Tatsuo Nakahama

Intermolecular Coupling Enhancement of the Molecular Hyperpolarizability in Multichromophoric Dipolar Dendrons

Nonlinear optical dendritic macromolecules, called azobenzene dendrons, were synthesized, and their conformational properties and molecular nonlinear optical properties were clarified by second-order nonlinear optical measurement. The synthesized molecules were modified by introducing 1−15 numbers of azobenzene branching units as the nonlinear optical chromophore and by placing aliphatic chains at the end the dendritic chains. In these topologically complex molecules, each chromophore contributed coherently to the macroscopic nonlinear optical activity. The first-order molecular hyperpolarizability of the azobenzene dendron having 15 chromophoric units was measured to be 3010 × 10-30 esu using the hyper-Rayleigh scattering method. This level of molecular hyperpolarizability was much higher than that for an azobenzene monomer (150 × 10-30 esu). The polarized nonlinear optical measurement provided structural information on the dendrons and indicated that each chromophore was oriented noncentrosymmetrically ...

Supernarrowing mirrorless laser emission in dendrimer-doped polymer waveguides

A supernarrowing laser spectrum is observed for mirrorless polymer waveguide lasers containing rhodamine B cored dendrimers. The moderate scattering from the aggregated dendrimers in the host polymer matrix is responsible for the mirrorless lasing. However, the long amplification path through the waveguide produces a directional beam and supernarrowing a spectrum in scattering lasers.

Second harmonic generation of dipolar dendrons in the assembled thin films

Abstract Dendritic macromolecules, called ‘dendrons’, having a branching structure modified with an electron donor/acceptor functionalized azobenzene chromophore have been synthesized. The structurally different dendrons possess a different number, 1, 3, 7, and 15, of chromophore units. The second harmonic generation (SHG) of dendrons was measured in the molecular assembled thin films prepared by the Langmuir–Blodgett (LB) transfer technique. The SHG results indicated that dendrons could be highly organized in these films, where film structure and molecular orientation were analogous to those in the conventional SHG active LB films. The SHG intensity for these molecular organized thin films revealed that the individual chromophores in dendrons were oriented non-centrosymmetrically and coherently contributed to the increase in their molecular hyperpolarizability. The SHG activity became larger as the number of branching units of dendrons increased. The highest molecular hyperpolarizability was found to be 4000×10 −30 esu for a dendron containing 15 chromopholic units. The SHG result suggested that the structure of dendrons synthesized was uniaxally dipolar, and that there was a particular merit in this structure for the generation of SHG.

Two-photon-induced polymerization in a laser gain medium for optical microstructure

We have fabricated a polymeric solid-state microcavity in the laser gain media by a two-photon-induced polymerization technique. The photopolymerization resin contains conventional laser dye and dendrimer. A dendrimer can encapsulate the laser dye, and therefore increase its concentration up to 4 wt % with limited energy transfer during the photopolymerization process. The microcavity consisted of <0.4 μm linewidth polymerized strips arranged in layer-by-layer structure. The periodic variation in the refractive index gave rise to Bragg reflection. A laser emission was measured in the microcavity under optical excitation. The spectral linewidth was about 0.15 nm above the lasing threshold. The laser emission is attributed both to the distributed Bragg reflection and to the high gain of a polymeric medium containing laser dye with a high concentration.

Spatial photon confinement and super-radiation from dye cored dendrimer

Abstract This paper describes the preparation of a dendritic macromolecule having a rhodamine B chromophore, called Rh-dendrimer, and the emission properties from the neat films due to the spontaneous emission and super-radiative emission. An important concept to be investigated for material preparation, is the surrounding of a fluorescent chromophore by the dendritic branches. Therefore, a dendrimer becomes a core-shell structure, achieving a high efficiency of fluorescence with little energy transfer and self-quenching. The optical properties from dendrimer were very different from that of a bare chromophore. The Rh-dendrimer showed a high efficiency of fluorescence, while a bared rhodamine chromophore formed a charge transferred complex and exhibited less fluorescence. Under optical excitation with sufficient intensity (I0), the emission intensity was non-linearly increased as a function of the square of the number of molecules (N), giving relation of I∝N2I0. This property can not be adequately explained within the simple picture of spontaneous emission. Results were discussed in terms of cooperative emission.

Dendrimers for Optoelectronic Applications

This manuscript describes the dendritic macromolecules for optical and optoelectronic applications, particularly stimulated emission, laser emission, and nonlinear optics. Dendrimers have been designed and synthesized for these applications based on simple concepts. A core-shell structure, through the encapsulation of active units by dendritic branches, or a cone-shaped structure, through the step-by-step reactions of active units, can provide particular benefits for the optical high-gain media and nonlinear optical materials. It also described experimental results that support the methods presented for designing and fabricating functionalized dendrimers for optoelectronic applications, and theoretical results that reveal the intermolecular electronic effect of the dendritic structure.

Fabrication of three-dimensional microstructure in optical-gain medium using two-photon-induced photopolymerization technique

An approach using a laser-dye-doped dendrimer as a two-photon-induced photocurable resin was demonstrated for the fabrication of a three-dimensional microstructure. A dendrimer can encapsulate laser-dye and, as a result, increases its concentration up to 4 wt.% with limited energy transfer during the photopolymerization process. Confocal microscopic images showed a polymerized structure with a precise control of dimensions and spatial resolution within a submicrometer-scale. We fabricated a polymeric solid-state microcavity, which consisted of integrated strips arranged in a layer-by-layer structure. The microcavity had a periodic variation in the refractive index, which gave rise to Bragg reflection for laser feedback. Laser emission with a spectral linewidth of 0.15 nm was measured in the microcavity under optical excitation. This laser emission is attributed both to the distributed Bragg reflection and to the high gain of a polymeric medium containing a high concentration of laser-dye.

Functional Dendritic Macromolecules: Preparation and Optical Properties

Abstract Repetitive coupling reactions of branched azobenzene derivatives resulted in dendritic macromolecules, or “wedges” and “dendrimers”. These molecules were designed to incorporate aliphatic and polar units in order to prepare Langmuir-Blodgett (LB) films. Atomic force microscopy (AFM), absorption spectroscopy, and second harmonic generation (SHG) measurements revealed structural properties of the LB films in terms of size, aggregation, and orientation. AFM measurements revealed that growth of the branches increased as a function of the generation. In absorption spectroscopy and SHG measurements, results showed that wedges had an unsymmetric orientation in the LB films, while dendrimers had a symmetrically functionalized structure.

Effect of the metal/organic interface phenomena on the current–voltage characteristics of organic single electron tunneling device

The current-voltage (I-V) characteristic of Au/PI/(Rh-G2 + PI)/PI/Au (or Al) junctions was examined. An asymmetrical I-V characteristic with several steps was observed for Au/PI/PI:Rh-G2/PI/Al junctions, whereas a symmetric I-V characteristic with steps was obtained for Au/PI/PI:Rh-G2/PI/Au junctions. It was concluded that the presence of metal/organic interfacial states made a significant contribution to the I-V characteristics of these junctions.

Remarkable optical properties of dendrimers for laser applications

We have investigated optical properties of dye-encapsulated dendrimers for photonic applications and observed several interesting behavior, such as controlled intermolecular interactions, super-radiance, and supernarrowing laser emission. We confirmed limited inter molecular interactions between encapsulated dyes in fluorescence lifetime measurement of rhodamine B-cored dendrimers with different shell sizes. Also, photochemical stability against singlet oxygen was studied for fluorescent dyes encapsulated in a dendrimer box and the lifetime was improved by 60 times due to the shielding effect of the dendrimer shell. A supernarrowing laser spectrum was observed without using any resonator mirrors due to moderate scattering caused by densely doped dendrimer aggregates in a polymer waveguide. At the optimum operation level, the spectral linewidth is found to be 0.55 nm, which is limited by the resolution of the spectrograph used. Weak scattering from the aggregated dendrimers in the host polymer matrix with gain is responsible for photon localization. However, in contrast to conventional random lasers with strong scattering, weak scattering enables the long amplification path through the waveguide producing a directional beam and a supernarrowing spectrum. The dependence of supernarrowing lasing behavior on the density of the scattering moiety was studied by changing the dendrimer doping ratios in the polymer matrix.