Dong Kim

Magneto-Optical Thin Films for On-Chip Monolithic Integration of Non-Reciprocal Photonic Devices

Achieving monolithic integration of nonreciprocal photonic devices on semiconductor substrates has been long sought by the photonics research society. One way to achieve this goal is to deposit high quality magneto-optical oxide thin films on a semiconductor substrate. In this paper, we review our recent research activity on magneto-optical oxide thin films toward the goal of monolithic integration of nonreciprocal photonic devices on silicon. We demonstrate high Faraday rotation at telecommunication wavelengths in several novel magnetooptical oxide thin films including Co substituted CeO2−δ, Co- or Fe-substituted SrTiO3−δ, as well as polycrystalline garnets on silicon. Figures of merit of 3~4 deg/dB and 21 deg/dB are achieved in epitaxial Sr(Ti0.2Ga0.4Fe0.4)O3−δ and polycrystalline (CeY2)Fe5O12 films, respectively. We also demonstrate an optical isolator on silicon, based on a racetrack resonator using polycrystalline (CeY2)Fe5O12/silicon strip-loaded waveguides. Our work demonstrates that physical vapor deposited magneto-optical oxide thin films on silicon can achieve high Faraday rotation, low optical loss and high magneto-optical figure of merit, therefore enabling novel high-performance non-reciprocal photonic devices monolithically integrated on semiconductor substrates.

Visualization study on the transient liquid film behavior and inner gas flow after rupture of a soap bubble

This study examined the transient behavior of liquid films and the flow of inner gas. Olive oil particles were inserted into a soap bubble through a Laskin nozzle for visualization, and the inner gas flow fields were measured by time-resolved particle image velocimetry. A pulse laser was used for contactless rupturing of the soap bubble. The transient behavior of the liquid film after the soap bubble ruptured was captured using a high-speed camera at 3,600 frames per second. After rupturing the soap bubble, the inner gas flowed out to the outside through the crack. This is called the primary flow. The removal velocity of the upper liquid film was faster than that of the bottom liquid film. The Kelvin–Helmholtz vortex was generated at the upper and bottom boundaries of the liquid film. A series of Kelvin–Helmholtz vortices, which arise in shear flow along a contact discontinuity, were formed around the bubble sphere. Secondary flow was generated due to a change in momentum after impinging the soap film to a point, and was faster than primary flow.Graphical Abstract

Numerical simulation on the opto-electro-kinetic patterning for rapid concentration of particles in a microchannel

This paper presents a mathematical model for laser-induced rapid electro-kinetic patterning (REP) to elucidate the mechanism for concentrating particles in a microchannel non-destructively and non-invasively. COMSOL(®)(v4.2a) multiphysics software was used to examine the effect of a variety of parameters on the focusing performance of the REP. A mathematical model of the REP was developed based on the AC electrothermal flow (ACET) equations, the dielectrophoresis (DEP) equation, the energy balance equation, the Navier-Stokes equation, and the concentration-distribution equation. The medium was assumed to be a diluted solute, and different electric potentials and laser illumination were applied to the desired place. Gold (Au) electrodes were used at the top and bottom of a microchannel. For model validation, the simulation results were compared with the experimental data. The results revealed the formation of a toroidal microvortex via the ACET effect, which was generated due to laser illumination and joule-heating in the area of interest. In addition, under some conditions, such as the frequency of AC, the DEP velocity, and the particle size, the ACET force enhances and compresses resulting in the concentration of particles. The conditions of the DEP velocity and the ACET velocity are presented in detail with a comparison of the experimental results.

Two-dimensional thermographic phosphor thermometry in a cryogenic environment

In this study, lifetime-based thermographic phosphor thermometry was developed for 2D temperature measurements in a cryogenic temperature environment. A chamber was set up to provide such an environment with temperatures of 300–110 K and accuracy of ±3.5 K. Mg4FGeO6:Mn was used as a sensor material, which was excited by a pulsed UV LED. A high-speed camera with a frequency of 8000 Hz was used for the phosphor thermometry. Calibration was performed at temperatures ranging from 110 to 290 K. The calibration results clearly show variation in the lifetime at different temperatures, and the calibration error is within 1.7%. This measurement is demonstrated in a 2D temperature measurement of an aluminum plate with a heater for both steady and unsteady heat transfer conditions. The measurement results were compared with thermocouple measurements to validate the method.

Effect of surface moisture on chemically bonded phosphor for thermographic phosphor thermometry

This study examined the effect of surface moisture on the calibration lifetime in chemically bonded phosphor paint preparation. Mg4FGeO6:Mn was used as a sensor material, which was excited by a pulsed UV LED. A high-speed camera with a frequency of 8000 Hz was used to conduct phosphor thermometry. Five samples with different degrees of surface moisture were selected during the preparation process, and each sample was calibrated 40 times at room temperature. A conventional post-processing method was used to acquire the phosphorescent lifetime for different samples with a 4 × 4-pixel interrogation window. The measurement error and paint uniformity were also studied. The results showed that there was no obvious phosphorescence boundary between the wet parts and dry parts of phosphor paint. The lifetime increased by about 0.0345% per hour during the preparation process, showing the degree of surface moisture had almost no influence on the lifetime measurement. The lifetime changed only after annealing treatment. There was also no effect on the measurement error and uniformity. These results provide a reference for developing a real-time measurement method using thermographic phosphor thermometry. This study also provides a feasible basis for chemically bonded phosphor thermometry applications in humid and low-temperature environments.

Numerical simulation on the formation of a toroidal microvortex by the optoelectrokinetic effect

In this study, the formation of a toroidal microvortex by optoelectrokinetic effect was numerically simulated using COMSOL v4.2a multiphysics software. AC voltage was applied to the two parallel electrodes in a microchannel to generate temperature gradient in the fluids. In addition to the AC electrothermal effect, local heating by a laser illumination was also considered. Numerical simulations were conducted for dielectric fluids. The toroidal microvortex induced by the optoelectrokinetic effect shows that two counter-rotating vortices are produced above the bottom electrodes. Fluid motions in the middle of bottom boundary are cancelled out by flows in opposite directions and consequently producing stagnation. It is expected that micro/nano particles are deposited in bottom electrode. Local heating enhanced the intensity of microvortex substantially due to the additional temperature gradient, it was confirmed that the AC electrothermal effect with laser illumination can be used for rapid concentration of micro/nano particles in the spot area.

Numerical Simulation on the Formation of a Toroidal Microvortex by the Optoelectrokinetic Effect

Optoelectrokinetic effects were effectively used for rapid concentration of particles in microfluidics. In this study, we clarified detail mechanism of particle aggregation by numerical simulation using COMSOL v4.2a multiphysics software. A 3D simulation was conducted with axisymmetric boundary conditions. AC voltage was applied to the two parallel electrodes in a microchannel to generate temperature gradient in the fluids. In addition to the AC electrothermal (ACET) effect, local heating by a laser illumination was also considered. Numerical simulations were carried out for dielectric fluids. A toroidal microvortex induced by the optoelectrokinetic effect shows that fluid motions in the middle of bottom boundary are cancelled out by flows in opposite directions and consequently producing stagnation. It is expected that micro/nano particles can be deposited in the bottom electrode. Local heating by the laser illumination enhanced the intensity of microvortex substantially. It is confirmed that the dominant driving force for the microvortex is natural convection by the laser illumination, however AC voltage is necessary for particle aggregation in the spot area.Copyright