The dephasing rate of the A1g phonon increases linearly with increasing ion dose, which is explained by the additional dephasing process of the coherent phonon originated from scattering of phonons by the defects. Carrier dynamics are also found to be affected by additional scattering process mediated by point defects.

Temperature dependence of coherent A1g and Eg phonons of bismuth

Bismuth has been a model material in the study of femtosecond dynamics of coherent lattice oscillations. The generation mechanism was first proposed to be displacive for the symmetric A1g mode, which was the only mode observed as a coherent phonon. The absence of the other Raman active mode Eg has not been fully explained, but was phenomenologically attributed to the exclusive coupling of the hot electrons at k∼0 and high symmetry phonons. In the present study, we demonstrate that both A1g and Eg modes are excited as coherent phonons at low temperature and confirm that the coherent phonons are generated via a Raman process in bismuth. We found a puzzling π∕2 difference in the initial phases of the two coherent phonons, which suggests that the initial phase cannot be a clear-cut index for the generation mechanism in absorbing media.

Coherent A1g and Eg phonons of antimony

We report the ultrafast dynamics of the coherent A1g and Eg phonons of antimony as a function of temperature and optical polarization. Like in bismuth, the two phonon modes exhibit nearly π/2 difference in their initial phase, suggesting their different coupling strengths with photoexcited electrons. The dependence of the phonon amplitude on the optical polarization and temperature indicates the generation of the coherent A1g phonons through both displacive and Raman processes, rather than a purely displacive one. In contrast, the generation of the coherent Eg phonons can be understood within Raman framework alone.

Ultrafast carrier and phonon dynamics in ion-irradiated graphite

The effect of defects on the dynamics of photoexcited carriers and coherent acoustic phonon in graphite is investigated by means of reflectivity measurements with femtosecond time resolution. Point defects are introduced by irradiating graphite with 5 keV He+ ions. Introduction of the defects enhances the carrier relaxation by opening a decay channel via vacancy states, which competes efficiently with carrier–phonon scattering. The coherent phonon relaxation is also accelerated due to an additional scattering by defects. The linear fluence dependence of the decay rate is understood as scattering of propagating acoustic phonon by single vacancies.

Ab initio calculation of the hydrogen molecule in silicon

Abstract We have calculated potentials of hydrogen molecules in the silicon clusters (Si 10 H 16 ) by the ab initio Hartree-Fock method. A tetrahedral site for the hydrogen molecule is a stable trapping site and the calculated vibrational frequency of the hydrogen molecule is 4470 cm −1 , which agrees resonably with the experimentally obtained frequency of H 2 in silicon crystal.

Coherent Lattice Oscillations in Solids and Their Optical Control

Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50 THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation.

Tight-Binding Molecular Dynamics Study of Hydrogen Molecule Inside Silicon Crystal

Tight-binding molecular dynamics simulations were carried out to investigate the dynamics of a H2 molecule within a silicon crystal using a cluster model. The global minimum of the H2 molecules configuration was found to be at the tetrahedral interstitial site along the direction. This is in good agreement with the results of first-principles quantum calculations. The H2 molecule was trapped at this site up to a temperature of 600 K. At 900 K, the H2 molecule diffused into the silicon crystal through the hexagonal site of the silicon lattice while retaining the H–H bond. These results justify the stability of the H2 molecule inside the silicon crystal and the possibility of diffusion of the H2 molecule in the silicon crystal without dissociation.

Fully symmetric and doubly degenerate coherent phonons in semimetals at low temperature and high excitation : similarities and differences

We report the ultrafast dynamics of simultaneously excited coherent A1g and Eg phonons in semimetals Bi and Sb. At a low level of excitation, the doubly degenerate Eg phonons in pump–probe data occur at low temperature only, and they nearly vanish at room temperature. Their initial phase is shifted by π/2 with respect to that of the fully symmetric A1g phonons indicating that the Eg and A1g phonons are excited predominantly impulsively and displacively, respectively. At a high level of excitation, both phonons display an asymmetric (varying in time) line shape with more spectral weight at low frequencies testifying to quantum interference between the one-phonon state and a continuum of states. Furthermore, above a threshold fluence, these phonons of different symmetry exhibit collapse and revival, albeit with different characteristic times.