Browsing by Author "Kobayashi, T."

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A wide range THz pulses generation in ZnTe crystals
(2012)
Ku, S. A.
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Tu, C. -M.
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Luo, C. W.
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Wu, K. H.
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Yabushita, A.
The THz generation in ZnTe crystals by widely varying pumping fluences is reported. Some abnormal nonlinear effects were observed including the evolution of THz spectra and the phase shift of temporal waveforms. Moreover, the THz efficiency can enhance again at high pumping fluence due to the saturation of free carrier.
A widely tunable dual-wavelength CWTi : sapphire laser with collinear output
(2008-03-03)
Luo, C. W.
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Yang, Y. Q.
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Mak, I. T.
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Chang, Y. H.
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Wu, K. H.
We report a collinear output and tunable dual-wavelength CW Ti:sapphire laser with a simple cavity configuration. The wavelength splitting range is easily tuned from 10 nm to 110 nm, which provides 56 THz bandwidth for terahertz generation. The total output power of two colors with the spatial mode of TEM00 is between 700 mW and 300 mW, for small and large wavelength splittings, respectively, under 5 W argon-ion laser pumping. (C) 2008 Optical Society of America.
Anomalous phase behavior and apparent anharmonicity of the pump-probe signal in a two-dimensional harmonic potential system
(2007-11-15)
Taneichi, T.
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Kobayashi, T.
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先進超快雷射研究中心
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Adv Ultrafast Laser Ctr
Discussion on wavelength dependent "anharmonic" effects in a pump-probe signal for a system of wavepacket on one- and two-dimensional harmonic potentials was given. The Fourier power spectrum of the signal, calculated for a model composed of a three-state electronic system coupled to a set of displaced harmonic oscillators, depends on the pulse duration. Condition under which the wavepacket motion in the harmonic potential substantially deviates from that of the classical point mass is derived. The Fourier power spectrum has enhanced components with frequencies of harmonies even in a system composed of ideally harmonic potentials. Utility of the Fourier analysis of the spectrum for clarification of the squeezed molecular vibrational state is discussed. Calculated oscillatory behavior in phase of a pump-probe signal, as a function of probe frequency, was discussed in terms of a two-dimensional effect on a pump-probe signal. (C) 2007 Elsevier B.V. All rights reserved.
B-side Electron Transfer in Bacterial Photosynthetic Reaction Centers Revealed by a Few-Cycle Pulse Laser
(2013-01-01)
Du, J.
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Kobayashi, T.
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Watanabe, K.
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Tamiaki, H.
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電子物理學系
B-side electron transfer in wide-type reaction centers from Rhodobacter sphaeroides 2.4.1 was investigated by broadband real-time vibrational spectroscopy. H-B(-) was found to be generated in less than 50 fs after the excitation of 6.3 fs laser pulse, and its decay lifetime was determined to be similar to 280fs and 1.4ps, which is extremely fast compared with the one occurring in A-side. Considering the ultrafast of the generation time of H-B(-), B-B rather than P was found to be the initial electron donor to the H. Hence, the initial charge-separated state is determined to be BB+HB- state.
Chirped molecular vibration after impulsive Raman excitation in a stilbene derivative molecule in solution
(2007)
Kobayashi, T.
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Colonna, A.
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Yabushita, A.
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Iwakura, I.
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Tokunaga, E.
The vibrational dynamics in 4-methoxy-4\'-nitrostilbene in a condensed phase was studied by fermosecond pump-probe spectroscopy and quantum-chemical calculation. Transient changes in absorbance due to bleaching exhibit oscillations reflecting chirped molecular vibrational on the potential surface.
Coherent acoustic phonons in hexagonal HoMnO(3) probed by femtosecond spectroscopy
(2010)
Luo, C. W.
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Shih, H. C.
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Wu, K. H.
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Lin, J-Y
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Juang, J. Y.
We have observed the coherent acoustic phonons in the hexagonal manganite HoMnO(3) single crystals using wavelength-dependent femtosecond pump-probe spectroscopy. The oscillatory component of the photoinduced transient reflectivity change (Delta R/R) at various pump-probe wavelengths is consistent with the model of strain pulse propagation. Furthermore, the oscillation period of the oscillation is strong temperature- and wavelength-dependent. In particular, the period of all oscillations in Delta R/R for various wavelengths are very similar in the vicinity of the Neel temperature (T(N)), which associates with the appearance of the antiferromagnetic (AFM) long-range ordering.
Controllable subwavelength-ripple and -dot structures on YBa2Cu3O7 induced by ultrashort laser pulses
(2012-11-01)
Luo, C. W.
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Tang, W. T.
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Wang, H. I.
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Liao, L. W.
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Lo, H. P.
The evolution of periodic ripples and dots on an YBa2Cu3O7 surface irradiated by single-beam and dual-beam femtosecond laser pulses is investigated. The density and size of the dots can be controlled by the number of pulses. The spatial period of these ripples does not show a close relation to the pulse number. However, the orientation and geometrical period of the ripples are strongly dependent on the polarization and incident angle of the laser pulses, respectively. The formation processes are also discussed based on the dependence.
Dichotomy of photoinduced quasiparticle on CuO2 planes of YBa2Cu3O7 directly revealed by femtosecond polarization spectroscopy
(2007-08-01)
Luo, C. W.
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Chen, L. Y.
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Lee, Y. H.
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Wu, K. H.
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Juang, J. Y.
The dichotomy of photoinduced quasiparticle dynamics in the anisotropic high T, superconductors (YBa2CU3O7) is revealed by using the orientation-resolved ferntosecond reflection spectroscopy in the specially designed thin films. The transient reflectivity Delta R/R along the ab diagonal and b axis are totally different in the slow relaxation process which relates to the opening of the superconducting gap. In addition, the distribution of the temperature evolution of Delta R/R on the CuO2 planes of YBa2Cu3O7 may indicate the d-wave symmetry of the superconducting gap in high T, superconductors. (c) 2007 American Institute of Physics.
Doping dependence of the ultrafast electronic dynamics of Y(1-x)Pr(x)Ba(2)Cu(3)O(7-delta) thin-film superconductors from femtosecond optical spectroscopy
(2010-09-17)
Luo, C. W.
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Lo, H. P.
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Su, C. H.
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Wu, I. H.
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Chen, Y. -J.
Time-domain spectroscopy, which probes the dynamics of the electronic states near Fermi surface that are associated with superconductivity, has proven to be a powerful method for providing insights into the fundamental nature of both pseudogap and superconducting gap. This study shows that the unique T-x phase diagram with the time-evolving ultrafast dynamics of Y(1-x)Pr(x)Ba(2)Cu(3)O(7-delta) can be used to identify clearly the Nernst, pseudogap, and superconducting regions. All of the orders appear together at a specific delay time, such as 1.2-3 ps, after pumping by an ultrashort pulse. These discoveries suggest that the Nernst effect, the pseudogap and even superconductivity may have the same physical origin.
Doping dependence of the ultrafast electronic dynamics of Y1-xPrxBa2Cu3O7-delta thin-film superconductors from femtosecond optical spectroscopy
(2010-09-17)
Luo, C. W.
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Lo, H. P.
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Su, C. H.
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Wu, I. H.
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Chen, Y. -J.
Time-domain spectroscopy, which probes the dynamics of the electronic states near Fermi surface that are associated with superconductivity, has proven to be a powerful method for providing insights into the fundamental nature of both pseudogap and superconducting gap. This study shows that the unique T-x phase diagram with the time-evolving ultrafast dynamics of Y1-xPrxBa2Cu3O7-delta can be used to identify clearly the Nernst, pseudogap, and superconducting regions. All of the orders appear together at a specific delay time, such as 1.2-3 ps, after pumping by an ultrashort pulse. These discoveries suggest that the Nernst effect, the pseudogap and even superconductivity may have the same physical origin.
Eliminate coherence spike in reflection-type pump-probe measurements
(2009-07-06)
Luo, C. W.
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Wang, Y. T.
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Chen, F. W.
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Shih, H. C.
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Kobayashi, T.
The coherence spike of femtosecond laser pulses in the reflection-type pump-probe measurements has been systematically studied in the semiconductor (100) InP. By varying the setup of the pump-probe measuring system, i.e. the polarizations of pump and probe pulses, the incident angles of pump and probe beams, and the interval of delay time between pump and probe pulses, the dramatic changes in the strength of coherence spike could be clear observed. Furthermore, the proposed methods to remove the coherence spike from the transient reflectivity curves have been demonstrated in the time-domain measurements. (C) 2009 Optical Society of America
Excitonic and vibrational nonlinear processes in a polydiacetylene studied by a few-cycle pulse laser
(2008-06-30)
Kobayashi, T.
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Iwakura, I.
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Yabushita, A.
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電子物理學系
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Department of Electrophysics
High-density excitation of exciton induces the faster decay of absorbance change due to exciton-exciton interaction originated from the Auger recombination process. Experimental results clarified the dynamics of the exciton and the bimolecular quenching rate and the jump rate of exciton were determined to be 5.2 x 10(-9) cm(3) s(-1) and 5.2 x 10(-9) s(-1), respectively. The intensity modulation of electronic transition under high-density excitation with sub-5-fs pulse was analyzed to observe the Fourier power spectrum of the molecular vibration modes and revealed the existence of a combination tone (2726 cm(-1)) of the C-C (1241 cm(-1)) and the C=C (1485 cm(-1)) stretching modes. This indicates that the highly excited state induced by the Auger process results in the fusion of vibrational quantum states. Difference frequency generation of the vibrational modes also takes place in the same process due to the fission of the vibrational quanta stimulated by the interaction of the two vibrational modes. It was also found that the intensities of several prominent peaks corresponding to the modes of C=C and C-C stretching were proportional to the squared intensity of the probe pulse. Furthermore, the frequencies of the modes were shifted to low frequencies at high probe intensity. The process can be considered to be fifth-order cascaded Raman scattering via real excitation of vibronic excitons.
Frequency domain approach for time-resolved pump-probe microscopy using intensity modulated laser diodes
(2014-09-01)
Miyazaki, J.
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Kawasumi, K.
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Kobayashi, T.
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電子物理學系
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Department of Electrophysics
We present a scheme for time-resolved pump-probe microscopy using intensity modulated laser diodes. The modulation frequencies of the pump and probe beams are varied up to 500 MHz with fixed frequency detuning typically set at 15 kHz. The frequency response of the pump-probe signal is detected using a lock-in amplifier referenced at the beat frequency. This frequency domain method is capable of characterizing the nanosecond to picosecond relaxation dynamics of sample species without the use of a high speed detector or a high frequency lock-in amplifier. Furthermore, as the pump-probe signal is based on the nonlinear interaction between the two laser beams and the sample, our scheme provides better spatial resolution than the conventional diffraction-limited optical microscopes. Time-resolved pump-probe imaging of fluorescence beads and aggregates of quantum dots demonstrates that this method is useful for the microscopic analysis of optoelectronic devices. The system is implemented using compact and low-cost laser diodes, and thus has a broad range of applications in the fields of photochemistry, optical physics, and biological imaging. (C) 2014 AIP Publishing LLC.
Magnetization dynamics and the Mn(3+) d-d excitation of hexagonal HoMnO(3) single crystals using wavelength-tunable time-resolved femtosecond spectroscopy
(2009-07-01)
Shih, H. C.
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Lin, T. H.
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Luo, C. W.
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Lin, J. -Y.
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Uen, T. M.
We analyze the photo-induced carrier and magnetization dynamics in antiferromagnetic (AFM) HoMnO(3) single crystals, delineated by wavelength-dependent femtosecond spectroscopy. The emergence of long-range and short-range magnetic order are unambiguously revealed in association with an abnormal blueshift of Mn(3+) 3d level around the Neacuteel temperature and the slope variation in the temperature-dependent transient reflectivity change (Delta R/R) near the Neacuteel transition, respectively. Furthermore, the disturbance (in ps time scale) of the AFM ordering and its reordering time (in a few 100 ps time scale) are clearly identified in the negative Delta R/R..
Magnetization dynamics and the Mn3+ d-d excitation of hexagonal HoMnO3 single crystals using wavelength-tunable time-resolved femtosecond spectroscopy
(2009-07-01)
Shih, H. C.
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Lin, T. H.
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Luo, C. W.
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Lin, J. -Y.
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Uen, T. M.
We analyze the photo-induced carrier and magnetization dynamics in antiferromagnetic (AFM) HoMnO3 single crystals, delineated by wavelength-dependent femtosecond spectroscopy. The emergence of long-range and short-range magnetic order are unambiguously revealed in association with an abnormal blueshift of Mn3+ 3d level around the Neacuteel temperature and the slope variation in the temperature-dependent transient reflectivity change (Delta R/R) near the Neacuteel transition, respectively. Furthermore, the disturbance (in ps time scale) of the AFM ordering and its reordering time (in a few 100 ps time scale) are clearly identified in the negative Delta R/R..
Marked enhancement of Neel temperature in strained YMnO(3) thin films probed by femtosecond spectroscopy
(2011-04-01)
Wu, K. H.
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Chen, H-J
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Chen, Y. T.
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Hsieh, C. C.
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Luo, C. W.
In this paper, the (001)-oriented hexagonal YMnO(3) (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111) and YSZ(111) substrates by pulsed laser deposition. The ultrafast dynamics probed by wavelength-tunable femtosecond pump-probe spectroscopy was performed to disclose the effects of the epitaxial strain on the electronic structure and associated magnetism. Analyses based on the measured transient-reflectivity-change (Delta R/R) curves revealed that while the magnitude of the on-site Mn d-d transition energy E(dd)(T) changes only slightly with the variation of the lattice constant ratio c/a, a marked shift in the antiferromagnetic (AFM) Neel temperature T(N) was observed and the direction of shift was dependent on the type of strain. The possible mechanism for the observed strain effect on the AFM ordering is discussed. Copyright (C) EPLA, 2011
Marked enhancement of Neel temperature in strained YMnO3 thin films probed by femtosecond spectroscopy
(2011-04-01)
Wu, K. H.
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Chen, H-J
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Chen, Y. T.
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Hsieh, C. C.
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Luo, C. W.
In this paper, the (001)-oriented hexagonal YMnO3 (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111) and YSZ(111) substrates by pulsed laser deposition. The ultrafast dynamics probed by wavelength-tunable femtosecond pump-probe spectroscopy was performed to disclose the effects of the epitaxial strain on the electronic structure and associated magnetism. Analyses based on the measured transient-reflectivity-change (Delta R/R) curves revealed that while the magnitude of the on-site Mn d-d transition energy E-dd(T) changes only slightly with the variation of the lattice constant ratio c/a, a marked shift in the antiferromagnetic (AFM) Neel temperature T-N was observed and the direction of shift was dependent on the type of strain. The possible mechanism for the observed strain effect on the AFM ordering is discussed. Copyright (C) EPLA, 2011
Microstructure and magnetic properties of oxidized titanium nitride thin films in situ grown by pulsed laser deposition
(2013-02-20)
Chen, S. C.
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Sung, K. Y.
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Tzeng, W. Y.
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Wu, K. H.
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Juang, J. Y.
Different oxidation states of titanium nitride thin films, including pure TiN(h 0 0), TiN1-xOx(h 0 0), Ti2O3(0 0 l) and pure anatase TiO2(0 0 l), were prepared by pulsed laser deposition with various oxygen pressures (P-O2) using a TiN target. Elaborative evolutions of the crystal and electronic structures of the obtained films were examined systematically by x-ray diffraction and x-ray absorption spectroscopy. We found that the Ti2O3(0 0 l) film, which was prepared at oxygen pressures P-O2 = 10(-4) Torr, exhibited the maximum room temperature ferromagnetism (RTFM) behaviour. The bound magnetic polaron model is used to clarify the origin of RTFM in these films.
Multilayered optical storage in Sm(DBM)(3)Phen-doped and un-doped polymethylmethacrylate by a femtosecond laser
(2011)
Nie, Z. G.
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Jang, W. Y.
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Lim, K. S.
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Lee, M. K.
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Lee, H. Y.
We report on multilayered optical storage in Sm(DBM)(3)Phen-doped and un-doped polymethylmethacrylate read out by fluorescence and reflection modes. The detection of fluorescence signal enables retrieval of the stored bits with a higher signal-to-noise ratio. (C) 2011 Optical Society of America
Multiplex stimulated Raman imaging with white probe-light from a photonic-crystal fibre and with multi-wavelength balanced detection
(2014-08-30)
Seto, K.
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Okuda, Y.
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Tokunaga, E.
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Kobayashi, T.
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電子物理學系
A straightforward method for spectral stimulated Raman scattering (SRS) microscopy is to measure the scanned gain/loss spectrum of a white probe light from a photonic crystal fibre (PCF). However, the intensity of the white light noise is a serious problem for SRS microscopy. In this study, we have demonstrated simultaneous two-wavelength SRS microscopy with PCF through balanced detection suitable for spectroscopy with a modification of an auto-balance scheme. The developed auto-balance detection system suppresses the degradation of noise cancellation performance caused by a sample, and is suitable for spectral SRS imaging with simple and robust optics.