Browsing by Author "Nie, Zhaogang"

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Coherent phonon coupled with exciton in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser
(2014-08-01)
Kobayashi, Takayoshi
;
Nie, Zhaogang
;
Du, Juan
;
Xue, Bing
;
電子物理學系
The vibrational wavepackets dynamics of single-walled carbon nanotubes (SWCNTs) are studied through the modulation of the transition probability in the visible spectral range of the systems. The modulations corresponding to the radial breathing mode (RBM), observed in the time traces for the four chiral systems (6,4), (6,5), (7,5), and (8,3), have been analyzed. The vibrational modes of the coherent phonon spectra are identified from the two-dimensional distribution of probe photon energy versus Fourier frequency. The present study pointed out that the observed probe photon energy dependence is due to both the imaginary and real parts of the third-order susceptibility, corresponding to derivative type dependence of the absorbed photon energy spectrum due to molecular-phase modulation, Raman (and Raman-like) gain and loss processes, and molecular phase modulation, respectively. (C) 2014 Elsevier B.V. All rights reserved.
Coherent phonon generation in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser
(2013-01-01)
Kobayashi, Takayoshi
;
Nie, Zhaogang
;
Du, Juan
;
Kataura, Hiromichi
;
Sakakibara, Youichi
Coherent phonon dynamics in single-walled carbon nanotubes with several chiralities is investigated by 7.1-fs pump-probe experiments. Vibrational wave-packets corresponding to the radial breathing mode (RBM) and G mode can be detected for four chiral systems, (6,4), (6,5), (7,5) and (8,3). Coherent phonon generation of RBMs is in-depth studied by analyzing the probe photon energy dependent amplitude profiles, which indicates that the real and imaginary parts of the third-order susceptibility can both contribute to the modulation of the probed difference absorbance. (c) 2011 Elsevier B.V. All rights reserved.
Coherent phonon in single-walled carbon nanotubes by using a few-cycle pulse laser
(2011)
Nie, Zhaogang
;
Li, Xinzhong
;
Kobayashi, Takayoshi
;
電機學院
;
College of Electrical and Computer Engineering
We studied coherent phonon dynamics in Co-Mo-catalyst grown single-walled carbon nanotubes by using sub-5-fs visible pulses. Vibrational wave-packets corresponding to radial breathing mode for four chiral systems, (6,4), (6,5), (7,5) and (8,3), can be separately observed and achieved exact chirality assignments without ambiguity in coherent phonon spectra. Coherent phonon generation of radial breathing modes is in-depth studied by analyzing the probe energy dependent amplitude profiles, which indicates that the real and imaginary parts of the third-order susceptibility can both contribute to the modulation of the probed difference absorbance.
Electronic relaxation and coherent phonon dynamics in semiconducting single-walled carbon nanotubes with several chiralities
(2013-07-15)
Kobayashi, Takayoshi
;
Nie, Zhaogang
;
Du, Juan
;
Okamura, Kotaro
;
Kataura, Hiromichi
Coherent phonon (CP) dynamics and electronic relaxation in single-walled carbon nanotubes (SWNTs) are investigated in femtosecond pump-probe experiments. Using a sensitive multichannel lock-in amplifier, chirality-specific electronic relaxation and vibrational dynamics are resolved in SWNT ensembles composed of several chiral systems without the need for selective isolation of the different species by purification. The dynamics of vibrational wave packets are studied based on oscillatory changes in the absorbance of the systems. Modulations corresponding to the radial breathing mode (RBM), observed in the time traces of the absorbance change for the four chiral systems (6,4), (6,5), (7,5), and (8,3), have been analyzed in detail. The vibrational modes of the CP spectra are identified from the two-dimensional distribution of probe photon energy versus Fourier frequency. Resonance conditions and mode frequencies lead to definite chirality assignments. Coherent RBM phonon generation is analyzed using the probe photon energy-dependent amplitude profiles as a result of the spectral shift induced by wave-packet motion on the potential surface. The present study clarifies that the observed probe photon energy dependence is due to both the imaginary and real parts of the third-order susceptibility, corresponding to Raman (and Raman-like) gain and loss processes and to molecular phase modulation, respectively. The imaginary part is the dominant contribution to the modulation in the difference absorbance. It shows probe photon energy dependence in the form of a difference in absorbed photon energy between the spectra that are shifted and unshifted with vibrational frequency. The size of the Huang-Rhys factors from the difference fitting to the (6,4), (6,5), and (8,3) systems are 0.26, 0.32, and 0.75, respectively. The trend of the factors originates in the stiffness differences of the SWNT structures. The real part depends on the derivative of the absorbed photon energy spectrum due to cross-phase modulation, resulting from the change in refractive index during the molecular vibrations. This process induces a probe spectral change, as evidenced by first-derivative fitting using a small number of data points of probe photon energies. The effective nonlinear refractive index for each chiral system is determined to range from 0.2 to 3.1 x 10(-17) cm(2)/W.
Femtosecond laser induced photoluminescence in poly(methyl methacrylate) and three-dimensional optical storage
(2011-02-01)
Nie, Zhaogang
;
Lim, Ki-Soo
;
Lee, Heungyeol
;
Lee, Myeongkyu
;
Kabayashi, Takayoshi
We report on a femtosecond-laser induced photoluminescence (PL) in poly(methyl methacrylate) and its potential application to three-dimensional optical storage Irradiation with a focused 800 nm 1 kHz 100 fs pulsed laser induced a strong PL change in UV-visible region Absorption spectra and Fourier-transform Infrared spectra before and after laser irradiation indicate the PL may result from the emissive oxidized products of photo-degradation reaction of PMMA This makes it possible to read out the stored data by detecting the PL change The pulse energy threshold of the light-induced PL change of PMMA is found to be at similar to 2 mu J/pulse and the optimal recording energy is similar to 3 mu J/pulse A ten-layer pattern inside the bulk sample recorded by tightly focusing a pulsed laser beam was read out by a reflection-type fluorescent confocal microscope which detected the emission in visible range as the signal High-contract fluorescent images with a much higher signal-to-noise ratio were obtained without crosstalk in comparison with the ordinary reflection mode (C) 2010 Elsevier B V All rights reserved
Real-Time Spectroscopy of Single-Walled Carbon Nanotubes for Negative Time Delays by Using a Few-Cycle Pulse Laser
(2014-02-13)
Kobayashi, Takayoshi
;
Nie, Zhaogang
;
Xue, Bing
;
Kataura, Hiromichi
;
Sakakibara, Youichi
Several investigations using the femtosecond pump-probe technique have been made for coherent phonon dynamics in single-walled carbon nanotubes (SWNTs), which are promising candidates as devices for nanoelectronics and photonics due to their unique properties. However, the most relevant spectroscopic analyses by far have been performed in the positive time range, with the pump pulse coming before the probe. We studied the real-time coherent phonon in SWNTs using 7.1-fs pump-probe measurements for negative time delays in which vibronic coherence between the ground state and exciton state can be observed. Coherent vibrations corresponding to the radial breathing mode caused by the wave packet generated in the exciton state were detected, in contrast to the ground-state wave packet dynamics for positive delays previously reported. The present results show that excited-state wave packets contribute greatly to radial breathing modes in four, (6,5), (6,4), (7,5), and (8,3), chiral systems, but very little to G modes, even though the oscillation period of the latter is 20 fs, which is much shorter than the 100 fs of the exciton life. We discussed the differences in dephasing times in the four different chiral systems.