Special Presentation: Takayoshi Kobayashi

    Thursday, October 23, 2014 - 2:00pm - 3:00pm
    Meinel 547

    "Development of Ultrafast Laser and Application to Ultrafast Spectroscopy of Novel Materials"


    We have developed a sub-three-femtosecond visible pulse laser and sub-10-femtosecond ultraviolet- and deep-ultraviolet-pulse lasers and applied them to the study of the mechanism of ultrafast dynamics in carbon nanotubes, topological insulators and DNA bases. Coherent phonon dynamics of single-walled carbon nanotubes with several chiralities were investigated by sub-five-femtosecond pump-probe experiments. Dynamics of vibrational wave-packets corresponding to the radial breathing mode and gravity mode found in the time traces of modulated absorbance change due to RBMs observed for three chiral systems. Coherent phonon generation of RBMs was studied by analyzing the probe photon energy-dependent amplitude profiles, which indicates that the Raman gain-loss process induced by coherent phonons due to the imaginary part of the third-order susceptibility and spectral shift represented by the first-derivative of absorption spectrum due to the real part of the susceptibility. This work is in collaboration with Z. Nie.

    An ultrafast optical pump mid-infrared probe was used to explore the dynamics of Dirac fermions near the Dirac point. The Dirac fermion phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was approximately one millielectron volt per picosecond. These results are crucial to the design of Dirac fermion devices. This work is in collaboration with C.H. Luo in National Chiao Tung University (Taiwan).

    Single 9.6-femtosecond DUV pulses with a spectral range of 255-290 nanometers are generated by a chirped-pulse four-wave mixing technique for use as pump and probe pulses to study DNA bases, which have unusually short excited-state lifetimes. Electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of thymine over the full spectral range using a 128-channel lock-in detector. Vibrational modes of the electronic ground state and excited states can be observed as well as the decay dynamics of the electronic excited state. From the moment analyses we could conclude the coherent coupling between pi-pi* and n-pi* are relaxing during well-known ultrafast relaxation of allowed state form the ground state. This work is in collaboration with Xue Bong.

    Speaker Bio(s): 

    Takayoshi Kobayashi works with the Advanced Ultrafast Laser Research Center at the University of Electro-Communications in Tokyo, Japan, and the Advanced Ultrafast Laser Research Center at National Chiao Tung University in Hsinchu, Taiwan.