Browsing by Author "Du, Juan"

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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
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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.
Development of real-time vibrational spectroscopy of molecules in electronic excited states: toward mapping molecular potential energy hypersurfaces
(2011-05-01)
Teramoto, Takahiro
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Du, Juan
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Wang, Zhuan
;
Liu, Jun
;
Tokunaga, Eiji
We have developed an experimental system for real-time vibrational spectroscopy of molecules in a specific electronic excited state. A synchronized UV light source and a few-cycle visible laser pulse were utilized to prepare and investigate the ultrafast dynamics of molecules in the electronic excited states. A multichannel lock-in amplifier detection system operating in a tandem double lock-in detection mode was used to extract the signal correlated only to the UV and visible pump pulses. Real-time vibrational spectroscopy of chrysene in the triplet state and 1', 3'-dihydro-1', 3', 3'-trimethyl-6-nitrospiro [2H-1-benzopyran-2, 2'-(2H)-indole] in a photochromic state was demonstrated. (C) 2011 Optical Society of America
Electronic relaxation and coherent phonon dynamics in semiconducting single-walled carbon nanotubes with several chiralities
(2013-07-15)
Kobayashi, Takayoshi
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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.
Excited-state molecular vibration observed for a probe pulse preceding the pump pulse by real-time optical spectroscopy
(2008-07-18)
Kobayashi, Takayoshi
;
Du, Juan
;
Feng, Wei
;
Yoshino, Katsumi
;
電子物理學系
It is shown experimentally that the absorbance change observed in the "negative" time range, where probe pulse precedes pump pulse in real-time vibrational spectroscopy is induced only by the excited-state wave-packet motion as theoretically expected. Coherent molecular vibration of a polymer in the excited state was observed in the real-time trace without the effect of wave-packet motion in the ground state, which usually makes it difficult to ascribe the signal either to the ground state or to the excited state.
Low-threshold and compact multicolored femtosecond laser generated by using cascaded four-wave mixing in a diamond plate
(2013-03-01)
He, Jinping
;
Du, Juan
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Kobayashi, Takayoshi
;
電子物理學系
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Department of Electrophysics
Low-threshold and compact multicolored laser generated by cascaded four-wave mixing in a diamond plate is demonstrated. The spectra and output energy of the generated sidebands up to 2nd order on the Stokes side and > 12th on the anti-Stokes side covering from near infrared to UV, are shown. The 2-D multicolor arrays were also observed in diamond plate with low threshold and explained briefly. The compact wavelength-tunable multicolored laser setup is promising to be used in spectroscopy experiments, for example multicolor pump-probe experiments. (C) 2012 Elsevier B.V. All rights reserved.
Observation of breather and soliton in a substituted polythiophene with a degenerate ground state
(2011)
Kobayashi, Takayoshi
;
Du, Juan
;
Feng, Wei
;
Yoshino, Katsumi
;
Tretiak, Sergei
The ultrafast dynamics of a unique polythiophene derivative that has a degenerate ground state due to a quantum mechanical resonance was investigated using a sub-5-fs pulse laser. The method allowed us to study the electronic relaxation and vibrational dynamics in completely same conditions at the same time. The dynamics of a breather composed of a dynamic bound state of solitons generated immediately after photoexcitation was time-resolved to reveal coupling between the vibrational modes and the solitons. The C-C and C=C stretching modes were found to be modulated by the breather, whose lifetime was determined to be 30-50 fs. The results of quantum-chemical excited-state molecular dynamics simulation are consistent with experimental results. Our modeling results allow to identify related vibrational normal modes strongly coupled to the electronic degrees of freedom. Moreover, analysis of calculated trajectories of excited state shows appearance of short-lived breather excitation decaying due to intramolecular vibrational energy equilibration on a timescale of hundreds of femtoseconds, which also agrees well with the experimental results. [GRAPHICS] Contour maps of the two-dimensional Fourier power of the vibrational components obtained by spectrogram calculation for real-time data covering 680-690 nm (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Observation of breather excitons and soliton in a substituted polythiophene with a degenerate ground state
(2010-02-01)
Kobayashi, Takayoshi
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Du, Juan
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Feng, Wei
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Yoshino, Katsumi
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Tretiak, Sergei
We investigated the ultrafast dynamics of a unique polythiophene derivative that has a degenerate ground state due to an alternating benzenoid-quinoid resonance. We probed 128 different wavelengths at the same time by using a sub-5-fs pulse laser and a detection system composed of a polychromator and a multichannel lock-in amplifier. The method allowed us to study the electronic relaxation and vibrational dynamics in completely same conditions at the same time. Because the polythiophene derivative has degenerate ground state, solitons are expected to be generated after photoexcitation. The dynamics of a breather composed of a dynamic bound state of solitons generated immediately after photoexcitation was time-resolved to reveal coupling between the vibrational modes and the solitons. The C-C and C=C stretching modes were found to be modulated by the breather, whose lifetime was determined to be 30-50 fs. The results of quantum-chemical excited-state molecular dynamics simulation are consistent with experimental results. Our modeling results allow to identify related vibrational normal modes strongly coupled to the electronic degrees of freedom. Moreover, analysis of calculated trajectories of excited state shows appearance of short-lived breather excitation decaying due to intramolecular vibrational energy equilibration on a time scale of hundreds of femtoseconds, which also agrees well with the experimental results.
Real-time observation of dynamic coupling between the stretching and bending modes in a polythiophene
(2009-10-28)
Du, Juan
;
Kobayashi, Takayoshi
;
電子物理學系
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Department of Electrophysics
By real-time vibrational spectroscopy using 6.3 fs pulses, both instantaneous frequency and amplitude modulated with frequency of 120-130 cm(-1) were observed for the C-C stretching mode with the central frequency of 1343 cm(-1) in a poly-(substituted thiophene). This provided the evidence that a stretching mode dynamically couples with another stretching mode mediated by a low-frequency bending mode. Instantaneous bond-order changed during the bending motion due to the change in the distribution of pi-electrons. The coupling between the stretching and bending modes was found to be maintained for longer than 1200 fs after the start of the coherent molecular vibration. (C) 2009 Elsevier B.V. All rights reserved.
Real-Time Vibrational Dynamics in Chlorophyll a Studied with a Few-Cycle Pulse Laser
(2011-08-17)
Du, Juan
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Teramoto, Takahiro
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Nakata, Kazuaki
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Tokunaga, Eiji
;
Kobayashi, Takayoshi
We use a 6.8-fs laser as the light source for broad-band femtosecond pump-probe real-time vibrational spectroscopy to investigate both electronic relaxation and vibrational dynamics of the Q(y)-band of Chl-a at 293 K. More than 25 vibrational modes coupled to the Q(y) transition are observed. Eleven of them have been clarified predominantly due to the excited state, and six of them are concluded to be nearly exclusively resulting from the ground-state wave-packet motion. Moreover, thanks to the broad-band detection over 5000 cm(-1), the modulated signals due to the excited state vibrational coherence are observed on both sides of the 0-0 transition with equal separation. The corresponding nonlinear process has been studied using a three-level model, from which the probe wavelength dependence of the phase of the periodic modulation can be calculated. The probe wavelength dependence of the vibrational amplitude is interpreted in terms of the interaction between the "pump" or "laser," Stokes, and anti-Stokes field intermediated by the molecular vibrations. In addition, an excited state absorption peak at similar to 709 nm has been observed. To the best of our knowledge, this is the first study of broad-band real-time vibrational spectroscopy in Chl-a.
Simultaneous measurement of electronic and vibrational dynamics to clarify a geometrical relaxation process in a conjugated polymer
(2008-05-01)
Du, Juan
;
Wang, Zhuan
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Feng, Wei
;
Yoshino, Katsumi
;
Kobayashi, Takayoshi
The real-time vibrational spectroscopy was utilized to obtain both electronic and vibrational dynamics in a conjugated polymer under the same excitation and probing condition by using the same sample and observing at the same time. Most of the conjugated polymers are in an amorphous phase, in which there is a relatively broad distribution of chain length, conjugation length, and degree of interaction strength between neighboring chains. Because of the varieties, a simultaneous measurement of electronic and vibrational dynamics in a broad spectral range is considered to be most powerful to study such systems as conjugated polymers. In the present paper, we performed this type of experiment of the simultaneous probing of electronic and vibrational relaxations at 128 wavelengths. The sample studied here is an amino-moiety-containing conjugated polymer, poly{[3-hexylthiophene-2,5-diyl]-[p-dimethylaminobenzylidenequinoidmethene]} (PHTDMABQ), whose monomer is a derivative of a thiophene oligomer. The light source is a few-cycle pulse laser with an ultimate time resolution of 0.2 fs in the visible and near IR ranges. The data containing both electronic relaxation and vibrational dynamics were analyzed to obtain a more reliable relaxation mechanism of the excitations than a combination of individual studies of electronic and vibrational relaxations. By utilizing the ultrafast pump-probe spectroscopic system, we could identify the exciton state with a pi-electron delocalized in benzilidene ring. In the polymer PHTDMABQ, the geometrical relaxation from the free exciton to the exciton polaron takes place in 60-100 fs. This time is close to that observed in several polydiacetylenes previously reported. The C-C single bond stretching frequencies of the free exciton and exciton polaron are about 1300 and 1350 cm(-1). The coherent molecular vibration after the geometrical relaxation is considered to be a kind of reaction induced coherence. This coherent vibration decays with the time constant of about 450 fs.
Spectral modulation observed in Chl-a by ultrafast laser spectroscopy
(2011-11-07)
Du, Juan
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Nakata, Kazuaki
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Jiang, Yongliang
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Tokunaga, Eiji
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Kobayashi, Takayoshi
Broadband real-time dynamic vibronic coupling in Chl-a were experimentally studied using few cycle laser pulses of 6.8fs duration and a 128-channnel lock-in amplifier. Thanks to the extreme temporal resolution benefitting from the ultrashort laser pulse, the real-time modulation of the electronic transition energy induced by the molecular vibrations were calculated by the time dependent first moments of the bleaching band. The transition energy was found to be modulated periodically with the same frequencies of molecular vibration found in the Fourier amplitude spectrum of the difference absorbance real-time traces. This was interpreted to be due to the difference in the effective transition energy associated with the wavepacket motion induced by the equilibrium positions of potential curves between the ground state and the excited state. Using the values, Huang-Rhys factors for several vibrational modes involved in the spectral modulation at the room-temperature have been determined. (C)2011 Optical Society of America
Sub-10 fs spectroscopy of K-TCNQ crystal for observation of intramolecular vibration modulation in melting of the Peierls dimer
(2014-08-14)
Nakata, Kazuaki
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Tokunaga, Eiji
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Du, Juan
;
Xue, Bing
;
Miyazaki, Jun
K-TCNQ crystals show photoinduced phase transition (PIPT) between a Mott insulator and a spin Peierls (SP) phase, having T-c = 395 K (= and the crystal has T-c = 395 K of the PIPT). We report the intra-molecular vibrations in the excited K-TCNQ crystal after impulsive excitation using a sub 9.4 fs pulse laser in the reflection spectrum. The frequencies of the vibrations are close to the Raman frequencies of molecules. The frequencies increase with melting of the SP phase, and then decrease with rearrangement of the dimerization. In addition, the frequencies of the vibrations are time-resolved to be modulated by the intermolecular optical phonon with 510 fs period corresponding to 65 cm(-1). This modulation is associated with the periodical change of coupling strengths of intermolecular charge transfer and/or the dipolar coupling between the counterpart molecules in the neighboring chains.
Surface-Enhanced Impulsive Coherent Vibrational Spectroscopy
(2016-11-04)
Du, Juan
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Harra, Juha
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Virkki, Matti
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Makela, Jyrki M.
;
Leng, Yuxin
Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process. On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in "real time". Here, we combine these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The vibrational modes of the ground and excited states of poly[2-methoxy5-( 2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), spin-coated on a substrate covered with monodisperse silver nanoparticles, are impulsively excited with a sub-10 fs pump pulse and characterized with a delayed broad-band probe pulse. The maximum enhancement in the spectrally and temporally resolved vibrational signatures averaged over the whole sample is about 4.6, while the realtime information about the instantaneous vibrational amplitude together with the initial vibrational phase is preserved. The phase is essential to determine the vibrational contributions from the ground and excited states.
Ultrafast electronic relaxation and vibrational dynamics in a polyacetylene derivative
(2013-04-19)
Kobayashi, Takayoshi
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Iiyama, Tsugumasa
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Okamura, Kotaro
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Du, Juan
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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.
Ultrafast UV laser induced dynamics in dielectric coating materials before laser damage
(2014-01-01)
Du, Juan
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Li, Zehan
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Kobayashi, Takayoshi
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Zhao, Yuanan
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Leng, Yuxin
The laser induced ultrafast dynamics in fused silica under few-cycle UV laser excitation is investigated in the present study. Using sub-10 fs UV laser pulses, we characterize the free carrier dynamics in the fused silica before laser damage. After laser excitation, free carrier in the conductive band is found to turn into self-trapped excitons within about 300fs. It is possible that the trapped exciton will result in the incubation effect under the condition of ultrafast high-frequency pulsed UV laser exposure.
Vibrational and electronic relaxation in MEH-PPV using few cycle pulses
(2013-07-30)
Kobayashi, Takayoshi
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Yamashita, Munenori
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Du, Juan
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Zhang, Jun
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Iwakura, Izumi
The electronic population decay time is determined to be about 500 fs in (poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylenel (MEH-PPV) using 5.7 fs pulses. The ultrafast electronic dephasing time and the self-trapping time of free exciton to form an exciton polaron is determined as 25 +/- 2 fs and 50 +/- 23 fs, respectively. Real-time spectrogram analysis reveals dynamic mode coupling among the high-frequency and low-frequency modes. The energy-ladder descent vibrational process is clarified using the first moment of the difference absorption spectrum. The phase relaxation time is calculated from the widths of the Fourier spectra of several strongly coupled modes. (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
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Nakata, Kazuaki
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Tokunaga, Eiji
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Okamura, Kotaro
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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.