Jun-Whee Kim

FDTD analysis of the light extraction efficiency of OLEDs with a random scattering layer

The light extraction efficiency of OLEDs with a nano-sized random scattering layer (RSL-OLEDs) was analyzed using the Finite Difference Time Domain (FDTD) method. In contrast to periodic diffraction patterns, the presence of an RSL suppresses the spectral shift with respect to the viewing angle. For FDTD simulation of RSL-OLEDs, a planar light source with a certain spatial and temporal coherence was incorporated, and the light extraction efficiency with respect to the fill factor of the RSL and the absorption coefficient of the material was investigated. The design results were compared to the experimental results of the RSL-OLEDs in order to confirm the usefulness of FDTD in predicting experimental results. According to our FDTD simulations, the light confined within the ITO-organic waveguide was quickly absorbed, and the absorption coefficients of ITO and RSL materials should be reduced in order to obtain significant improvement in the external quantum efficiency (EQE). When the extinction coefficient of ITO was 0.01, the EQE in the RSL-OLED was simulated to be enhanced by a factor of 1.8.

Flexible polymer waveguide tunable lasers

A flexible polymeric Bragg reflector is fabricated for the purpose of demonstrating widely tunable lasers with a compact simple structure. The external feedback of the Bragg reflected light into a superluminescent laser diode produces the lasing of a certain resonance wavelength. The highly elastic polymer device enables the direct tuning of the Bragg wavelength by controlling the imposed strain and provides a much wider tuning range than silica fiber Bragg gratings or thermo-optic tuned polymer devices. Both compressive and tensile strains are applied within the range from -36000 microepsilon to 35000 microepsilon, so as to accomplish the continuous tuning of the Bragg reflection wavelength over a range of up to 100 nm. The external feedback laser with the tunable Bragg reflector exhibits a repetitive wavelength tuning range of 80 nm with a side mode suppression ratio of 35 dB.

Optical Current Sensors Consisting of Polymeric Waveguide Components

Optical current sensors are demonstrated based on polarization rotated reflection interferometry by incorporating polymeric optical waveguide components. Polarization maintaining 3-dB couplers, TE-pass waveguide polarizers, and thermo-optic phase modulators are designed and fabricated in this work in order to provide essential building blocks for constructing the current sensors. The phase difference between the two circularly polarized waves imposed by the Faraday effect of the optical fiber is detected using the interferometric optical sensor consisting of the polymeric components. To remove the bending induced birefringence, the optical fiber wound around a ceramic frame is annealed at 850°C for 24 hours. The reflection interferometer comprising the polymer waveguide components operates with good linearity proportional to the monitoring current.

Polymer Waveguide Label-Free Biosensors With Enhanced Sensitivity by Incorporating Low-Refractive-Index Polymers

Label-free biosensors consisting of polymeric waveguides with a Bragg reflection grating are demonstrated for the purpose of highly sensitive protein detectors. Due to the unique nanoimprinting and injection molding processes, polymer waveguide devices have the potential to provide cost-effective disposable biosensors as long as a robust compatible immunoassay process is developed. A fluorinated polymer material with the lowest refractive index, which is close to that of the saline solution, is used for the sensor waveguide, in order to increase the effective index modulation efficiency. The large contrast waveguide exhibits a 1.9 times higher sensitivity for the detection of the variation of the cover index compared to low-contrast sensors. Bragg reflectors with a 3-dB bandwidth of 0.9 nm are fabricated using laser interferometry and dry etching of the core layer by 50 nm. An immunoassay experiment to detect the concentration of antibiotin is performed by measuring the peak wavelength shift of the Bragg reflection sensors. By using a blocking buffer, it is possible to obtain a stable saturated output signal by preventing the nonspecific binding. In the experiment to measure the antibiotin concentration, the sensor exhibits linear correspondence of peak wavelength shift for the concentrations.

Near-infrared tunable lasers with polymer waveguide Bragg gratings.

Wavelength tunable lasers operating at near infrared (NIR) wavelength are demonstrated through the thermo-optic (TO) refractive index tuning of polymer waveguide Bragg reflectors. The polymer-waveguide device has superior TO efficiency for substantially changing the refractive index, and it enables direct tuning of the Bragg reflection wavelength over a wide range. The waveguide is optimized for NIR wavelengths, and a third-order Bragg reflector is incorporated for facilitating fabrication of the grating. The laser exhibits an output power of 0 dBm, a side-mode suppression ratio of 40 dB, and a tuning range of 21 nm.

Optical Pressure Sensors Based on Vertical Directional Coupling With Flexible Polymer Waveguides

An optical pressure sensor based on the radiation coupling in vertical direction between a channel waveguide and a flexible polymer waveguide is proposed. The flexible waveguide is bent by an applied pressure to reduce the separation distance between the two waveguides so that the light propagating in the channel waveguide is radiated into the flexible planar waveguide. By applying a pressure ranging from 100 to 500 kPa, the output intensity modulation of 20 dB is obtained by virtue of the efficient phase matched coupling between the waveguides made of the same polymer material. The large extinction ratio is useful to measure the pressure precisely for a wide range.

Polymer waveguide integrated-optic current transducers

Various functional optical devices are integrated on a single chip in order to construct optical current transducers based on polarization rotated reflection interferometry, which consists of polarization maintaining 3-dB couplers, TE-pass polarizers, TE/TM polarization converters, and thermo-optic phase modulators. By virtue of the device integration, the sensor exhibited good linearity, and excellent accuracy with an error less than 0.2%. The integrated-optic device provides inherent polarization maintaining characteristics and precise controllability of the optical path length in the interferometric sensor. Single chip integration reduces the complexity of the interferometry, and enables mass-production of low-cost high performance current sensors.

Integrated-optic polarization controllers incorporating polymer waveguide birefringence modulators

Polarization controllers based on polymer waveguide technology are demonstrated by incorporating thermo-optic birefringence modulators (BMs) and thin-film wave plates. Highly birefringent polymer materials are used to increase the efficiency of birefringence modulation in proportion to the heating power. Thin-film quarter-wave plates are fabricated by using a crosslinkable liquid crystal, reactive mesogen, and inserted between the BMs to produce static phase retardation and polarization coupling. By applying a triangular AC signal to one BM and a DC signal to another, the polarization states of the output light are modulated to cover the entire surface of the Poincaré sphere.

Polarization-Splitting Waveguide Devices Incorporating Perfluorinated Birefringent Polymers

Low-loss perfluorinated polymers with a controllable optical birefringence were synthesized for the purpose of fabricating polarization-splitting waveguide devices, which have become an essential part of polarization-multiplexed coherent optical communications. The birefringent polymer was embedded on one branch of the asymmetric Y-branch waveguide to produce polarization-dependent mode evolution due to the effective birefringence. The device with a branch angle of 1/300 rad and birefringence of 0.007 showed a crosstalk of -25 dB and an insertion loss of 1.5 dB from fiber to fiber. The polarization splitters based on adiabatic mode evolution exhibited negligible wavelength dependence and large tolerance in waveguide dimension.

Polarization Converting Waveguide Devices Incorporating UV-curable Reactive Mesogen

Reactive mesogen (RM) is an organic liquid crystal molecule that can be self-aligned to have an optic axis of birefringence when coated over a polyimide alignment film. A free-standing optical wave-plate film consisting of RM and low-loss optical polymers was fabricated in this work, and the film was inserted across the polymer waveguide to form an integrated optical polarization converter. For convenient evaluation of the polarization converters, a waveguide polarizer and analyzer were fabricated in series. The polarization conversion efficiency was measured to be 25 dB for the wavelength range from 1520 to 1580 nm. The wave plate exhibited a temperature-dependent retardation of 4.5° for a temperature change from 25 to 100℃.