Goro Suzaki

Enhancement of Light Extraction from Organic Light-Emitting Diodes with Two-Dimensional Hexagonally Nanoimprinted Periodic Structures Using Sequential Surface Relief Grating

Electric characteristics of organic light-emitting diodes (OLEDs) with two-dimensional hexagonally nanoimprinted periodic structures were investigated, and enhanced light extraction from this device was confirmed. A master mold was fabricated from an azobenzene polymer thin film patterned by sequential surface relief grating formation into a three-directional structure. The device shows a higher current density and a higher current efficiency, although leakage current is also higher than that of a device without periodic structures. These results are caused by the extraction of diffracted light from waveguided and substrate-guided light even when incident light has larger angles than the critical angle for total reflection.

Low threshold lasing from dye-doped cholesteric liquid crystal multi-layered structures

We fabricated novel hybrid structures composed of a dye-doped low-molecular-weight cholesteric liquid crystal sandwiched by multi-layered polymer cholesteric liquid crystal films and evaluated their lasing characteristics. Lasing was observed with an extremely reduced threshold (12 nJ/pulse) by a factor of 10 compared with that in a simple dye-doped low-molecular-weight cholesteric liquid crystal cell. Lasing characteristics experimentally obtained were discussed by comparing them with the simulated photonic density of states spectra.

First observation of a smectic A–cholesteric phase transition in a thermotropic liquid crystal consisting of a rigid-rod helical polysilane

A smectic A–cholesteric phase transition for a rigid-rod helical polymer, poly[n-decyl-(S)-2-methylbutylsilane] (PD2MBS), with a narrow molecular weight distribution, has been observed for the first time. Polarizing optical microscopy showed that the fan-shaped texture of the smectic A phase turned into the characteristic planar texture of the cholesteric phase upon heating. The positive CD band, which corresponds to the reflection band of the cholesteric phase, gradually decreased in intensity within a range 30°C below the transition temperature on cooling, while the peak maximum shifted towards shorter wavelengths. It was concluded that the system has a very wide temperature region over which the cholesteric–smectic A phase transition occurs and in which the cholesteric pitch varies with temperature.

Smectic Liquid Crystal Observed in Thermotropic System of Rigid-Rod Poly(γ-octadecyl L-glutamate)

We investigated the thermotropic phase behavior of rigid-rod poly(γ-octadecyl L-glutamate) (PG18) with an average molecular weight Mw=36,000. Two polymers with narrow molecular weight distributions (Mw/Mn), PG18-A with Mw/Mn=1.12 and PG18-B with Mw/Mn=1.05, were prepared by gel permeation chromatography (GPC) fractionation. Both were found to form a smectic A phase by means of microscopic observations of the focal conic texture and by the small-angle X-ray method. The smectic A phase of PG18-A was transformed into a cholesteric phase at approximately 140–170°C while that of PG18-B stably existed in the entire thermotropic temperature range from 60°C to 220°C (decomposition temperature). This dependence of smectic stability on the molecular weight polydispersity qualitatively corresponds to the theoretical prediction based on the excluded volume effect.

Small-Angle X-Ray Analysis of Smectic a Cholesteric Liquid Crystal Phase Transition in Rigid-Rod Helical Polysilane

The smectic A Cholesteric liquid crystal phase transition of rigid-rod helical polysilane was studied by means of synchrotron radiation small-angle X-ray scattering (SR-SAXS). The information on the smectic trans-layer electron density distribution was clarified by fitting the model of electron density distribution to the diffraction data. Each of the layers was approximated by the boxlike function to introduce the molecular form factor of the polymer. The distribution of the molecular center of gravity within the layers due to the thermal motion was taken into consideration by convolution with the Gaussian function. The elucidated smectic layer structure was not like the conventional structure with their ends packed into the interlayer space. The layers with bumpy surfaces interdigitate in a key-and-keyhole manner to pack the molecules with different lengths into layers due to their finite molecular weight distribution.