Hae Yong Park

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.

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.

Determination of conduction band tail and Fermi energy of heavily Si‐doped GaAs by room‐temperature photoluminescence

A line‐shape analysis of room temperature photoluminescence (PL) spectra was carried out on Si‐doped GaAs samples grown by molecular beam epitaxy. The electron concentration n of the samples ranges from 1.0×1017 to 4.2×1018 cm−3. It was found that the conduction band tail ηc and the Fermi energy ef measured from the conduction band minimum can be expressed as ηc=2.0×10−8n1/3(eV) and ef=−0.074+1.03×10−7n1/3(eV), respectively. The PL peak energy, at which the electron concentration per unit energy in the conduction band is maximum, can also be expressed as 1.426+2.4×10−14n2/3(eV).

Asymmetric Mach-Zehnder filter based on self-collimation phenomenon in two-dimensional photonic crystals

A two-dimensional photonic crystal asymmetric Mach-Zehnder filter (AMZF) based on the self-collimation effect is studied by numerical simulations and experimental measurements in microwave region. A self-collimated beam is effectively controlled by employing line-defect beam splitters and mirrors. The measured transmission spectra at the two output ports of the AMZF sinusoidally oscillate with the phase difference of pi in the self-collimation frequency range. Position of the transmission peaks and dips can be controlled by varying the size of the defect rod of perfect mirrors, and therefore this AMZF can be used as a tunable power filter.

Essential role of impedance in the formation of acoustic band gaps

We investigate acoustic band gaps (ABGs) in a two-dimensional lattice of cylinders for the cases of constant impedance, Z, and constant velocity, v. ABGs become wider for the case of constant v (varying Z), and become smaller, eventually disappearing in the opposite case. As the volume fraction increases, the upper (bottom) edge of the stop band increases (decreases) and then decreases (increases) in composites with impedance variation only, so that the midgap frequency changes very little and a larger ABG can be created. The upper (bottom) edge of the stop band increases (decreases) when the impedance ratio increases, so that the midgap frequency decreases slightly and the size of the ABG increases.

Duplexer using microwave photonic band gap structure

We propose a frequency selective duplexer using microwave photonic band gap (PBG) structures. It uses two different PBGs to control the propagation of electromagnetic waves in the microwave region. In this structure, an additional narrow reflection band appears in the transmission spectrum when the PBG structure is not properly located relative to the T junction. By considering multiple reflections, it is proved that this additional reflection band in each PBG structure results from the interference between the input wave and the reflected wave from the other PBG structure. An effective way to prevent this interference effect is also discussed.

Experimental demonstration of reflection minimization at two-dimensional photonic crystal interfaces via antireflection structures

We experimentally confirm that the antireflection structures effectively minimize unnecessary reflections of self-collimated microwave beams at the interfaces of a two-dimensional photonic crystal, which is composed of cylindrical alumina rods. Optimized design parameters for the antireflection structures are obtained from the one-dimensional antireflection coating theory and the finite-difference time-domain simulations. Measured transmittance through the photonic crystal samples with and without the antireflection structures agree well with the simulation results. The measured results show that the photonic crystal with an antireflection structure yields about 90% transmission of incident power on the average in the frequency range of 12.0 to 13.0 GHz.

High-efficiency surface-emitting channel drop filters in two-dimensional photonic crystal slabs

We analyze theoretically the output efficiencies of surface-emitting channel drop filters using channel drop tunneling processes in two-dimensional photonic crystal slabs. By using the channel drop tunneling processes, the output efficiencies of the filters can be easily improved to the values much higher than 50%, the maximum output efficiency of surface-emitting channel drop filters using one single-mode cavity, and the output efficiency of 100% is possible theoretically. In order to demonstrate the theoretical results, we present a surface emitting channel drop filter using the channel drop tunneling processes in a two-dimensional photonic crystal slab. The output efficiency of the presented filter is about 94% in good agreement with the theoretical analysis.

Surface plasmon polariton resonance and transmission enhancement of light through subwavelength slit arrays in metallic films

In this study, we present experimentally measured transmission enhancement of microwaves through periodic slit arrays in metallic films. Enhanced transmission peaks and sharp transmission dips are clearly observed around the theoretically expected surface plasmon polariton(SPP) resonance frequencies. Dependence of the transmittance spectra on the geometrical properties of slits is also demonstrated by varying the slit width, slit periodicity and the thickness of metallic films. Transmission peaks and dips are originated from the coupling between the incident light and SPPs which are caused by the slit array that acts like a grating coupler. The obtained results are theoretically explained by solving the Maxwells equations and by the diffraction theory with appropriate boundary conditions, and they are in good agreement with those calculated by the finite-difference time-domain method.