Browsing by Author "Okamura, Kotaro"

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A new reaction mechanism of Claisen rearrangement induced by few-optical-cycle pulses: Demonstration of nonthermal chemistry by femtosecond vibrational spectroscopy
(2013)
Iwakura, Izumi
;
Yabushita, Atsushi
;
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.
Carrier-envelope phase measurement by all-optical poling with a polar side-chain polymer
(2008-12-01)
Okamura, Kotaro
;
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.
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
;
Okamura, Kotaro
;
Kida, Yuichiro
;
Teramoto, Takahiro
;
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
Determination of multidimensional hyper potential surface configuration in polyacetylene derivative
(2014)
Kobayashi, Takayoshi
;
Iiyama, Tsugumasa
;
Okamura, Kotaro
;
Juan, Du
;
Masuda, Toshio
We have for the first time obtained multi-dimensional Huang Rhys factors, which determine the potential hypersurface structures in complex polymeric system by observing the transition-energy modulations separately among many modes induced by impulsive excitation.
Direct observation of denitrogenation process of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, using a visible 5-fs pulse laser
(2012-02-27)
Abe, Manabu
;
Iwakura, Izumi
;
Yabushita, Atushi
;
Yagi, Shingo
;
Liu, Jun
The mechanism of denitrogenation for 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives was investigated using a visible 5-fs pulse laser. The time-dependent wavenumber changes observed in this study revealed that the concerted denitrogenation occurs in the parent DBH. However, a stepwise denitrogenation mechanism was found for the 7,7-diethoxy-substituted DBH derivative. (C) 2012 Elsevier B. V. All rights reserved.
Direct observation of the change in transient molecular structure of 9,9'-bianthryl using a 10 fs pulse UV laser
(2018-08-17)
Hashimoto, Sena
;
Yabushita, Atsushi
;
Kobayashi, Takayoshi
;
Okamura, Kotaro
;
Iwakura, Izumi
Photoexcitation of 9,9'-bianthryl has been considered to produce a locally excited (LE) state and then a charge transfer (CT) state by intramolecular charge transfer (ICT) in polar solvents. In contrast, the CT state is considered to be unstable in non-polar solvents and triggers a dipole flip-flop between the two anthracene rings. The central C-C bond between the two anthracene rings is considered to twist during relaxation in the LE state and CT state, and this was visualized in the present work using an ultrashort ultraviolet (UV) pulse. The C-C stretching frequency was recovered in a period of ca. 500 fs, which implies that the C-C bond undergoes repeated torsion back and forth in the electronic excited state. Investigation of samples in a polar solvent and a non-polar solvent indicated that ICT does not affect the torsion around the central C-C bond. (C) 2018 Elsevier B.V. All rights reserved.
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 pulses cleaning by transient-grating process Kerr-optical media
(2011-05-10)
Liu, Jun
;
Okamura, Kotaro
;
Kida, Yuichiro
;
Kobayashi, Talcayoshi
;
電子物理學系
We use a transient-grating (TG) process in a Kerr bulk medium to clean a femtosecond laser pulse. Using the technique, the temporal contrast of the generated TO signal is improved by more than two orders of magnitude in comparison with the incident pulse in a 0.5-mm-thick fused silica plate. The laser spectrum is smoothed and broadened, and the pulse duration is shortened simultaneously. We expect to extend this technique to a clean pulse with broadband spectral bandwidth at a wide spectral range because it is a phase-matched process.
Femtosecond pulses cleaning by transient-grating process Kerr-optical media
(2011-05-10)
Liu, Jun
;
Okamura, Kotaro
;
Kida, Yuichiro
;
Kobayashi, Talcayoshi
;
電子物理學系
We use a transient-grating (TG) process in a Kerr bulk medium to clean a femtosecond laser pulse. Using the technique, the temporal contrast of the generated TO signal is improved by more than two orders of magnitude in comparison with the incident pulse in a 0.5-mm-thick fused silica plate. The laser spectrum is smoothed and broadened, and the pulse duration is shortened simultaneously. We expect to extend this technique to a clean pulse with broadband spectral bandwidth at a wide spectral range because it is a phase-matched process.
Octave-spanning carrier-envelope phase stabilized visible pulse with sub-3-fs pulse duration
(2011-01-15)
Okamura, Kotaro
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
The visible second harmonic of the idler output from a noncollinear optical parametric amplifier was compressed using adaptive dispersion control with a deformable mirror. The amplifier was pumped by and seeded in the signal path by a common 400 nm second-harmonic pulse from a Ti:sapphire regenerative amplifier. Thus, both the idler output and the second harmonic of the idler were passively carrier-envelope phase stabilized. The shortest pulse duration achieved was below 3 fs. (C) 2011 Optical Society of America
Output Energy Stabilization of Non-collinear Optical Parametric Amplifier
(2009-07-01)
Okamura, Kotaro
;
Kobayashi, Takayoshi
;
電子物理學系
;
Department of Electrophysics
Active stabilization loops were implimented in a non-collinear optical parametric amplifier (NOPA) to reduce output pulse energy fluctuation. Proportional-integral-derivative feedback stabilizations of pump and seed energy reduced fluctuation of each controlled parameters more than 80% and resulted in more than 60% reduction of the final NOPA output energy fluctuation. This simple stabilization scheme is expected not to affect the capability of NOPA to generate ultrashort pulses, and hence this scheme can be applied to improve signal-to-noise ratio of the data obtained in experiments using NOPA. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.070214
Photo-impulsive reactions in the electronic ground state without electronic excitation: non-photo, non-thermal chemical reactions
(2012)
Iwakura, Izumi
;
Yabushita, Atsushi
;
Liu, Jun
;
Okamura, Kotaro
;
Kobayashi, Takayoshi
Allyl phenyl ether has an absorption band in the ultraviolet region (lambda < 400 nm); therefore, irradiation with few-optical-cycle ultraviolet pulses (lambda = 360-440 nm) causes a transition to the ultraviolet band, which leads to an electronic state and a photo-Claisen rearrangement (radical reaction) in the electronic excited state. However, the reaction scheme of allyl phenyl ether under irradiation with few-optical-cycle visible pulses (lambda = 525-725 nm) was determined to be same as that of the thermal Claisen rearrangement ([3,3]-sigmatropic rearrangement), which is symmetry-allowed in the electronic ground state. Photo-excitation with few-optical cycle visible pulses below the absorption band induces a photo-impulsive reaction in the electronic ground state without electronic excitation, of which the trigger scheme is different from that of photoreaction or thermal-reaction. The photo-impulsive reaction in the electronic ground state is highly possible as a novel reaction scheme.
Sub-10-fs deep-ultraviolet light source with stable power and spectrum
(2012-09-10)
Kida, Yuichiro
;
Okamura, Kotaro
;
Liu, Jun
;
Kobayashi, Takayoshi
;
電子物理學系
In some applications of ultrafast spectroscopy that employ sub-10-fs pulses, the pulse spectrum and power need to be stable for several tens of minutes. In this study, we generate sub-10-fs deep-ultraviolet (DUV) pulses with such stabilities by chirped-pulse four-wave mixing. A power fluctuation of less than 3% rms was realized by employing stabilization schemes that employ a power stabilizer. The pulses generated in this study have been applied to transient absorption spectroscopy in the DUV with a sub-10-fs time resolution [Phys. Chem. Chem. Phys. 14, 6200 (2012).]. This sub-10-fs DUV source has a similar performance to widely used noncollinear optical parametric amplifiers. (c) 2012 Optical Society of America
Temporal contrast enhancement of femtosecond pulses by a self-diffraction process in a bulk Kerr medium
(2010-10-11)
Liu, Jun
;
Okamura, Kotaro
;
Kida, Yuichiro
;
Kobayashi, Takayoshi
;
電子物理學系
We demonstrated for the first time the application of a self-diffraction (SD) process in a bulk Kerr medium to improve the temporal, spectral, and spatial qualities of femtosecond laser pulses. A proof-of-principle experiment succeeded in improving the temporal contrast of a femtosecond pulse by four orders of magnitude even in the picosecond region using a 0.5-mm-thick fused silica glass plate by this technique. The energy conversion efficiency from the incident pulses to the two first-order SD signals is about 12%. By the SD process, a laser pulse with smoother spectral shape, higher beam quality, and shorter pulse duration than those of the input pulse was generated. This technique is expected to be used to design background-free petawatt laser system in the future. (C) 2010 Optical Society of America
Ultrafast electronic relaxation and vibrational dynamics in a polyacetylene derivative
(2013-04-19)
Kobayashi, Takayoshi
;
Iiyama, Tsugumasa
;
Okamura, Kotaro
;
Du, Juan
;
Masuda, Toshio
Real-time vibrational spectra in a polyacetylene derivative, poly[o-TFMPA([o-(trifluoromethyl)phenyl]acetylene)] in a broad electronic spectral region were observed using a sub-7-fs laser. Using the frequencies and initial phases of vibrational modes obtained by the spectroscopy, the assignment of the wavepackets was made. From the first moment, Huang-Rhys parameters were determined for six most prominent modes, which characterize the potential hypersurface composed of multi-dimensional vibrational mode spaces. (C) 2013 Elsevier B. V. All rights reserved.
Vibrational Energy Flow between Modes by Dynamic Mode Coupling in THIATS J-Aggregates
(2013-11-14)
Hasegawa, Daisuke
;
Nakata, Kazuaki
;
Tokunaga, Eiji
;
Okamura, Kotaro
;
Du, Juan
We performed ultrafast pump-probe spectroscopy of J-aggregates of 3,3'-disulfopropyl-5,5'-dichloro-9-ethyl thiacarbocyanine triethylammonium (THIATS), one of the most typical cyanine dyes, and detected excited molecular vibrations, using a sub-10 fs pulse laser. The time-resolved two-dimensional difference absorption (Delta A) spectra are observed between -314 and 1267 fs. By performing the Fourier transform and spectrogram analysis, vibrational modes in THIATS are observed at 285, 485, 555, 824, and 1633 cm(-1) and there was a modulation of the vibrational frequencies around 1633 cm(-1) which depend on the delay time, respectively. By the analysis of the modulation, energy flow is found to take place from other modes to the 1633 cm(-1) mode through the low frequency mode with similar to 50 cm(-1). Also, by fitting the real-time traces of Delta A with the sum of two exponential functions and a constant term, the average lifetimes of three electronically excited states were found to be tau(1) = 52 +/- 5 fs and tau(2) = 540 +/- 78 fs. By performing single-exponential fitting around the stationary absorption peak at 1.990 eV, in the negative time range, the electronic dephasing time, T-2(ele), is determined to be 18.30 fs.