Tetsuhiko Nagahara

Near-field optical imaging of plasmon modes in gold nanorods

We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calculated local density-of-states maps and were attributed to spatial characteristics of plasmon modes inside the nanorods. Dispersion relation for plasmons in gold nanorods was obtained by plotting the resonance frequencies of the plasmon modes versus the wave vectors obtained from the transmission images.

Photoluminescence from gold nanoplates induced by near-field two-photon absorption

We have investigated two-photon-induced photoluminescence (PL) properties of single gold nanoplates by using an apertured scanning near-field optical microscope. The nanoplates show PL much stronger than nanorods. The near-field PL images show characteristic spatial features, which depend on the incident polarization. These PL images are in good agreement with the calculated spatial distribution of the electric fields adjacent to the particles at the excitation wavelength. We attribute the observed images to spatial characteristics of plasmon modes.

Time-resolved scanning near-field optical microscopy with supercontinuum light pulses generated in microstructure fiber

A novel apparatus for time-resolved near-field optical microscopy is described. The apparatus consists of a mode-locked Ti:sapphire laser, a microstructure fiber, and a scanning near-field optical microscope equipped with an apertured optical fiber probe. The probe pulses in visible to near-infrared regions are generated by focusing laser pulses in the microstructure fiber. The broadband continuum can be used as a wavelength-tunable light source for fluorescence excitation as well as for probing absorption of excited states at arbitrary wavelengths by applying pump-probe scheme, in high spatial-resolution (∼100 nm) predominantly determined by the aperture size of the tip. Time resolution obtained with 740–830 nm probe pulses was in 1–2 ps range, while that with 570 nm pulses was 5 ps without precompensation of the group delay dispersion for the probe pulses. Results on the excited-state dynamics of molecular aggregates are presented.

Chiral sum frequency spectroscopy of thin films of porphyrin J-aggregates.

Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering.

Pulse compression based on coherent molecular motion induced by transient stimulated Raman scattering

A novel method for compressing a laser pulse, using a combination of transient stimulated Raman scattering and a pump-probe technique, is proposed. The approach does not require a short laser pulse, in contrast to a reported method based on impulsive stimulated Raman scattering. The observed spectrum was sufficiently broad to generate a sub-10 fs pulse. In fact, a 100-fs pulse in the near-ultraviolet region was compressed to the sub-30 fs. Further compression of the laser pulse would be achieved by compensating for phase distortion, as suggested from the observed data of the spectral phase.

Self-Assembled Monolayers of Double-Chain Disulfides of Adenine on Au: An IR-UV Sum-Frequency Generation Spectroscopic Study

We have synthesized double-chain disulfides of adenine with different chain lengths (n = 2, 4, 5, 9, and 10) and studied their self-assembled monolayers (SAM) on gold surface by IR-UV doubly resonant sum frequency generation spectroscopy with the help of DFT calculation. A versatile way to investigate the orientation angle of functional groups of SAMs and their surface coverage has been demonstrated. It was revealed that the IR dipoles of the band at around 1630 cm(-1), which were almost parallel to the long molecular axis of the adenine ring, were less tilted with respect to the substrate surface in the SAMs with longer chains (n = 9 and 10) in comparison to those with shorter chains (n = 2, 4, and 5).

Excited-state dynamics of naphthoylnaphthvalene: valence isomerization of the lowest excited singlet state and intermolecular hydrogen-atom abstraction of the lowest excited triplet state

Abstract By measurements of phosphorescence and transient absorption spectra as well as steady-state photolysis of the title compound (NNV), it is concluded as follows: (1) In benzene and ethanol, intersystem crossing from the lowest excited singlet ( 1 NNV ∗ ) to triplet ( 3 NNV ∗ ) states competes withvalence isomerization of 1 NNV ∗ yielding ground-state naphthoylnaphthalene (NN). (2) In ethanol, the NNV ketyl radical (generated by hydrogen-atom abstraction of 3 NNV ∗ from the solvent molecule) also undergoes rapid valence isomerization and recombination of two NN ketyl radicals thus formed yields 1,1,2,2-tetranaphthyl-1,2-ethanediol (naphthopinacol).

Excited-state dynamics for interpretation of the solvent-dependent photochemistry of 9,10-dichloroanthracene in the presence of 2,5-dimethylhexa-2,4-diene

Abstract In connection with quenching of the lowest excited singlet state of 9,10-dichloroanthracene (DCA) by ground-state 2,5-dimethylhexa-2,4-diene (DMHD), measurements of the fluorescence spectra reveal formation of an exciplex ((DCA–DMHD) * ) irrespective of the dielectric constant ( e ) of a solvent. By means of subpicosecond and nanosecond laser photolysis as well as steady-state photolysis, however, the following results are obtained: (a) in acetonitrile, methanol or acetone, decomposition of (DCA–DMHD) * generates the DCA radical anion (DCA − ) as an intermediate for the formation of 9-chloroanthracene (CA), but its rate ( v ) decreases in the order of v (acetonitrile, e =37.5)> v (acetone, e =20.7)> v (methanol, e =32.7); (b) in 1-octanol ( e =10.3), diethyl ether ( e =4.34) or heptane ( e =1.92), decomposition of (DCA–DMHD) * generates an open-form transient (OFT, i.e. a ground-state singlet biradical or its zwitterionic form (ZIF)) as an intermediate for formation of the [4+2] adduct; (c) in ethanol ( e =24.6) or 2-propanol ( e =19.9), not only OFT (major) but also DCA − (minor) are generated from (DCA–DMHD) * and formation of two compounds (the [4+2] adduct and CA) in ethanol (or only the former compound in 2-propanol) can be seen. We thus conclude that not only the dielectric constant of a solvent but also its hydrogen-bonding ability affects the photochemistry of DCA originating from (DCA–DMHD) * .

Laser-induced shockwave chromatography: a separation and analysis method for nanometer-sized particles and molecules.

A microscopic chromatography has been developed where nanometer-size molecules or particles are separated according to their size by the laser-induced shockwave in a water-filled capillary. As the shockwave passed through the mixture of molecules/particles in solution, they move to the direction of the propagation of the shockwave. The distance from the point of shockwave generation depends on the particle size or molecular weight. This technique has some advantages compared to conventional chromatography, in terms of quick analysis of molecular weight and applicability to sticky and adsorbing polymers. Experimental results obtained for proteins, their aggregates, and inorganic nanoparticles are presented.

Femtosecond Time-Resolved Fluorescence Spectroscopy Utilizing Optical Kerr Shutter: Direct Observation of a Temporal Fluorescence Band Shape

A novel femtosecond fluorescence spectrum measurement system using optical Kerr shutter has been developed. Wide range of 400nm wavelengths are obtained at once. Temporal changes in fluorescence spectra of a styryl dye DASPI are demonstrated.