Juro Oshima

High-order nonlinear optical response of a polymer nanocomposite film incorporating semiconducotor CdSe quantum dots

We report on observation of high-order optical nonlinearities in our recently developed photopolymerizable semiconductor CdSe quantum dot (QD)-polymer nanocomposite films at various volume fractions of CdSe QDs as high as 0.91 vol.% (3.6 wt.%). We performed Z-scan and degenerate multi-wave mixing (DMWM) measurements using a 532-nm picosecond laser delivering single 35 ps pulses at a repetition rate of 10 Hz. Using the uniformly cured polymer nanocomposite films, we observed the third- and fifth-order nonlinear optical effects in closed-aperture Z-scan measurements by which it was found that saturable nonlinear absorption (light-induced transparency) and large negative nonlinear refraction were induced. We also measured dependences of the effective third- and fifth-order nonlinear refraction constants on CdSe QD volume fraction. Based on the Maxwell-Garnett model, we estimated the third- and fifth-order nonlinear optical susceptibilities of CdSe QD and discussed a contribution of the third-order effect to the fifth-order one due to the cascaded (local-field) effect. Coexistence of the third- and fifth-order nonlinear refraction was also confirmed by DMWM.

Holographic assembly of semiconductor CdSe quantum dots in polymer for volume Bragg grating structures with diffraction efficiency near 100

We report on the fabrication of centimeter-size transmission Bragg gratings in semiconductor CdSe quantum dots dispersed 50 μm thick photopolymer films. This was done by holographic assembly of CdSe quantum dots in a photopolymerizable monomer blend. Periodic patterning of CdSe quantum dots in polymer was confirmed by a fluorescence microscope and confocal Raman imaging. The diffraction efficiency from the grating of 1 μm spacing was near 100% in the green with 0.34 vol % CdSe quantum dots, giving the refractive index modulation as large as 5.1×10−3.

Uniform and refreshable liquid electroluminescent device with a back side reservoir

We fabricated a refreshable organic light-emitting diode with a liquid emitting layer. This device has a mesh-structured cathode and a liquid reservoir in the back side of the cathode for easy convection of the liquid emitter. The small uniform gap between the electrodes was fabricated by means of micro-electro-mechanical systems (MEMS) processing. Although the device luminance decreased because of decomposition of the liquid emitters under the drive current, the decreased emission was quickly recovered to the initial state by convectional replacement of the decomposed emitters with fresh emitters through the holes of the mesh cathode.

Microfluidic White Organic Light-Emitting Diode Based on Integrated Patterns of Greenish-Blue and Yellow Solvent-Free Liquid Emitters.

We demonstrated a novel microfluidic white organic light-emitting diode (microfluidic WOLED) based on integrated sub-100-μm-wide microchannels. Single-μm-thick SU-8-based microchannels, which were sandwiched between indium tin oxide (ITO) anode and cathode pairs, were fabricated by photolithography and heterogeneous bonding technologies. 1-Pyrenebutyric acid 2-ethylhexyl ester (PLQ) was used as a solvent-free greenish-blue liquid emitter, while 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb)-doped PLQ was applied as a yellow liquid emitter. In order to form the liquid white light-emitting layer, the greenish-blue and yellow liquid emitters were alternately injected into the integrated microchannels. The fabricated electro-microfluidic device successfully exhibited white electroluminescence (EL) emission via simultaneous greenish-blue and yellow emissions under an applied voltage of 100 V. A white emission with Commission Internationale de l’Declairage (CIE) color coordinates of (0.40, 0.42) was also obtained; the emission corresponds to warm-white light. The proposed device has potential applications in subpixels of liquid-based microdisplays and for lighting.

Multi-color microfluidic organic light emitting device using electroluminescence and electrochemiluminescence

We developed a multi-color microfluidic organic light emitting device, which consists of liquid organic light emitting diodes (OLEDs) and electrochemiluminescence (ECL) devices. A 3 × 3 matrix of emitting pixels was fabricated in SU-8 microchannels sandwiched between polyethylene naphthalate (PEN) and glass substrates with indium tin oxide (ITO) anode and cathode pairs. Liquid organic semiconductor and ECL solutions as liquid emitters were injected into the microchannels. The electroluminescence (EL) and ECL were successfully obtained at the emitting pixels in the microchannels. The proposed structure can be applicable for multi-color liquid-based display.

Simple and high-throughput fabrication of large-area and multilayer flexible polymer film structures for microfluidic organic light emitting diode

A simple and high-throughput fabrication of large-area and multilayer polymer film structures was developed and applied for microfluidic organic light emitting diodes (OLED). We employed following three distinguished technologies, a) screen-printing technique for transparent electrodes, b) novel belt-transfer exposure technology for SU-8 microchannels, and c) heterogeneous low temperature bonding technology using self-assembly monolayer. A Liquid organic semiconductor as a liquid emitter was injected to each microchannel and photoluminescence and electroluminescence were obtained not only in a straight state, but also in a bending state. This large-area flexible microfluidic structure for OLED is applicable to nondisposable posters or displays which can be adopted around the curved surfaces of the various structures.

Deep-blue light emission with a wide-bandgap naphthalene-derivative liquid organic semiconductor host

We developed a novel naphthalene-derivative to function as a wide-bandgap liquid organic semiconductor (LOS) host material for the limited range of liquid deep-blue light-emitting materials that have been developed to date. The naphthalene-derivative, 1-naphthaleneacetic acid 2-ethylhexyl ester (NLQ) was synthesized as a LOS, by introducing an ethylhexyl group into naphthalene. 9,10-Diphenyl anthracene (DPA) was doped into NLQ as a guest deep-blue dye. From the absorption spectrum, the bandgap energy of NLQ was estimated to be 4.13 eV, indicating that NLQ has the widest bandgap energy of any such host material so far as we know. Deep-blue electroluminescence (EL) emission in a liquid state was obtained by doping DPA into NLQ. Light emission could be achieved by a combination of Förster resonance energy transfer and direct recombination of trapped holes and electrons because the bandgap energy of DPA is straddle by that of the wide-bandgap NLQ. Thus, NLQ is shown to be a promising wide-bandgap LOS host material, which allows deep-blue light emission and may have applications in liquid organic light-emitting diodes.

Flexible organic light emitting diode ribbons using three liquid organic semiconductors

We propose ribbon type flexible organic light emitting diodes (OLEDs), which can be integrated into textiles, using liquid organic semiconductors (LOSs). A linear flexible microchannel, which consists of a liquid light-emitting layer with LOS sandwiched between two indium tin oxide (ITO) electrodes, was fabricated with photolithography and heterogeneous bonding. LOSs were injected into the SU-8-based linear microchannels of 70 mm in length. Three different colors of photoluminescence (PL) emission were observed from flexible linear microchannels. Liner electroluminescence (EL) emission was successfully obtained in a bending state as well as straight state. We expect that the proposed microfluidic OLED ribbons will have high potential for future free-formable wearable devices such as electronic textiles.

Observation of higher-order optical nonlinearities in photopolymerizable semiconductor CdSe quantum dot-polymer nanocomposites

Semiconductor quantum dots (QDs), also referred to as nanocrystals, that confine optically excited electron-hole pairs in all three space dimensions are also of considerable interest in science and engineering. Tailored patterning of QDs in a thick polymer matrix is of great interest for photonic and biomedical applications. For example, using enhanced third-order nonlinear optical responses of QDs [1], one expects optical multi-stability action from periodic QDs structures embedded in a polymer film [2]. In this work we report on the nonlinear optical properties of semiconductor CdSe QDs dispersed in an optically transparent polymer film. Z-scan and degenerate multiwave mixing (DMWM) measurements were performed to obtain effective nonlinear optical susceptablilities of the film at a wavelength of 532nm for various concentrations of CdSe QDs. It was found that the film exhibited 5th- and 7th-order nonlinearities. We also observed nonlinear Bragg diffraction from holographically patterned [3] CdSe QD-polymer nanocomposite films that exhibited the diffraction efficiency near 100% [4].