• Journal of the Korean Physical Society
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• v.73 no.10
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• pp.1502-1505
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• 2018

We present a design for hybrid circular Bragg gratings (hCBGs) for efficiently extracting single-photons emitted by InAs quantum dots (QDs) embedded in GaAs. Finite-difference time-domain simulations show that a very high photon collection efficiency (PCE) up to 96% over a 50 nm bandwidth and pronounced Purcell factors up to 19 at cavity resonance are obtained. We also systematically investigate the geometry parameters, including the $SiO_2$ thickness, grating period, gap width and the central disk radius, to improve the device performances. Finally, the PCEs and the Purcell factors of QDs located at different positions of the hCBG are studied, and the results show great robustness against uncertainties in the location of the QD.

• Journal of Sensor Science and Technology
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• v.32 no.1
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• pp.62-65
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• 2023

The wave mode calculation of a large-scale optical system in comparison to the working wavelength is practically impossible because the computational cost increases exponentially. In this paper, we propose a method that can obtain the optical mode in a large-scale optical system. The method carries out simulations by dividing the calculation area into blocks and moving along the light axis along which the light propagates. By applying this method to the calculation of resonant modes in a ring-type optical resonator, which is mainly used for ring laser optical gyro sensors, the efficiency of the proposed method was verified.

• Current Optics and Photonics
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• v.7 no.5
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• pp.496-503
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• 2023

We present a detailed analysis of the light extraction efficiency (LEE) of a core-shell nanorod light emitting diode (LED) using finite-difference time-domain (FDTD) simulations. We found that the LEE has a deep dependence on source positions and polarization directions based on the calculated LEE results for every x and z position inside the core-shell nanorod structure. The LEEs are different for the upper part (pyramid) and the lower part (sidewall) of the core-shell nanorod owing to total internal reflection (TIR) and the generated optical modes in the structure. As a result, the LEE of sidewall is much larger than that of pyramid. The averaged LEE of the core-shell nanorod LED is also investigated with variable p-GaN thickness, n-GaN thickness, and height for the design guidelines for the optimized LEE of core-shell nanorod LEDs.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.9 no.6
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• pp.761-767
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• 1998

The Perfectly Matched Layer(PML) provide good performance in absorption over a wide frequency range and is an appropriate ABC for waveguides with high dispersion. In this paper, a novel algorithm is proposed to improve the computational efficiency of the PML. In the input and output ports, the fields are decomposed into a series of modes, and then an appropriate ABC is applied to each mode. CPU time and memory storage requirements are greatly reduced, since the computational region is analyzed in one dimension. A WG-90 rectangular waveguide with a thick asymmetric iris is analyzed by Finite-Difference Time-Domain(FDTD) simulations with the conventional PML and the proposed one-dimensional (1-D) PML. Numerical results show that the computational efficiency is significantly improved by the proposed method.

• Current Photovoltaic Research
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• v.5 no.2
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• pp.55-58
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• 2017

We investigated the influences of dielectric thin film coating on the optical characteristics of c-Si wafers with nanocone (NC) arrays using finite-difference time-domain (FDTD) simulations. Dielectric thin films on high-refractive-index surface can lower optical reflection and reflection dips appear at the wavelengths where destructive interference occurs. The optical reflection of the NC arrays was lower than that of the dielectric-coated planar wafer in broad wavelength range. Remarkable antireflection effects of the NC array could be attributed to beneficial roles of the NCs, including the graded refractive index, multiple reflection, diffraction, and Mie resonance. Dielectric thin films modified the optical reflection spectra of the NC arrays, which could not be explained by the interference alone. The optical properties of the dielectric-coated NC arrays were determined by the inherent optical characteristics of the NC arrays.

• Current Photovoltaic Research
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• v.4 no.2
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• pp.64-67
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• 2016

We investigated the influences of Ag nanoparticle (NP) arrays and surface nanohole (NH) patterns on the optical characteristics of 10-${\mu}m$-thick c-Si wafers using finite-difference time-domain (FDTD) simulations. In particular, we comparatively studied the plasmonic effects of both monomer arrays (MA) and heptamer arrays (HA) consisting of identical Ag NPs. HA improved the optical absorption of the c-Si wafers in much wider wavelength range than MA, with the help of hybridized plasmon modes. The light trapping capability of the NH array pattern is superior to that of the Ag plasmonic NPs. We also found that the addition of the Ag HA on the wafers with surface NH patterns further enhanced optical absorption: the expected short-circuit current density was as high as $34.96mA/cm^2$.

• Journal of Biomedical Engineering Research
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• v.31 no.1
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• pp.20-26
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• 2010

The aim of this study was to test the feasibility of a wireless transceiver that can be inserted into the ear canal. The wireless technology could minimize the cosmetic problems of patients, and it can be applied to binaural hearing aids for improving speech perception. In order to implement the ear canal insertable transceiver, simple finite-difference time-domain (FDTD) simulations were carried out to determine the feasibility, and the hardware of the transceiver was implemented within the ear shell. The size of the implemented transceiver was only $7{\times}7mm$, and it could successfully transmit signals to external devices. In order to measure the radiation pattern, a simple RF phantom was used, and the maximum attenuation from the phantom was observed to be 23 dB when the reference antenna was placed at a distance of 2 m from the transmitter.

• Korean Journal of Optics and Photonics
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• v.27 no.6
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• pp.229-232
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• 2016

Metamaterials operating in the terahertz frequency range show promising potential for use in highly sensitive microbial sensors that are capable of effectively detecting microorganisms in the ambient environment. We were able to detect extremely small numbers of microorganisms by measuring the differential transmission spectra (DTS) of the metamaterial resonances. This was possible because their sizes are on the same scale as the microgaps of the terahertz metamaterials. DTS depend critically on the number of microorganisms placed in the gap area, and their dielectric constant. In addition, these metamaterial microbial sensors are reusable, because the microorganisms can be completely removed by fungicide solution. Finite-difference time-domain simulations successfully reproduce our experimental data.

• Investigative Magnetic Resonance Imaging
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• v.7 no.2
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• pp.137-143
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• 2003

Purpose : B1 field of birdcage RF (radiofrequency) coil that is used most for brain imaging in magnetic resonance imaging (MRI) decreases toward endring from the coil center. We investigated how much RE B1 homogeneity effect the endcap shield brings form the coil center as it towards to endcap region. Materials and Methods : We compared RF B1 field distribution by each finite difference time domain (EDTD) simulations for lowpass, highpass and hybrid birdcage RF coils. We selected the highpass birdcage RF coil that was the highest RF B1 field condition as simulation result, and studied how much RF B1 homogeneity effect was occurred when endcap shield was applied to endring area. Results : B1 field of the highpass birdcage RF coil was higher than other birdcage RF coil types as simulation result. However, the RF B1 homogeneity was lower than other coil types. RE B1 field of highpass birdcage RF coil with endcap shield is similar with RF B1 field of hybrid birdcage RF coil and the overall RE B1 homogeneity in sagittal direction was better. Conclusion In this paper, proposed method can apply improving RF B1 homogeneity of RF coil in clinical examination.

• Journal of Biomedical Engineering Research
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• v.29 no.2
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• pp.99-106
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• 2008

To improve $B_1$ homogeneity in high field MRI, the RF power is applied to the transmit array coil elements sequentially in the time-multiplexed way. Since only a single coil element is activated in a time-multiplexing slot, the global standing wave formation in the human body is greatly suppressed. The time-multiplexing slot width is on the order of micro seconds, hence, high-order-harmonic slices can be placed far from the transmit coil and simultaneous multiple slice selection can be avoided. The $B_1$ homogeneities of a birdcage coil and an eight-channel transmit array coil have been compared through finite difference time domain simulations. The simulation results indicate that the proposed technique can reduce the peak-to-peak $B_1$ inhomogeneity down to one fourth of the transmission with a birdcage coil on the central plane of the human head model at 3 T. The mimicking experiments at 3 T, eight separate experiments with a single coil element activated and image reconstruction by combining the eight images, also show promising results. It is expected that the proposed technique has some advantages over other $B_1$ improving methods in real practice since simple RF switching circuitries are only necessary and electromagnetic coupling between the coil elements is out of concern in its realization.