Takayoshi Kobayashi

Femtosecond nonlinear optical dynamics of excitons in J-aggregates

Time-resolved difference absorption spectra of J-aggregates were measured by femtosecond pump-probe spectroscopy. The induced absorption near the J-band at 20 K was assigned to the transitions from n-exciton states to (n + 1)-exciton states (n⩾1). The decay time of the n(⩾2)-exciton states is determined to be about 200 fs. The ultrafast dynamics of the exciton states is explained by a confined exciton model in individual coherent aggregates, which incoherently constitute the whole structural aggregate. The average size of the exciton system defined for each coherent aggregate is estimated to be about 20 molecules.

New fabrication method for highly oriented J aggregates dispersed in polymer films

We present a new fabrication method, called ‘‘vertical spin coating,’’ to prepare highly oriented J aggregates dispersed in polymer films. Linear dichroic spectra of the oriented J aggregate of 1,1’‐diethyl‐2,2’‐quinocyanine bromide PIC‐Br were measured at 5 K. The dichroic ratio at the peak of J band was 5 to 10, dependent on the preparation conditions. Precise measurement of the dichroism at the J band revealed that the J band is composed of two bands with transition dipole moments perpendicular to each other. The films are stable even at room temperature, and have applications as nonlinear optical devices.

Superradiance quenching by confined acoustic phonons in chemically prepared CdS microcrystallites

We report the luminescence spectrum and picosecond dynamics of excitons in CdS microcrystallites embedded in an acrylonitrile–styrene copolymer film by a new method. The intensity ratio of band‐edge emission due to the direct recombination of excitons to red‐shifted emission from the trapped states was much larger than any other samples previously reported. This indicates much higher quality of our microcrystallites. From the comparison of luminescence spectrum of samples prepared by different cadmium salts as raw materials, we propose a ‘‘site–substitution’’ model for the origin of the trapped states. According to the model, the trapped states yielding the red‐shifted emission are impurities substituting the sulfur sites. The band‐edge emission was found to be emitted mainly from the larger microcrystallites among the size distribution. The luminescence lifetime of the band‐edge emission was almost constant at 60 ps below a threshold temperature (45±5 K) and increases nearly proportionally with temperatu...

Giant static dipole moment change on electronic excitation in highly oriented J-aggregates

Abstract A large change in the static dipole moment upon electronic excitation was unexpectedly observed in highly oriented one-dimensional J-aggregates of 1,1′-diethyl-2,2′-quinocyanine bromide (PIC-Br) prepared by a method developed by our group. Difference absorption spectra due to the Kerr effect were induced by changes in a static dipole moment and a polarizability separated by the dual-phase lock-in detection. The change in the static dipole moment associated with the transition from the ground state to the exciton state was measured with an applied ac field ( F ) of 300 Hz and polarization ( e ) parallel and perpendicular to the one-dimensional axis ( d ) of the oriented J-aggregates, and obtained in each configuration to be 18 ( F || d / e || d ), 12 ( F ⊥ d / e || d ), 27 ( F || d / e ⊥ d ), and 14 ( F ⊥ d / e ⊥ d ) D.

Femtosecond Sagnac interferometer for phase spectroscopy

We developed a novel femtosecond Sagnac interferometer that enables us to take single-shot measurements of both the difference transmission and the difference phase spectra. Owing to the excellent stability of the Sagnac interferometer, a difference phase spectrum was obtained with a high phase change sensitivity of ~lambda/100 after a 1000-shot accumulation. The spectral range, wavelength, and time resolution were 530-650 nm, 1 nm, and 200 fs, respectively.

Femtosecond phase spectroscopy by use of frequency-domain interference

We present the principles, experimental procedures, applications, and theoretical analyses of femtosecond phase spectroscopy, which is complementary to femtosecond absorption spectroscopy. In femtosecond phase spectroscopy difference spectra of both phase and transmission are simultaneously measured with a frequency-domain interferometer, which is only slightly modified from the conventional pump–probe method. Femtosecond time-resolved dispersion relations for CdSxSe1−x-doped glass and CS2 are obtained with transform-limited pulses of 60-fs duration and 620-nm center wavelength. The results are theoretically analyzed and are well reproduced by numerical simulations. Although time-resolved data are not expected to satisfy the Kramers–Kronig (K–K) relations, the degree of discrepancy from the K–K relations is more substantial for CS2 than for CdSxSe1−x-doped glass. These results arise from the difference in the linear susceptibility and in the excited-population dynamics. The conditions for which the K–K relations are applicable to time-resolved spectra are obtained theoretically and verified experimentally. It is shown that induced amplitude and phase modulations of the probe pulses cause a deviation from the K–K relations.

Exciton transition energy and temperature dependence of ultrafast relaxation of self-trapped excitons in polydiacetylenes

Abstract Exciton transition energies and temperature dependence of ultrafast relaxation processes have been investigated in polydiacetylene (PDA-(12,8) Langmuir-Blodgett films with several exciton transition energies using femtosecond absorption spectroscopy. The relaxation from self-trapped exciton to the ground state in PDA with 1.88 eV exciton energy is faster than that relaxation in PDA with larger exciton energy. The decay kinetics in PDAs can be explained in terms of tunneling in the configuration space from the self-trapped exciton to the ground state.

Wavelength and polarization dependence of spectral hole-burning efficiency in highly oriented J-aggregates

Abstract From the concentration dependence of the dichroic spectrum of highly oriented J -aggregates as well as the strong dependences of the spectral hole-burning efficiency on the burning wavelength and polarization, a system of cyanine dye aggregates was concluded to have a hierarchic structure of mesoscopic and macroscopic aggregates. The observed low efficiency, by a factor of 10 −3 , on the higher-energy side of the J -band is explained in terms of the inefficient conformational change in a macroscopic aggregate, which has a large enough size to be aligned fluid-dynamically by the centrifugal force in the process of the spin-coating method.

Transport properties of photoexcited carriers in a fibonacci superlattice

Abstract We have investigated the electronic structure and the perpendicular transport properties of photoexcited carriers in a GaAs/AlAs Fibonacci superlattice (SL) with an enlarged well (EW) by means of photoluminescence excitation (PLE) spectroscopy and picosecond luminescence measurements. The PLE spectrum of the SL emission shows the structure corresponding to the subbands obtained in the Fibonacci system by splitting of the level of n =1 in the isolated well. Difference in the PLE spectrum between the SL and EW emission also shows that the only very few carriers generated in the lowest subbands in the Fibonacci SL reach the EW at 4 K, and comparison with results for a random and a periodic SLs suggests that the degree of localization in the Fibonacci SL is intermediate among the three systems. It is also found from the temporal evolution behaviors of the photoluminescence that about a half of the carriers created in higher energy states travel to EW within 50 ps and the phonon-assisted transport of the other half at 50 K in the Fibonacci and the periodic SLs is faster than that in the random SL.

Femtosecond continuum interferometer for transient phase and transmission spectroscopy

We measure difference phase spectra (DPS) over the whole visible spectrum by frequency-domain interferometry (FDI), using chirped femtosecond continuum pulses. The effects of the probe-pulse chirp on time-resolved dispersion relations are studied. Because of the correspondence between time and frequency in the chirp, temporal evolution of the optical Kerr response in CS2 is projected into DPS. In addition, it is found that the chirped continuum shows unexpected frequency shifts owing to induced phase modulation even when the continuum has a flat spectrum. The chirp character can be readily obtained from the projected traces, and the potential application to the single-shot pulse-shape measurement by FDI is discussed. It is shown that the delay-time-corrected spectra satisfy the Kramers–Kronig relations if the continuum has a flat spectrum and does not have higher chirp than the linear chirp but that the distortion caused by the induced modulation of the continuum remains unremoved in the corrected spectra.