Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

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 E-2 and A(1)(LO) modes, as well as the 2A(1)(LO) overtone are observed. Doping of acceptor and donor atoms enhances the dephasing of the polar A(1)(LO) phonon via coupling with plasmons, with the effect of donors being stronger. Doping also enhances the relative amplitude of the coherent A(1)(LO) phonon with respect to that of the high-frequency E-2 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.

DOI： 10.1088/0953-8984/25/20/205404

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Language：English   Publishing type：Research paper (international conference proceedings)  

We have used a femtosecond time-resolved core-level surface PES system based on the 92-eV harmonic source to study the surface carrier dynamics that induces the transient SPV on semiconductor surfaces. We clarified the temporal evolution of the transient SPV characterized by the time of the photo-generated carrier separation and recombination. This result demonstrates the potential of this technique for clarifying the initial stage of the surface carrier dynamics after photoexcitation. © Owned by the authors, published by EDP Sciences, 2013.

DOI： 10.1051/epjconf/20134104017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

By considering analytical expressions for the self-energies of intervalley and intravalley phonons in graphene, we describe the behavior of D, 2D, and D' Raman bands with changes in doping (mu) and light-excitation energy (E-L). Comparing the self-energy with the observed mu dependence of the 2D bandwidth, we estimate the wave vector q of the constituent intervalley phonon at (h) over bar vq similar or equal to E-L/1.6 (v is the electron's Fermi velocity) and conclude that the self-energy makes a major contribution (60%) to the dispersive behavior of the D and 2D bands. The estimate of q is based on a concept of shifted Dirac cones in which the resonance decay of a phonon satisfying q >omega/v (omega is the phonon frequency) into an electron-hole pair is suppressed when mu < (<(h)over bar>vq - h omega)/2. We highlight the fact that the decay of an intervalley (and intravalley longitudinal optical) phonon with q = omega/v is strongly suppressed by electron-phonon coupling at an arbitrary mu. This feature is in contrast with the divergent behavior of an intravalley transverse optical phonon, which bears a close similarity to the polarization function relevant to plasmons.

DOI： 10.1103/PhysRevB.86.201403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

We have developed a time-resolved surface photoelectron spectroscopy system based on a 1.5 eV laser-pump and 92 eV high-order harmonic probe configuration that enables us to measure the electron and hole dynamics on a semiconductor surface. Core-level photoelectron spectra of GaAs(001) were successfully obtained with a single 59th harmonic, which was selected by using a pair of Mo/Si multilayer mirrors with a narrow reflection bandwidth. We observed transient energy shifts of the Ga 3d core level photoelectron peak of approximately 200 meV towards a higher binding energy, indicating that the surface potential changed due to the spatial separation of electron--hole pairs generated by the laser irradiation. The relatively slow recovery of the Ga 3d core level shift with a time constant of a few ns is dominated by the recombination process of the excess surface charges generated by the laser irradiation.

DOI： 10.1143/JJAP.51.072401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

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 A(1)-symmetry longitudinal optical (A(1)-LO) phonon mode. In the n-type SiC, the A(1)-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 E-2-symmetry transverse optical and A(1)-LO modes in both the SI and n-type 4H-SiC. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4725479]

DOI： 10.1063/1.4725479

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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.

DOI： 10.1103/PhysRevB.85.075437

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DOI： 10.1007/978-3-642-28948-4_62

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Other Link： http://orcid.org/0000-0003-4643-7614

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Matrix elements of electron-light interactions for armchair and zigzag graphene nanoribbons are constructed analytically using a tight-binding model. The changes in wave number (Delta n) and pseudospin are the necessary elements if we are to understand the optical selection rule. It is shown that incident light with a specific polarization and energy induces an indirect transition (Delta n = +/- 1), which results in a characteristic peak in the 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.

DOI： 10.1103/PhysRevB.84.085458

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We studied the dephasing dynamics of coherent phonons in n-type, p-type, and intrinsic Si using time-resolved reflectivity measurements with sub-10 fs laser pulses. The dephasing time of the coherent phonons increases (decreases) for n-type (p-type) doping compared with that of intrinsic Si, while the frequencies of the coherent phonons exhibit a redshift for both types of doping. These doping-induced changes in the coherent phonon dynamics are observed when the carrier concentration exceeds 10(19) cm(-3). The doping-type dependent changes in the dephasing time are attributed to the interconduction and intervalence band transitions in n-type and p-type Si, respectively. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3574533]

DOI： 10.1063/1.3574533

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigated the ultrafast dynamics of coherent phonons with sub-10 fs laser pulses in metallic single-walled carbon nanotubes (m-SWCNTs). We identified radial breathing mode (RBM), D-mode, and G-mode coherent phonons in m-SWCNTs. We also resolved G(-)- and G(+)-mode coherent phonons, which correspond to the longitudinal and transverse optical phonon modes, respectively. The frequencies of the RBM coherent phonons shift with increases in laser intensity, suggesting that photocarriers modify the phonon frequency. (C) 2010 American Institute of Physics. [doi:10.1063/1.3491220]

DOI： 10.1063/1.3491220

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Publishing type：Research paper (international conference proceedings)   Publisher：OSA  

DOI： 10.1364/up.2010.tue42

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Other Link： http://orcid.org/0000-0003-4643-7614

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Femtosecond time-resolved reflectivity measurement is performed on YBa2Cu3O7-delta films. Coherent phonons of both the Ba-O and Cu-O modes are observed at frequencies of 3.4 and 4.3 THz, respectively. Amplitudes of both Ba-O and Cu-O modes are optically manipulated by using a pair of femtosecond pulses, the separation time of which is controlled. Coherent phonons of the Ba-O and Cu-O modes are completely suppressed at the double-pulse separation times of 135.0 and 108.5 fs and those amplitudes are enhanced at 270 and 217 fs, respectively. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2009.07.037

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOCIETY APPLIED PHYSICS  

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. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.100205

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DOI： 10.1109/CLEOE-EQEC.2009.5192661

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Other Link： http://orcid.org/0000-0003-4643-7614

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

Sub-10-fs pulses allow real time observation of coherent phonons in aligned bundles of single-walled carbon nanotubes. A complex polarization dependence of the G band phonon amplitude in the transient reflectivity is explained by the superposition of G-band phonons with different symmetries.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

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.

DOI： 10.1021/nl801200p

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/0953-4075/39/4/007

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.cplett.2004.08.123

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Carbon nanotubes have potential to make breakthrough achievement in nano-electronics, photonics, and ultrafast devices. We report real-time observations of coherent phonons in aligned single-walled carbon nanotubes through the time-resolved reflectivity measurements with sub-10 fs laser pulses. In the isotropic reflectivity measurements, we observe the radial breathing modes, G and even D modes, while in the anisotropic reflectivity measurements, only the G mode appears. A complex polarization dependence of the G band phonon amplitude in the isotropic reflectivity indicates the superposition of G band phonons with different symmetries.

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