Ken-ichi Hashizume

Onset of exciton-polariton behavior observed in individual fibers of pseudoisocyanine J-aggregates by sub-micron scale reflectance spectroscopy

Abstract We report results on reflection microscopy and local spectroscopy of aggregates of pseudoisocyanine chloride prepared in thin film of polyvinylsulfate. The aggregates assemble into fiber-like structures of relatively high reflectivity of up to 20%. Locally, the fibers show a large variety of reflectance spectra from sharp peaks at the position of the absorption J-band to broad bands with further structures near their maxima. Polarization experiments confirm that transition dipoles responsible for the J-band are oriented along the fibers. A simple theoretical model taking into account polariton-like excitations and finite dimensions of the fibers is used to simulate the experimental results.

Mapping the orientation of exciton transition dipoles along individual nanostructures of molecular J-aggregates

Modulation of the polarization state of incident light in reflection microspectroscopy is used to study locally the orientations of exciton transition dipole moments of fiberlike nanostructures of J-aggregates of pseudoisocyanine dyes. The lowest exciton state assumes a wide range of directions with respect to the long axis of the fibers, from parallel to perpendicular. This behavior is correlated with the onset of polariton-like character of the local reflectance spectra. The results imply that a new structural model has to be conceived to explain the observed phenomena.

Inner structure of intact chloroplasts observed by a low temperature laser scanning microscope

Inner structure of isolated intact chloroplasts was observed for the first time by a method of laser scanning microscopy at the temperature of liquid nitrogen at 77 K. The microscope, based on gradient index optics, has a maximum resolution of 440 nm at the wavelength of 650 nm. Chloroplasts were excited into the Q-band of chlorophyll b by a krypton laser line at 647.6 nm and fluorescence was detected using two different interference filters. The 680 nm interference filter detects the regions where photosystem (PS) 2 mainly occurs, the 730 nm interference filter detects domains with predominant location of PS1. Since PS1 occurs mainly in stroma lamellae, whereas PS2 occurs mainly in grana regions we were able to view the structure of thylakoid membrane in isolated intact chloroplast that is the closest to in vivo state.

Optical properties of individual nanostructures of molecular J-aggregates

Simultaneous atomic force microscope and reflection micro-spectroscopic study of nanostructures of pseudoisocyanine J-aggregates shows morphology related variety of optical properties in the form of exciton- and polariton-like reflectance spectra. Modulation of the polarization state of incident light is used to study locally the absolute orientations of exciton transition dipole moments. The lowest exciton state assumes a wide range of directions with respect to the long axis of the fibers, from parallel to perpendicular. This behavior is correlated with the onset of polariton-like character of the local reflectance spectra.

Micro-spectroscopy of individual nanostructures of molecular aggregates: relationship between morphology and optical properties

Abstract Results on microscopic reflection imaging and spectroscopy of J-aggregates of pseudoisocyanine dyes in thin film matrices of polyvinyl sulfate, polyvinyl alcohol and polystyrene are reported. Despite different aggregation mechanisms in the three polymers, reflection images show formation of similar, highly reflecting fiber-like nanostructures in all samples. Local reflectance spectra reveal large inhomogeneities of local optical properties that are determined by an onset of exciton–polariton character.

Sub-micron scale spectroscopy and microscopy of individual mesoscopic systems at cryogenic temperatures

We report on a design of a laser scanning solid immersion microscope based on gradient index optics that is suitable for low-temperature excitation spectroscopy on the single molecule level. Further, we present a hole-burning study of a system consisting of molecular J-aggregates and associated trap states. These two studies are pre-requisites for microscopic high-resolution excitation spectroscopy performed on individual fibers of J-aggregates of pseudoisocyanine in a polymer film. Preliminary results of the microscopic study are further compared with microscopic reflection spectroscopy measured on identical systems at room temperature.

Sub-micron scale spectroscopy localized on individual fibers of molecular J-aggregates of pseudoisocyanine dyes in thin polymer film

Abstract Local optical properties of molecular J-aggregates on sub-micrometer scale are studied by means of low-temperature high-resolution excitation spectroscopy and by room temperature reflectance microscopy and spectroscopy. Differences in local properties are demonstrated as distributions of absorption line peak positions and line widths

Laser scanning gradient index optics microscope: submicrometre scale spectroscopy and imaging at cryogenic temperatures

A laser scanning far‐field optical microscope for low‐temperature imaging and spectroscopy based on gradient index optics is presented. A rod‐shaped gradient index microlens is used as a zero‐working‐distance solid immersion objective lens. The obtained lateral resolution is 310 nm of the FWHM at a wavelength of 545 nm. A laser scanning mechanism located outside an optical cryostat enables one to achieve large scanning ranges independent of temperature. The use of the microscope for submicrometre‐scale spectroscopy and low‐temperature photochemistry performed on molecular J aggregates in thin polymer films is presented.

Novel Implementations In Mesoscopic-Structured Excitonic Systems: CdSe/ZnS/TOPO Nanocrystals and Molecular J-Aggregate Fibrils

Recent results of two types of mesoscopic-structured excitonic systems are reported. First is the synthesis of CdSe and CdSe/ZnS nanocrystals by pyrolysis of organometallic reagents in coordinating solvents TOPO-TOP and/or TOPO-TOP-HDA mixtures and their optical properties are characterized. Microscopic passivation by ZnS overcoating provides enhancement of their radiative recombination process, i.e. maximum quantum efficiency reaches over 50% for the moment. To increase the range of the sizes of obtained nanocrystals, 2-step injection of Cd/Se precursors seems effective. The second is J-aggregates by PIC-Cl dye molecules prepared in thin PVA polymer films. In specific condition, also in PVA as in ionic PVS, coherent J-segments seem gathered together to form sub-micron width fibril-like structures. Microscopy and spectroscopy of individual fibril structures are described.

Sub‐wavelength region spectroscopy and local morphology of individual mesoscopic quantum systems

Results of reflection microscopy and local reflection spectroscopy of J‐aggregates of two pseudoisocyanine dyes in a thin film polymer matrix are reported. The individual J‐aggregates assemble into fibre‐like shapes of large structural heterogeneity. Reflectance spectra obtained at different samples, different locations on one sample and even at different positions of the same aggregate fibre reveal a wide distribution of optical properties. The shapes and absolute reflectivities of the spectra are suggested to originate from varying strengths of exciton–photon interaction and from the effect of finite thickness of the aggregate fibres. Polarization dependence measurements of local reflectivities provide orientations of the exciton transition dipole moments at 572 nm and 540 nm with respect to the orientation of the aggregate fibres. Furthermore, modified synthesis of capped semiconductor nanocrystals of CdSe is presented. The method yields high quality quantum dots suitable for microscopic imaging and spectroscopy.