Kazuhiko Misawa

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.

Size effects on luminescence dynamics of CdS microcrystallites embedded in polymer films

Abstract We investigated the size dependence of picosecond dynamics of the band-edge emission from CdS microcrystallites. Excitonic superradiance was observed from CdS microcrystallites with larger diameters than 50 A, while microcrystallites smaller than 35 A yield only shallow-trap emission from surface-trapping states. The superradiant decay rate increases with particle diameter, is nearly constant below a threshold temperature, and decreases above that temperature. The threshold temperatures of microcrystallites of 25 and 29 A in radius are 45 and 30 K, respectively, in good agreement with the calculated radial-compression-mode frequencies of 43.2 and 37.2 K, using the elastic-sphere model.

Generation of a 10.6-THz ultrahigh-repetition-rate train by synthesizing phase-coherent Raman-sidebands.

A train of highly-stable, high-beam-quality ultrashort pulses is successfully produced by synthesizing phase-coherent rotational-Raman-sidebands in parahydrogen. The-intensity-waveform of this ultrashort-pulse-train is directly evaluated in time domain based on a sum-frequency-generation autocorrelation-technique. It is shown that a 10.6-THz ultrahigh-repetition-train of short pulses is formed with an effective-duration of 20 fs and a high peak-power of 2 MW.

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.

ULTRAFAST EXCITON AND EXCITED-EXCITON DYNAMICS IN J-AGGREGATES OF THREE-LEVEL PORPHYRIN MOLECULES

We observed the ultrafast response of exciton (S1-exciton) and excited-exciton (S2-exciton) in one-dimensional J-aggregates of three-level porphyrin molecules by femtosecond pump–probe spectroscopy. The decay profiles of the nonlinear response can be fitted to a sum of instantaneous response and two exponential decay components with time constants of 1.3±0.1 and 40±1 ps. The former and latter were found to correspond to the lifetimes of S2- and S1-excitons, respectively. The origins of the nonlinearity were attributed to the following three contributions: (1) coherent effects between the pump and probe via one-photon virtual S1-exciton, (2) induced absorption of real S2-excitons generated by two photons, and (3) induced absorption of real S1-exciton.

Femtosecond x-ray absorption spectroscopy with hard x-ray free electron laser

We have developed a method of dispersive x-ray absorption spectroscopy with a hard x-ray free electron laser (XFEL), generated by a self-amplified spontaneous emission (SASE) mechanism. A transmission grating was utilized for splitting SASE-XFEL light, which has a relatively large bandwidth (ΔE/E ∼ 5 × 10−3), into several branches. Two primary split beams were introduced into a dispersive spectrometer for measuring signal and reference spectra simultaneously. After normalization, we obtained a Zn K-edge absorption spectrum with a photon-energy range of 210 eV, which is in excellent agreement with that measured by a conventional wavelength-scanning method. From the analysis of the difference spectra, the noise ratio was evaluated to be ∼3 × 10−3, which is sufficiently small to trace minute changes in transient spectra induced by an ultrafast optical laser. This scheme enables us to perform single-shot, high-accuracy x-ray absorption spectroscopy with femtosecond time resolution.

Femtosecond time-resolved X-ray absorption spectroscopy of liquid using a hard X-ray free electron laser in a dual-beam dispersive detection method

We present femtosecond time-resolved X-ray absorption spectroscopy of aqueous solution using a hard x-ray free electron laser (SACLA) and a synchronized Ti:sapphire laser. The instrumental response time is 200 fs, and the repetition rate of measurement is 10 Hz. A cylindrical liquid beam 100 μm in diameter of aqueous ammonium iron(III) oxalate solution is photoexcited at 400 nm, and the transient X-ray absorption spectra are measured in the K-edge region of iron, 7.10 - 7.26 keV, using a dual X-ray beam dispersive detection method. Each of the dual beams has the pulse energy of 1.4 μJ, and pump-induced absorbance change on the order of 10(-3) is successfully detected. The photoexcited iron complex exhibits a red shifted iron K-edge with the appearance time constant of 260 fs. The X-ray absorption difference spectra, with and without the pump pulses, are independent of time delay after 1.5 ps up to 100 ps, indicating that the photoexcited species is long-lived.