Chul-Sik Kee

Highly efficient photonic crystal-based multichannel drop filters of three-port system with reflection feedback

We have derived the general condition to achieve 100% drop efficiency in the resonant tunneling-based channel drop filters of a threeport system with reflection feedback. According to our theoretical modeling based on the coupled mode theory in time, the condition is that the Q-factor due to coupling to a bus port should be twice as large as the Q-factor due to coupling to a drop port and the phase retardation occurring in the round trip between a resonator and a reflector should be a multiple of 2pi. The theoretical modeling also shows that the reflection feedback in the threeport channel drop filters brings about relaxed sensitivity to the design parameters, such as the ratio between those two Q-factors and the phase retardation in the reflection path. Based on the theoretical modeling, a fivechannel drop filter has been designed in a two-dimensional photonic crystal, in which only a single reflector is placed at the end of the bus waveguide. The performance of the designed filter has been numerically calculated using the finite-difference time domain method. In the designed filter, drop efficiencies larger than 96% in all channels have been achieved.

Tunable omnidirectional reflection bands and defect modes of a one-dimensional photonic band gap structure with liquid crystals

We have theoretically investigated the tunability of the omnidirectional reflection band (ORB) of a one-dimensional (1D) photonic crystal (PC) consisting of alternating isotropic dielectric and nematic liquid crystal layers by an external electric field. The width of the ORB becomes wide as the external voltage increases, but the center frequency is changed little. The frequency of the defect mode created by inserting a layer of liquid crystals into a 1D dielectric PC is also found to be tunable by the application of external voltage. These properties can be applied to tunable optical filters or optical switches.

Line-defect-induced bending and splitting of self-collimated beams in two-dimensional photonic crystals

We show that line defects can give rise to the bending and splitting of self-collimated beams in two-dimensional photonic crystals from the equifrequency contour calculations and the finite-difference time-domain simulations. The power ratio between two split self-collimated beams can be controlled systematically by varying the radii of rods or holes in the line defect. We also show that the bending and controllable splitting of self-collimated beams can be useful in steering the flow of light in photonic crystal integrated light circuits.

Reflection minimization at two-dimensional photonic crystal interfaces

We propose a method to design antireflection structures to minimize the reflection of light beams at the interfaces between a two-dimensional photonic crystal and a homogeneous dielectric. The design parameters of the optimal structure to give zero reflection can be obtained from the one-dimensional antireflection coating theory and the finite-difference time-domain simulations. We examine the performance of a Mach-Zehnder interferometer utilizing the self-collimated beams in two-dimensional photonic crystals with and without the optimal antireflection structure introduced. It is shown that the optimal antireflection structure significantly improves the performance of the device.

Elliptical defected core photonic crystal fiber with high birefringence and negative flattened dispersion

We propose a novel design of photonic crystal fiber (PCF) using an elliptical air hole in the core as a defected core in order to enhance the performance of modal birefringence and to control the properties of chromatic dispersion at the same time. From the simulation results, it is shown that the proposed fiber has high birefringence up to the order of 10(-2), negative flattened chromatic dispersion in a broad range of wavelengths, and low confinement loss less than that of the single mode fiber. The outstanding advantage of the proposed PCF is that high birefringence, negative flattened dispersion, and low confinement loss can be achieved just by adding a small sized elliptical air hole in the core to the elliptical air hole PCF, especially at the same time.

Fabrication of polymer photonic crystals using nanoimprint lithography

Dense two-dimensional periodic photonic bandgap structures are produced in poly(methyl methacrylate) thin films using nanoimprint lithography (NIL). The stamp original was made by electron beam lithography. Then, a high versatility of the process was achieved by fabricating copies of the stamp original via NIL, which enables varying and optimizing both fill factors and aspect ratios independently. For reliable stamp copying over the whole structural areas, the nanorheological behaviour had to be considered. Using those stamp copies, polymeric photonic band gap structures with an aspect ratio as high as 2 were successfully replicated.

Roles of wave impedance and refractive index in photonic crystals with magnetic and dielectric properties

We investigated the roles of wave impedance and refractive index in photonic crystals by means of analytical expressions for edges frequencies of a photonic bandgap (PBG) in a one-dimensional photonic crystal with magnetic and dielectric properties. The analytical expressions were derived when the optical thicknesses of layers are the same. The wave impedance governs the formation of PBGs and the intensity of defect modes. Meanwhile, the position of PBGs and the creation of defect modes are related to the refractive index.

Dispersion properties of dual-core photonic-quasicrystal fiber

We propose a photonic quasi-crystal fiber with a dual-core structure. The circular-like outer core caused by the quasi-periodic arrangement gives rise to an interesting dispersion property that is different from that of a photonic crystal fiber with a dual-core structure. The absolute value of negative dispersion for the dual-core photonic quasi-crystal fiber increases as the distance between the nearest holes increases, while the absolute value of negative dispersion for the dual-core photonic crystal fiber decreases. The dispersion property can be useful in reducing the coupling loss between the compensating dispersion fiber and a conventional single mode fiber.

Novel optical properties of six-fold symmetric photonic quasicrystal fibers.

We have investigated optical properties of an optical fiber having a six-fold symmetric quasiperiodic array of air holes in cladding, a six-fold symmetric photonic quasicrystal fiber. The photonic quasicrystal fiber exhibits larger cutoff ratio for endlessly single mode operation than that of a triangular photonic crystal fiber having six-fold symmetry and almost zero ultra-flattened chromatic dispersion, 0+/-0.05 ps/km/nm, in the range of wavelength from 1490 to 1680 nm. The dispersion value is much less than those of the proposed dispersion flattened PCFs.

Modified rectangular lattice photonic crystal fibers with high birefringence and negative dispersion.

We propose a modified rectangular lattice PCF and numerically investigate its birefringence and dispersion. Based on the plane wave expansion method, it is shown that the proposed structure provides high birefringence and negative dispersion. Numerical results show that the birefringence of the modified PCF reaches 10(-2) and the leakage loss is about 1000 times smaller than that of an original rectangular PCF because the modification gives rise to the strong confinement of guided modes. Dispersion and its slope are also negative over the C band.