Heuk Park

Enhanced quantum efficiency in polymer electroluminescence devices by inserting a tunneling barrier formed by Langmuir–Blodgett films

Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly(methyl methacrylate) Langmuir–Blodgett films were used as the thin tunneling barrier. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing the effective barrier for hole injection. The effects improve the balanced injection of electrons and holes into the light‐emitting devices.

Light-emitting electrochemical cells based on poly(9,9-bis(3,6-dioxaheptyl) -fluorene-2,7-diyl)

To enhance color stability of light-emitting electrochemical cell (LEC) made with poly(9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl) (BDOH-PF), BDOH-PF end-capped with anthracene was prepared by Ni(0) mediated polymerization. Moreover, a copolymer of 9,9-bis(3,6-dioxaheptyl)-2,7-dibromofluorene and 9,10-dibromoanthracene which was also end-capped with anthracene, was synthesized by the same polymerization method. Less excimer emission in photoluminescence (PL) spectra were observed in these polymers compared to the previous BDOH-PF. LECs were fabricated using blends of lithium triflate and the prepared polymers. These LECs showed high brightness at low applied voltages. In the electroluminescence (EL) spectra, narrow blue emission was observed initially in these LECs and the EL spectra were getting broader because of excimer formation. End capping with anthracene and/or copolymerization with anthracene led to strong inhibition of excimer formation though it was not suppressed completely.

Polymeric 2 x 2 electrooptic switch consisting of asymmetric Y junctions and Mach-Zehnder interferometer

2/spl times/2 electrooptic switches consisting of a pair of asymmetric Y junctions and Mach-Zehnder interferometer have been demonstrated in polymeric waveguides. The switching voltage is 15 V with 1.5 cm long electrode for TM polarized light at 1.3 /spl mu/m. When the branching angle of the asymmetric Y junction is 0.2/spl deg/, crosstalk of -27 to -22 dB are obtained for both input arms. The measured insertion loss by the lens coupling is about 9-10 dB.

High performance electro-optic polymer waveguide device

A high performance electro-optic polymer modulator is fabricated by utilizing a thermally curable cladding polymer. Half-wave modulation voltage as low as 3.7 and 4.8 V under 500 Hz operation is obtained with a 1.5 cm long electrode at wavelengths of 1.3 and 1.55 μm, respectively. The modulator also shows stable dc bias voltage to be traceable and low poling-induced optical loss. This work implies that proper selection of a cladding material is as important as the electro-optic materials in the electro-optic waveguide devices.

ORIGIN OF DIRECT CURRENT DRIFT IN ELECTRO-OPTIC POLYMER MODULATOR

Characteristics of the dc drift phenomenon in electro-optic polymer modulators have been analyzed. It is found that the dc drift originates from the difference between the dielectric relaxation times of the guiding and cladding layers. The dc drift is accelerated as the intensity of the guided light increases. Furthermore, it becomes faster when the device is exposed to visible light. The effect of the guided light and visible light on the dc drift is interpreted in terms of the photoconductivity of the guiding layer.

The initial state of dark spots in degradation of polymer lighting-emitting diodes

Abstract The initial state of dark spots in polymer light-emitting diodes (PLEDs) with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) layer as an emissive layer has been investigated by in situ measurement with the interferometer, fluorescence microscope, and other various analytical instruments. Degradation of the device initiates with the dark spot that is the non-emissive portion of the polymer layer. The cross-sectional profile of interferometer image of Al electrode changes with operating time from dip to con-shaped sharp peak. It is clear that the dark spot originates from the pinhole leading to the severe damages on the microstructure of the polymer layer, polymer/metal interface, and metal electrode. Interferometer is a powerful tool for the investigation of the short-term degradation behavior of the polymeric EL devices.

Polarization stabilizer using a polarization splitter and a thermooptic polymer waveguide device

For the stabilization of the time-varying light polarization in the optical fiber, we demonstrates a polarization stabilizer by using a fiber-optic polarization splitter, a passive polarization converter, and a power equalizing waveguide device. Because the ratio of the output light power from the polarization splitter is time-varying depending on the input polarization, we adopt a power equalizer consisting of a polymeric thermooptic (TO) phase modulator and an asymmetric X-junction waveguide. A feedback signal is obtained from the difference of the two outputs of the power equalizer. Then it is applied to the TO phase modulator. The output power is stabilized with the TE polarization independent of the time-varying polarization state of the input light.

End-face scattering loss in integrated-optical waveguides

An experimental technique to determine the end-face scattering loss in electro-optic polymer channel waveguides is presented. The technique combines the cut-back and the optimum end-fire coupling methods. A loss resulting from the scattering was a prominent source of waveguide coupling loss and was strongly dependent on the end-face roughness of the guiding and cladding layers induced by cleaving. With the use of our investigation methods, other losses could also be examined with ease and high reliability.

Tunable optical CDMA encoder/decoder based on modified PN code using FBG array

We propose a novel tunable OCDMA encoder/decoder based on modified PN code FBG array. Bragg wavelengths of the FBG array are tuned by strain/temperature control and selected by a band-pass filter to realize the full-set of user codes.

Improvement of quantum efficiency in polymer electroluminescence devices by inserting pmma langmuir-blodgett films

Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly(methyl methacrylate) Langmuir-Blodgett films were used as the thin tunneling barrier. The enhancement may result from the improved balance between the electron and hole injection into the light emitting devices.