Keiko Kato

Theory of optical transitions in graphene nanoribbons

Matrix elements of electron-light interactions for armchair and zigzag graphene nanoribbons are constructed analytically using a tight-binding model. The changes in wavenumber (

Coherent Phonon Anisotropy in Aligned Single-Walled Carbon Nanotubes

\Delta n

Anisotropy in Ultrafast Carrier and Phonon Dynamics in p-Type Heavily Doped Si

) and pseudospin are the necessary elements if we are to understand the optical selection rule. It is shown that an incident light with a specific polarization and energy, induces an indirect transition (

Pseudospin for Raman D Band in Armchair Graphene Nanoribbons

\Delta n=\pm1

The effect of n- and p-type doping on coherent phonons in GaN

), which results in a characteristic peak in absorption spectra. Such a peak provides evidence that the electron standing wave is formed by multiple reflections at both edges of a ribbon. It is also suggested that the absorption of low-energy light is sensitive to the position of the Fermi energy, direction of light polarization, and irregularities in the edge. The effect of depolarization on the absorption peak is briefly discussed.

Ultrafast carrier and coherent phonon dynamics in semi-insulated and n-type 4H-SiC

By time-resolved reflectivity measurements with sub-10 fs laser pulses at 395 nm, the coherent phonons of aligned bundles of single-walled carbon nanotubes are observed for various polarization directions of the pump and probe pulses. In the isotropic reflectivity measurement, we observe the radial breathing modes, G, and even D modes, while in the anisotropic reflectivity mode, only the G mode appears. A complex polarization dependence of the G band phonon amplitude in the isotropic reflectivity is explained by the superposition of G band phonons with different symmetries.

Time-Resolved Surface Photoelectron Spectroscopy of Photoexcited Electron and Hole Dynamics on GaAs Using 92 eV Laser Harmonic Source

We performed time-resolved reflectivity measurements in p-type heavily doped Si under non-resonant excitation. A large contribution from anisotropic state-filling is observed, indicating that the lowered Fermi energy due to the p-type heavy doping enhances the anisotropy in the hole distribution. The initial phase shift of coherent phonons induced by p-type doping is attributed to the anisotropic hole distribution.

Doping-type dependence of phonon dephasing dynamics in Si

By analytically constructing the matrix elements of an electron-phonon interaction for the D band in the Raman spectra of armchair graphene nanoribbons, we show that pseudospin and momentum conservation result in (i) a D band consisting of two components, (ii) a D band Raman intensity that is enhanced only when the polarizations of the incident and scattered light are parallel to the armchair edge, and (iii) the D band softening/hardening behavior caused by the Kohn anomaly effect is correlated with that of the G band. Several experiments are mentioned that are relevant to these results. It is also suggested that pseudospin is independent of the boundary condition for the phonon mode, while momentum conservation depends on it.

Determination of phonon decay rate in p-type silicon under Fano resonance by measurement of coherent phonons

The effect of doping on the carrier-phonon interaction in wurtzite GaN is investigated by pump-probe reflectivity measurements using 3.1 eV light in near resonance with the fundamental band gap of 3.39 eV. Coherent modulations of the reflectivity due to the E2 and A1(LO) modes, as well as the 2A1(LO) overtone are observed. Doping of acceptor and donor atoms enhances the dephasing of the polar A1(LO) phonon via coupling with plasmons, with the effect of donors being stronger. Doping also enhances the relative amplitude of the coherent A1(LO) phonon with respect to that of the high-frequency E2 phonon, though it does not affect the relative intensity in Raman spectroscopic measurements. We attribute this enhanced coherent amplitude to the transient depletion field screening (TDFS) excitation mechanism, which, in addition to impulsive stimulated Raman scattering (ISRS), contributes to the generation of coherent polar phonons even for sub-band gap excitation. Because the TDFS mechanism requires photoexcitation of carriers, we argue that the interband transition is made possible at a surface with photon energies below the bulk band gap through the Franz-Keldysh effect.

Observation of coherent phonons in metallic carbon nanotubes

We investigated ultrafast carrier and phonon dynamics in semi-insulated (SI) and n-type 4H-SiC using time-resolved reflectivity measurements. In the n-type 4H-SiC, carriers are excited by an inter-conduction band transition, and thermalized by electron-electron scattering within 20 fs. We observed coherent phonons of folded phonon modes, which become Raman active as a result of the zone folding, and those of A1-symmetry longitudinal optical (A1-LO) phonon mode. In the n-type SiC, the A1-LO coherent phonon forms a coupled mode with a plasmon, resulting in an asymmetrically broadened Fourier transform spectrum. The polarization dependence and the sine-type initial phase indicate that the impulsive stimulated Raman scattering is the mechanism generating coherent phonons of E2-symmetry transverse optical and A1-LO modes in both the SI and n-type 4H-SiC.