Browsing by Author "Kobayashi, Takayoshi"

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18.7-fs DUV pulses generated by using chirped-pulse four-wave mixing in bulk material
(2014)
He, Jinping
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Sub-20fs, wavelength tunable, uJ-level DUV pulses were generated by using nondegenerate, chirped-pulse four-wave mixing in CaF2 crystal. The DUV spectrum can be tuned from 257nm to 277nm with widest spectral width of 16.8nm. The compact DUV laser system is useful in pump-probe experiment for investigating ultrafast dynamics in biological molecules.
A new reaction mechanism of Claisen rearrangement induced by few-optical-cycle pulses: Demonstration of nonthermal chemistry by femtosecond vibrational spectroscopy
(2013)
Iwakura, Izumi
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Yabushita, Atsushi
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Liu, Jun
;
Okamura, Kotaro
;
Kezuka, Satoko
Time-resolved vibration spectroscopy is the only known way to directly observe reaction processes. In this work, we measure time-resolved vibration spectra of the Claisen rearrangement triggered and observed by few-optical-cycle pulses. Changes in molecular structure during the reaction, including its transition states (TSs), are elucidated by observing the transient changes of molecular vibration wavenumbers. We pump samples with visible ultrashort pulses of shorter duration than the molecular vibration period, and with photon energies much lower than the minimum excitation energy of the sample. The results indicate that the "nonthermal Claisen rearrangement" can be triggered by visible few-optical-cycle pulses exciting molecular vibrations in the electronic ground state of the sample, which replaces the typical thermal Claisen rearrangement.
Applications of parametric processes to high-quality multicolour ultrashort pulses, pulse cleaning and CEP stable sub-3fs pulse
(2012-04-14)
Kobayashi, Takayoshi
;
Liu, Jun
;
Okamura, Kotaro
;
電子物理學系
;
Department of Electrophysics
Our recent experimental results of three methods related to and useful for the generation of attosecond pulses are summarized. The pulses obtained by all of them have high qualities in terms of phase, temporal, spectral and spatial properties which are based on the physical principles associated with the parametric processes. First, carrier-envelope phase (CEP) stable sub-5 fs and sub-3 fs pulses by non-collinear optical parametric amplification (NOPA) in the near-infrared and visible spectral range will be described. The mechanism of the passive CEP stabilization is described. Passively stabilized idler and its second harmonic (SH) pulses from NOPAs are compressed to sub-5fs and sub-3fs, respectively. Compression of the idler output from a NOPA and its SH is attained with a specially designed characterization method during the compression. Second, generation of multicolour pulses by the cascaded four-wave mixing process in bulk media is discussed. As short as 15-fs multicoloured femtosecond pulses are obtained with two similar to 40 fs pulses incident to a fused-silica glass plate by this method. These broadband multicolour sidebands are expected to provide single-cycle or sub-fs pulses after the Fourier synthesis. Third, a new technique based on self-diffraction in the Kerr medium is used to clean and shorten the femtosecond laser pulse. The cleaned pulse with high temporal contrast is expected to be used as a seed for a background-free petawatt laser system and then used as the laser source for high-energy attosecond pulse generation in a solid target. The mechanisms of CEP stabilization, pulse spectral smoothening and pulse contrast enhancement are comparatively discussed.
Beamlike photon pairs entangled by a 2 x 2 fiber
(2011-08-01)
Lo, Hsin-Pin
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Yabushita, Atsushi
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Luo, Chih-Wei
;
Chen, Pochung
;
Kobayashi, Takayoshi
Polarization-entangled photon pairs have been widely used as a light source of quantum communication. The polarization-entangled photon pairs are generally obtained at the crossing points of the light cones that are generated from a type-II nonlinear crystal. However, it is hard to pick up the photon pairs coming out from the crossing points because of their invisible wavelength and low intensity. In our previous work, we succeeded in generating polarization-entangled photon pairs by overlapping two light paths for the photon-pair generation. The photon pairs could be entangled in all of the generated photon pairs without clipping the crossing points, even with some difficulty in its alignment to overlap the two light paths. In this paper, we have developed an optical system which generates polarization-entangled photon pairs using a beamlike photon pair, without the difficulty in alignment. The measured results show that the photon pairs generated in the system are entangled in their polarizations.
Beamlike photon-pair generation for two-photon interference and polarization entanglement
(2011-02-14)
Lo, Hsin-Pin
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Yabushita, Atsushi
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Luo, Chih-Wei
;
Chen, Pochung
;
Kobayashi, Takayoshi
Beamlike photon pairs were generated by spontaneous parametric down-conversion using a type-II beta-BaB2O4 crystal. A pump laser generated photon pairs when it transmitted through the crystal and was reflected back into the crystal by a mirror to generate more photon pairs. The photon pairs generated when the pump laser first transmitted through the crystal (first photon pairs) were also reflected back into the crystal to overlap with the light path of the photon pairs generated in the second transmission of the pump laser through the crystal (second photon pairs). We observed interference between the first and second photon pairs modulated with a half period of the wavelength of the photon pairs, which demonstrates two-photon interference using the beamlike photon pairs. The fringe period confirms that the observed interference is not classical interference but quantum two-photon interference. Through rotating the angles of quarter-wave plates in the light paths of the photon pairs, we generated beamlike photon pairs with entangled polarization. The phase between the first and second photon pairs could be tuned by changing the position of mirrors reflecting the pump pulses and photon pairs. The fringes of coincidence counts showed that the beamlike photon pairs have polarization entanglement.
Beat of Frequency Modes with an Artificial Negative Frequency in Spectrogram Analysis
(2010-12-05)
Yabushita, Atsushi
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Kao, Chih-Hsien
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Lee, Yu-Hsien
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Kobayashi, Takayoshi
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電子物理學系
Spectrograms have been used for time-resolved analysis of Fourier power spectra in various fields We have performed spectrogram analysis for two neighboring modes and a low frequency mode changing gate width of the time gate function The obtained spectrogram of the former showed both amplitude and frequency modulations between two neighboring modes A contamination appearing on the spectrogram of the latter reflects an artifact showing beat between the signal mode and a negative frequency mode whose sign was inverted from the signal mode Surprising results from the amplitude and phase modulations were observed in the simulation even in the case of single mode This puzzling result was explained in terms of the interference between the single mode and the "negative" frequency composing the single mode with the corresponding positive frequency"
Biological imaging with nonlinear photothermal microscopy using a compact supercontinuum fiber laser source
(2015-04-20)
He, Jinping
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Miyazaki, Jun
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Wang, Nan
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Tsurui, Hiromichi
;
Kobayashi, Takayoshi
Nonlinear photothermal microscopy is applied in the imaging of biological tissues stained with chlorophyll and hematoxylin. Experimental results show that this type of organic molecules, which absorb light but transform dominant part of the absorbed energy into heat, may be ideal probes for photothermal imaging without photochemical toxicity. Picosecond pump and probe pulses, with central wavelengths of 488 and 632 nm, respectively, are spectrally filtered from a compact supercontinuum fiber laser source. Based on the light source, a compact and sensitive super-resolution imaging system is constructed. Further more, the imaging system is much less affected by thermal blurring than photothermal microscopes with continuous-wave light sources. The spatial resolution of nonlinear photothermal microscopy is similar to 188 nm. It is similar to 23% higher than commonly utilized linear photothermal microscopy experimentally and similar to 43% than conventional optical microscopy theoretically. The nonlinear photothermal imaging technology can be used in the evaluation of biological tissues with high-resolution and contrast. (C) 2015 Optical Society of America
Broadband coherent anti-Stokes Raman scattering light generation in BBO crystal by using two crossing femtosecond laser pulses
(2008-07-01)
Liu, Jun
;
Zhang, Jun
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Kobayashi, Takayoshi
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電子物理學系
;
Department of Electrophysics
As broad as 12000 cm(-1) coherent anti-Stokes Raman scattering (CARS) light from ultraviolet to infrared was generated in a BBO crystal by using two crossing femtosecond laser pulses with 30% conversion efficiency. More than fifteenth-order anti-Stokes and second-order Stokes Raman sidebands were observed with nice Gaussian spatial mode. The effect of the crossing angle between two input beams on the spectrum and emitting angle of the Raman sidebands was studied in detail. Calculation shows that the phase-matching condition determines the frequencies and angles of the sidebands. (C) 2008 Optical Society of America.
Carrier-envelope phase measurement by all-optical poling with a polar side-chain polymer
(2008-12-01)
Okamura, Kotaro
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Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Dependence of all-optical poling efficiency on carrier-envelope phase (CEP) could be measured using photoisomerization of dye molecules which are covalently bound to a polymer main chain and have large difference in static dipole moment between the ground state and excited state. Increased chromophore density leads to an order of magnitude reduction in signal-detection time from a dye doped polymer. Analysis of all-optical poling experiments with CEP changes clearly showed the presence of polarization restoring force to zero polarization. This enables resetting of SH activity in the all-optical poling process to be used for fast response loop of CEP stabilization. Phenomenological model could explain well the difference in the growth-and-decay dynamics of poling between sample of dye doped in polymer studied previously and that grafted to a polymer main chain used in the present paper. (C) 2008 Elsevier B.V. All rights reserved.
Cascaded four-wave mixing and multicolored arrays generation in a sapphire plate by using two crossing beams of femtosecond laser
(2008-12-22)
Liu, Jun
;
Kobayashi, Takayoshi
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電子物理學系
;
Department of Electrophysics
Ultrabroadband signals were found to be generated by cascaded four-wave mixing in a bulk medium by using two crossing femtosecond laser pulses. As many as fifteen spectral upshifted pulses and two spectral downshifted pulses were obtained with spectral bandwidth broader than 1.5 octaves. Bright upshifted pulses were observed on both sides of the two crossing beams. Two dimensional multicolored arrays were observed for the first time in a sapphire plate medium. (c) 2008 Optical Society of America
Cascaded four-wave mixing in transparent bulk media
(2010-03-15)
Liu, Jun
;
Kobayashi, Takayoshi
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電子物理學系
;
Department of Electrophysics
Cascaded four-wave mixing was theoretically and experimentally studied in transparent bulk media. The obtained sidebands show small angle dispersion, but have good beam quality with M(2) factor better than 1.1 in the measurement. The center wavelength of the generated multicolor sideband can be conveniently tuned by replacing the nonlinear medium with one of several different materials. Theoretical analysis and calculation based on the phase-matching condition with the consideration of broadband spectra of the two incident pulses clarified the mechanism of this process. Self-compressed multicolor 20-fs pulses, which were nearly transform-limited, were obtained when one of the two input beams was appropriately negatively chirped and the other was positively chirped. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.
Chirped modulation of molecular vibration in quinoidal thiophene after sub-5 fs excitation
(2006-10-19)
Wang, Zhuan
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Otsubo, Tetsuo
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Kobayashi, Takayoshi
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電子物理學系
;
Department of Electrophysics
Fourier transformation of the real-time trace of sub-5 fs pump-induced molecular vibration in quinoidal thiophene in solution revealed coupled modes to the electronic excitation at 206, 447, and 1466 cm(-1). Spectrogram analysis of 1466 cm(-1) (C=C stretching) mode showed that the amplitude and frequency of the mode are modulated with another low-frequency mode at about 220 cm(-1), which is down chirped being induced by the coupling with modes of even lower frequency, resulting in the irreversible relaxation due to high mode density in the low-frequency range. This Letter thus reports on observation of the modulation of vibrational frequency with a chirped frequency. (c) 2006 Elsevier B.V. All rights reserved.
Chirped molecular vibration in a stilbene derivative in solution
(2007-11-15)
Colonna, Anne
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Yabushita, Atsushi
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Iwakura, Izumi
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Kobayashi, Takayoshi
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電子物理學系
The wave packet dynamics of 4-methoxy-4 '-nitrostilbene in solution was studied by femtosecond pump-probe spectroscopy. Transient changes in absorbance exhibit oscillations, which reflect vibrational motions on the ground-state potential surface. Modification of this oscillatory feature in three different solvents (benzene, methanol, and acetonitrile) can be explained in term of the strength of hydrogen bonding and the stabilization of charge-transfer state, confirmed by steady-state absorption and fluorescence spectra. (c) 2007 Elsevier B.V. All rights reserved.
Circa conservation of vibrational energy among three strongly coupled modes of a cyanine dye molecule studied by quantum-beat spectroscopy with a 7 fs laser
(2008-11-24)
Kobayashi, Takayoshi
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Wang, Ying
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Wang, Zhuan
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Iwakura, Izumi
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電子物理學系
Real-time energy exchange among the three vibrational modes forming a triplet with frequencies omega(1), omega(2), and omega(3) satisfying a 'circa sum-frequency relation', namely omega(1) + omega(2) congruent to omega(3) in a cyanine dye molecule has been studied with a 7-fs pulse laser. There are two triplets with average frequencies of the components of (146, 279, 432 cm (1)) and (279, 432, 738 cm (1)). Even though the circa sum-frequency relation is satisfied, circa- conservation of the vibrational energy holds among the three components only in the latter triplet. The difference between the two is ascribed to the symmetry of the molecular vibrational modes. (C) 2008 Elsevier B. V. All rights reserved.
Classification of dynamic vibronic couplings in vibrational real-time spectra of a thiophene derivative by few-cycle pulses
(2007-12-20)
Kobayashi, Takayoshi
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Wang, Zhuan
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Otsubo, Tetsuo
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電子物理學系
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Department of Electrophysics
Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and band-integrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zeroth-order derivatives due to quasi-pure non-Condon type and first- and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.
Clean sub-8-fs pulses at 400 nm generated by a hollow fiber compressor for ultraviolet ultrafast pump-probe spectroscopy
(2010-09-27)
Liu, Jun
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Okamura, Kotaro
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Kida, Yuichiro
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Teramoto, Takahiro
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Kobayashi, Takayoshi
Clean 7.5 fs pulses at 400 nm with less than 3% energy in tiny satellite pulses were obtained by spectral broadening in a hollow fiber and dispersive compensating using a prism pair together with a deformable mirror system. As an example, this stable and clean pulse was used to study the ultrafast pump-probe spectroscopy of photoactive yellow protein. Moreover, the self-diffraction signal shows a smoothed and broadened laser spectrum and is expected to have a further clean laser pulse, which makes it more useful in the ultrafast pump-probe spectroscopy in the future. (C) 2010 Optical Society of America
Coherent phonon coupled with exciton in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser
(2014-08-01)
Kobayashi, Takayoshi
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Nie, Zhaogang
;
Du, Juan
;
Xue, Bing
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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
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Nie, Zhaogang
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Du, Juan
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Kataura, Hiromichi
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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
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Li, Xinzhong
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Kobayashi, Takayoshi
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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.
Confined breather-type excitation in a quinoidal thiophene after sub-5 fs pulse excitation
(2006-07-28)
Kobayashi, Takayoshi
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Wang, Haibo
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Wang, Zhuan
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Otsubo, Tetsuo
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電子物理學系
A quinoidal thiophene molecule which is a prototype of the repeated unit of s-trans-cis-polyacetylene was studied by sub-5 fs spectroscopy. A breatherlike mode modulation of both the amplitude and frequency of the C = C stretching was observed for the first time. It generates two sidebands of 2238 and 700 cm(-1) with a separation of 769 cm(-1) corresponding to the modulation frequency. The latter mode is very close to the C-S-C ring deformation, which indicate that this mode mediates coupling between the breather mode and the C-beta-C-beta stretching mode. It was also found that the breather-type excitation has a longer lifetime (2-3 ps) than in all-trans-polyacetylene (50 fs) due to its confinement. (c) 2006 American Institute of Physics.