Sun-Goo Lee

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.

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.

Terahertz notch and low-pass filters based on band gaps properties by using metal slits in tapered parallel-plate waveguides

We present a tunable notch filter having a wide terahertz (THz) frequency range and a low-pass filter (LPF) having a 0.78 THz cutoff frequency. Single slit and multiple slits are positioned at the center of air gaps in tapered parallel-plate waveguides (TPPWG) to obtain the notch filter and LPF, respectively. The notch filter has a dispersion-free and low-loss transverse magnetic (TM) mode. The Q factor was proved to be 138, and the resonant frequency is easily tunable by adjusting the air gaps between TPPWG. On the other hand, the cut off frequency of the LPF was determined using a Bragg stop band, which depends on slit period. The LPF has a transition width of 68 GHz at the cutoff frequency and a dynamic range of 35 dB at stop bands. In addition, the characteristics of such filters were analyzed using finite-difference time-domain (FDTD) simulations.

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.

Magnetostriction of melt-spun Dy-Fe-B alloys

The magnetostriction of melt‐spun ribbons of Dyx(Fe1−yBy)1−x (x=0.2, 0.25, 0.3; 0≤y≤0.2) alloys is systematically investigated as a function of the wheel speed during melt quenching. As the wheel speed increases from 10 to 50 m/s, the magnetic softness improves with the wheel speed rather continuously for the alloys with the Dy content x=0.2 and 0.25 but it exhibits a maximum at the wheel speed of 30 or 40 m/s for the alloys with the highest B content (x=0.3). The softness also improves with the B content for a fixed wheel speed. Homogeneous and ultrafine grain structure is observed for the first time even in the as‐spun state when the ribbons of the alloy Dy0.3(Fe0.8B0.2)0.7 are fabricated at the wheel speed of 30 m/s. The ribbon having the ultrafine grain structure exhibits good magnetic softness together with a high strain.

Efficient beaming of self-collimated light from photonic crystals.

We propose a novel structure for achieving highly efficient beaming of self-collimated light from two-dimensional photonic crystals. The finite-difference time-domain simulations show that both enhanced transmission and highly directional emission of self-collimated beams from photonic crystals are achieved by using the bending and splitting of self-collimated beams in photonic crystals, and also by introducing an antireflection coating-like photonic crystal collimator to the exit surface of the structure. This structure is potentially important for highly efficient coupling of self-collimated beams from photonic crystals into conventional optical fibers and photonic crystal waveguides.

Ring-type Fabry-Pérot filter based on the self-collimation effect in a 2D photonic crystal

We propose a ring-type Fabry-Pérot filter (RFPF) based on the self-collimation effect in photonic crystals. The transmission characteristics of self-collimated beams are experimentally measured in this structure and compared with the results obtained with the simulations. Bending and splitting mechanisms of light beams by the line defects introduced into the RFPF are used to control the self-collimated beam. Antireflection structures are also employed at the input and output photonic crystal interfaces in order to minimize the coupling loss. Reflectance of the line-defect beam splitters can be controlled by adjusting the radius of defect rods. As the reflectance of the line-defect beam splitters increases, the transmission peaks become sharper and the filter provides a Q-factor as high as 1037. Proposed RFPF can be used as a sharply tuned optical filter or as a spectrum analyzer based on the self-collimation phenomena of photonic crystals. Furthermore, it is suitable for a building block of photonic integrated circuits, as it does not back reflect any of the incoming self-collimated beams owing to the antireflection structure applied.

Experimental demonstration of bending and splitting of self-collimated beams in two-dimensional photonic crystals

The authors have experimentally demonstrated the bending and splitting phenomena of self-collimated microwave beams in a two-dimensional square lattice photonic crystal composed of alumina rods. The bending and splitting were achieved by introducing a line defect in the photonic crystal. The power ratio of two split beams can be controlled by varying the radii of rods in the line defect.

Terahertz modulation on angle-dependent photoexcitation in organic-inorganic hybrid structures

The characteristics of terahertz (THz) modulation in organic copper phthalocyanine thin films deposited on a Si wafer were investigated by angle-dependent photoexcitation. We reveal that the efficiency of THz modulation reflects not only the angle-dependent reflectivity of the organic thin films that undergo a change of complex refractive index due to photoexcitation but also the laser-induced birefringence induced by a charge density grating in the direction normal to the polarization of the excitation beams.