Chul-Hyun Choi

Enhancement of electroluminescence in GaN-based light-emitting diodes by metallic nanoparticles

The enhanced electroluminescence of GaN-based light-emitting diodes (LEDs) with noble metallic nanoparticles (MNPs) is demonstrated. The sample with well-designed Ag MNPs has shown the best performance enhancement of 126% in electroluminescent intensity compared with a conventional LED sample, even though the MNPs are placed at least 200 nm away from the quantum-well active layer. The MNPs provide enhanced photon scattering and coupling between localized surface plasmon resonance (LSPR) modes and photon modes internally trapped in a device. To investigate this effect, the peculiarities of the LSPR and the corresponding structural properties of the MNPs are discussed through the effective medium approach.

A nonunitary transfer matrix method for practical analysis of racetrack microresonator waveguide

We report that a nonunitary transfer matrix (NU-TM) method that we have developed is highly accurate and effective in analyzing and designing racetrack microresonator devices. Relatively poor output efficiency of a fabricated racetrack microresonator, unexplained by the conventional unitary transfer matrix (U-TM) method, are analyzed successfully by the NU-TM method. The track-length dependence, which is not obvious in the U-TM analysis, is clearly shown in the NU-TM analysis and is found to stem from the nonorthogonal phase difference between two branching waves in the directional coupling process. The results show excellent agreement with the results obtained by the finite-difference time-domain simulation method, thus confirming the validity and superior capability of the NU-TM method.

The Generalized Characteristics of Extinction Ratio for a Directional Coupler and Design of Compact 1310/1550 nm Demultiplexer

In a directional coupler, the design process requires repeated calculation of the characteristics of every changed structure, because it is generally difficult to expect the extinction ratio to be optimized over the entire variation of design parameters. In this paper, we systematically simulated the extinction ratio as a function of the design parameters, and analyzed the general tendency of that characteristic. In other words, we could find the generalized extinction ratio curve if the separation distance is normalized by the waveguide width. Here, the extinction ratio is shown to be increased as the normalized frequency (v) and the ratio (d) of the separation distance over the waveguide width were increased. For various structures with same ratio d, all corresponding extinction ratio curves as a function of v coincide with each other. We showed the usefulness of the generalized extinction ratio curve by applying it to the design and the fabrication of 1310/l550 nm demultiplexer, as it was convenient to design a shorter directional coupler with targeted extinction ratio from this curve.

Residual Layer Distribution on Embossing Process for Photonic Devices

We report on the behavior and effect of the residual layer left on the substrate after embossing process for the fabrication of photonic devices. The PDMS material was used as mold, and thermal and UV curable polymers was adapted for the fabrication of demultiplexer and optical power splitter, respectively.

Photonic Crystal Electro-Optic Modulator Incorporating Hybrid Silicon/Polymer Material

The design and analysis of a novel photonic crystal (PhC) electro-optic (EO) modulator are presented in this paper. The device incorporates an EO polymer slot waveguide into the center of a silicon PhC waveguide. In this device, strong optical confinement in EO polymer core and small group velocity by PhC structure provide a surprise enhancement of EO effect.

Fabrication of Micron-scale Elliptical Structures for Vertical Optical Via Applications

Previously proposed shape of the optical via structures are simple 45 degree mirrors with or without metal coatings, and simple elliptical shapes. To improve alignment tolerance and coupling efficiency, various shape of the optical via structures are fabricated and optical characteristics of the shapes are measured.

Photonic crystal group delay device for optical code division multiple access

We propose and demonstrate a photonic crystal optical group delay device, in which two dimensional photonic crystal line defect waveguide with chirped hole, induces sequential optical group delays for time-spreading/wavelength-hopping optical code division multiple access (O-CDMA). The photonic crystal line defect waveguide allows ultrasmall size of device due to its strong optical confinement by the photonic bandgap. And chirped photonic crystal waveguides, in which radius of holes are gradually changed so that the photonic bands are smoothly shifted to the higher frequency side. When a short pulse signal with a wide spectrum comes into this device, the guided light is localized at specific position depending on wavelength. This concept is suitable to realize a pulse waveform synthesizer and an en/decoder for time-spreading/wavelength-hopping O-CDMA. We have confirmed that this device has a chromatic temporal dispersion of ~ 33-fs/nm, corresponding to the repetition rate of ~ 1-ps for pulses with wavelength-interval of 30-nm, by two dimensional finite difference time domain simulation.

Fabrication of polymer AWG demultiplexer using embossing technique

We fabricated an arrayed waveguide grating (AWG) demultiplexer for optical fiber communication systems and photonic integrated circuits. We also used an embossing technique to fabricated the AWG instead of traditional semiconductor technologies, such as photolithography and etch process. UV curable polymers (ZPU 12-47 and ZPU 12- 45) were used as the core and upper cladding layers. The polydimethylsiloxane (PDMS) mold used for the embossing process is manufactured by a photoresist structure formed on a silicon wafer. We tried the embossing onto a fused silica glass using the PDMS mold. After UV curing, the PDMS mold was peeled away carefully, and a pattern of the AWG demultiplexer was left on the surface of that substrate. The upper cladding layer was coated over the patterned structure. The fabrication of the AWG demultiplexer was completed by a cleaving process. The residual layer produced after an embossing process was adjusted by the volume of polymer droplet. The embossing technique will have the potential for broad applications in fabrication of photonic devices.

Investigation of Optimal input Angle for Directional Coupler and Field-profile Overshooting

We investigate a method to improve the extinction ratio of directional couplers by controlling the angle of approach to their input ports and controlling the error limit for improving the extinction ratio of the directional coupler. Although relatively large angle of approach to the input port usually causes an error in calculations made by mode propagation analysis(MPA), optimally designed angle of approach not only minimizes errors in MPA calculations but also can improve extinction ratio by precision control of coupling coefficients of optical modes. We show that abrupt changes in approaching angle, although not enough to cause modal leakage, give rise to field-profile overshooting, which degrades extinction ratio and other properties of the directional coupler. Using Beam Propagation Method (BPM) we calculate two types of input structures, linear and curved, for optimization of extinction ratio.

Design and fabrication of a polymer-based micron-scale semispherical vertical interconnecting structure for 3D photonic integrated circuit application

A vertically interconnected structure is a key feature of a photonic integrated circuit application, like an optical printed circuit board. Conventional vertical interconnecting structures have a 45 degree mirror, with or without metal coating to enhance its reflectance. We have designed a curved-shape semi-spherical type vertical interconnecting structure to enhance vertical interconnecting efficiency, and also developed simple fabrication procedure to realize it.