Suzushi Nishimura

Fabrication of two-dimensional photonic crystals using interference lithography and electrodeposition of CdSe

This letter describes a simple synthetic approach to fabricate two-dimensional midinfrared CdSe photonic crystals (PC) by electrodeposition of CdSe in a polymer template defined using interference lithography. Characterization of the transmission spectra of CdSe PCs with a hexagonal array of 1.3 μm diameter and 2.7 μm pitch air voids showed a well-defined drop in transmission at 4.23 μm. The drop in transmission increased with incident angle, reaching a maximum of approximately 2.6 dB at 40° relative to the surface normal. This two-step synthetic approach can be used to incorporate photonic crystals onto arbitrary substrates for integration into future advanced optical circuits.

Direct fabrication of two-dimensional titania arrays using interference photolithography

Two-dimensional (2D) titania arrays with periods of 0.8–2.0 μm were fabricated by polymerization of a photosensitive titanium-containing monomer film using interference photolithography. The 2D precursor arrays were prepared by exposing a mixture of methacrylic acid, ethyleneglycol dimethacrylate, and titanium ethoxide doped with photoinitiator to 355 nm, 15 ns pulses from a Nd-Yttrium–aluminum–garnet laser and then rinsing with methanol. Pure titania arrays were obtained from the precursor arrays by subsequent calcination at 575 °C. The structure of the arrays fabricated by this method was confirmed with optical microscopy and scanning electron microscopy.

Enhancement of normally directed light outcoupling from organic light-emitting diodes using nanoimprinted low-refractive-index layer

The authors have demonstrated increased light outcoupling from organic light-emitting diodes (OLEDs) based on nanoimprinted amorphous fluoropolymer, poly[perfluoro(4-vinyloxy-l-butene)] (PPFVB). Because PPFVB has a low refractive index and a high transmittance over wide wavelength range, the first-order Bragg diffracted light shows high directionality and decreased waveguide absorption. These characteristics provide an advantage for small- or medium-sized OLEDs, which are mainly viewed from the normal direction.

Fabrication technique for filling-factor tunable titanium dioxide colloidal crystal replicas

Filling-factor tunable titanium dioxide replicas of colloidal crystals were fabricated by using a liquid phase deposition (LPD) technique. Ammonium hexafluorotitanate [(NH4)2TiF6] was used as a precursor, which was converted slowly to titanium dioxide by hydrolysis with boric acid. After the oxide was formed in voids of latex colloidal crystals, the latex was removed by calcination. By applying the LPD process to the replica again, the filling factor of the replica could be increased in a controllable fashion. With 203 nm template spheres, the peak shift in the ∼490 nm stop band depended linearly on the deposition time for the first 20 min, at a rate of 4.4 nm/min. Thus the position of the Bragg reflection peak could be adjusted precisely by controlling the filling factor of the replica structure.

Defect mode lasing from a double-layered dye-doped polymeric cholesteric liquid crystal films with a thin rubbed defect layer

The authors have demonstrated defect mode lasing emission from a spin-coated double-layered dye-doped polymeric cholesteric liquid crystal (PCLC) films with a thin rubbed isotropic layer in between. The rubbed layer acts not only as an alignment layer for the second PCLC layer but also as an isotropic defect layer breaking periodicity of PCLC films. All the processes can be achieved in a wet process. Because of the process, the present polymeric solid-state laser is available as a freestanding thin film of only 5.5μm in thickness and an almost limitless large area, which enable one to use it as a disposable laser chip of any shapes.

Polarization Conversion in Surface-Plasmon-Coupled Emission from Organic Light-Emitting Diodes Using Spontaneously Formed Buckles

A surface plasmon (SP) polariton is an electromagnetic wave propagating along the interface between a dielectric and a metal, and its electromagnetic fi eld exponentially decays into the surrounding media. Because the wavevector of the SP mode is larger than that of a photon of the same frequency in vacuum, the SP mode on a fl at surface is nonradiative and its energy dissipates as heat in the metal layer. Organic light-emitting diodes (OLEDs) consisting of organic layers and metallic electrodes inevitably accompany the SP mode and the power loss to the SP mode signifi cantly limits the device effi ciency, particularly in small-moleculebased OLEDs. [ 1 , 2 ] Although wavelength-scale periodic gratings have been introduced in OLED structures to convert the dissipated energy to the SP mode into useful light, reported structures are effective only at a specifi c wavelength and angle, satisfying the Bragg diffraction condition. [ 1–6 ] In our previous study, we demonstrated that quasiperiodic buckling structures with broad distribution and directional randomness can effectively enhance the light-extraction effi ciency without introducing spectral changes and directionality by outcoupling the waveguide modes. [ 7 ] In that study, however, we could not differentiate the outcoupling of transverse electric (TE) mode from that of the SP mode (transverse magnetic (TM) mode) by buckles because of the broad periodicity of the buckling structure and the similar propagation vectors of the TE and SP modes. In this study, we report that a buckling structure is remarkably effective at outcoupling the SP mode over all emission wavelengths and angles through an OLED structure with a thin indium–tin oxide (ITO) layer, by which any other waveguide modes are suppressed and only the SP mode is excited. Interestingly, we found that the diffraction of the SP mode by buckles produces TE-polarized light in addition to TM-polarized light, which indicates polarization conversion from TM (SP) to TE mode.

Highly circularly polarized electroluminescence from organic light-emitting diodes with wide-band reflective polymeric cholesteric liquid crystal films

The authors have observed highly circularly polarized electroluminescence from organic light-emitting diodes (OLEDs) using wide-band reflector consisting of three-layered left-handed polymeric cholesteric liquid crystal (PCLC) films. By simply attaching the wide-band reflective PCLC reflector to a conventional OLED, the authors obtained a high degree of circular polarization, i.e., the ratio of brightness between right- and left-handed circularly polarized electroluminescences is over 10 over the whole emission band.

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.

Broadband Cavity-Mode Lasing from Dye-Doped Nematic Liquid Crystals Sandwiched by Broadband Cholesteric Liquid Crystal Bragg Reflectors

[*] Prof. H. Takezoe, Dr. H. Choi, Dr. F. Araoka, Prof. K. Ishikawa Department of Organic and Polymeric Materials Tokyo Institute of Technology 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 (Japan) E-mail: takezoe.h.aa@m.titech.ac.jp Prof. J. W. Wu, J. Kim Department of Physics and Division of Nano Sciences Ewha Womans University Seoul 120-750 (Korea) E-mail: jwwu@ewha.ac.kr Dr. S. Nishimura, T. Toyooka Central Technical Research Laboratory Nippon Oil Corporation 8 Chidori-cho, Naka-ku, Yokohama 231-0815 (Japan)

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.