Kwang-Ho Shin

Study on magnetophotonic crystals for use in reflection-type magneto-optical spatial light modulators

A magneto-optical spatial light modulator (MOSLM) is a real-time programmable device for modulating the amplitude and/or phase of two-dimensional optical signals at a high speed. Recently, we reported the development of the transmission-type MOSLM (T-MOSLM) based on a one-dimensional magnetophotonic crystal (MPC) with a high driving performance. However, further improvement of the T-MOSLM is required. The structure of the T-MOSLM has low optical efficiency due to weakening of transmittance through electrodes; furthermore, the fabrication of the driving electrodes is a very complicated process. In this study, we suggest a new design of the MPC for a reflection-type MOSLM (R-MOSLM). The idea of a reflection-type MPC (R-MPC) is to replace one of the Bragg mirrors with an aluminum reflection film. The structure is glass∕(Ta2O5∕SiO2)k∕Bi:YIG∕Al, where k is the repetition number. The approach of the optimal structure of the R-MPC was done by the calculation of the matrix approach method. In the calculation of t...

Magnetic field sensors using magnetophotonic crystals

In this work, we investigated the possibility of application of magnetophotonic crystals to the optical magnetic field sensor. The structure of 1D-MPC was (Ta2O5/SiO2)5/Bi:YIG/ (Ta2O5/SiO2)5 (magnetic material as a defect layer between two Bragg reflectors) on a fused quarts substrate using RF magnetron sputtering apparatus. We used Bismuth substituted yttrium iron garnet (Bi:YIG) polycrystal film as a defect layer, because Bi:YIG is well known as the magnetic material with effective MO properties, even if it is polycrystal. Due to specially designed structure, the localized mode appeared at the wavelength of 880 nm, which is tunable by the thickness of multi layers or defect layer. At the wavelength of localized mode, Faraday rotation was shown large enhancement of 1.5°, that is fifty times larger than for single Bi:YIG polycrystal film of the same thickness.

Enhancement of modulation rate of magneto-optical spatial light modulators with magnetophotonic crystals

Thin-film-type magneto-optical spatial light modulator has been developed by combining one-dimensional magnetophotonic crystal (MPC) and micropatterned drive lines. The use of MPC is expected to be effective because the apparent Faraday effect, which is directly proportional to pixel contrast, is largely enhanced by the light localization of light in the vicinity of the magnetic defect in magnetophotonic crystal. The magnetophotonic crystal had a multilayer structure of (Ta2O5∕SiO2)k∕Bi: yttrium iron garnet∕(SiO2∕Ta2O5)k formed on a glass substrate, where k is the repetition number and one of optimization parameters in this study. The transmittance T decreased with an increase of k, while Faraday rotation θF increased, indicating that the enhancement of light modulation rate would be achieved by optimizing the figure of merit F defined by F≈Tsin2θF. At k=6, the maximum F=8.8 was obtained, which is about 20 times larger than that of single layer structure.

Annealing Temperature Dependence of Magnetic and Optic Properties of Bi:YIG Films Deposited with Aerosol Deposition Method

Bismuth-substituted yttrium iron garnet (Bi:YIG, Bi 0.5 Y 2.5 Fe 5 O 12 ) films were deposited with aerosol deposition method and their magnetic and optical properties were investigated as a function of annealing temperature. Since the ceramic films deposited with aerosol deposition method have not a perfect crystal structure due to non-uniform internal stress o㏄urred by mechanical collision during their deposition, the post annealing could be a key process to release its internal stress and to improve its micro structure for optimizing the magnetic and magneto-optic properties of films. The crystallinity of Bi:YIG film was improved with increase of annealing temperature, and the saturation magnetization increased up to 87 emu/㏄ at 800℃. The Faraday rotation increased up to 1.4 deg/ ㎛ by annealing at 700℃ around the wavelength of 0.5 ㎛. The optical transmittance of the Bi:YIG film was also improved in visible region.

Magnetostrictive properties of polymer-bonded Fe-Co based alloy composites

Polymer-bonded composites of a Fe–Co based alloy are fabricated and their magnetic and mechanical properties are characterized. The use of flake-type powders with a thickness of 3–5 μm, instead of conventional spherical powders, is the key to good magnetostrictive properties and this is made possible by suitable design of the alloy (Fe36Co62Ge2) and the development of a pulverization process. The composites exhibit high magnetostriction of 105 ppm measured at an applied field of 4.3 kOe (70 ppm at 1 kOe). Large compressive strength of 42 MPa is also obtained for the composites, indicating solid bonding of flake-type powders by the polymer.

Mesoporous photonic crystals for sensor applications

Optical properties of mesoporous three-dimensional photonic crystals (3D PCs) based on thin opal films were found to be extremely sensitive to a humidity of the surrounding air. It was manifested that the internal structure of a single SiO2 sphere together with the net of voids between them in a thin opal film acts as a sponge for wet steams. Our experimental data have shown that hydrophilic internal structure of a mesoporous film sponges up (and lose) water (dry or wet steams) that influences dielectric permittivity, the latter causes significant changes in transmission spectra. High sensitivity, quick response and possibility of contactless measurements makes sensors based on optical effects in mesoporous PCs to be very promising. It concerns not only humidity sensors, but also sensors of various gases, temperature, deformation and other environmental impacts.

Anisotropy control of magnetostrictive film patterns

The magnetic properties of Fe-based amorphous films are thought to be highly sensitive to strain due to the large saturation magnetostriction and soft magnetic properties exhibited by the films. This is why patterned FeCoSiB films exhibit a large variation in their high-frequency impedance when strain is applied. To increase the sensitivity of strain sensors, it is very important to control the magnitude of magnetic anisotropy. We propose a method to control the magnetic anisotropy by stress bias annealing. By applying stress during annealing, we were able to control the anisotropy along the width of the patterned film. A strain sensor using a transverse-anisotropy-induced film pattern exhibited a figure of merit F/spl ap/8300, while a sensor without anisotropy exhibited F of only 500.

Optimization of Operation Frequency of Orthogonal Fluxgate Sensor Fabricated with Co Based Amorphous Wire

We present how to optimize the operation condition including frequency of the orthogonal fluxgate sensor in this paper. The orthogonal fluxgate sensor was fabricated with a Co-based amorphous wire with 10 mm long and 100 μm in the diameter and a 270-turn pickup coil wound on the amorphous wire. In order to investigate the frequency dependence of the sensitivity, output spectra of the sensor which was connected by using a coaxial cable with various lengths of 0.5-5 m were measured with a RF lock-in amplifier. The maximum sensitivities were obtained at different frequencies according to coaxial cable lengths. It was found that the optimal operation frequencies, at which maximum sensitivities were appeared, were almost identical to the frequencies of impedance resonance. The maximum sensitivity and optimal operation frequency were 1.1 V/Oe (≈ 11000 V/T) and 1.25 MHz respectively.

Efficient Cooling Method for a Cu Coil in an Induction Cooker by Using an Insulation Sheet

A silica aerogel sheet with a very low thermal conductivity is used to suppress the temperature increase of the Cu coil in an induction cooker by reducing the heat flow from the heat source (cooking pot). It is found that the temperature of the Cu coil is reduced significantly by the insertion of an insulation sheet between the heat source and the Cu coil, demonstrating the effectiveness of the insulation sheet in the suppression of the heat flow between the cooking pot and the coil. Furthermore, the temperature of the cooking pot increases more rapidly with the use of the insulation sheet, allowing for an increased efficiency of the induction cooker.

Efficiency of Exponential Deperm Protocol

Magnetic treatment of surface vessels and submarines (Deperm) is required to camouflage them against magnetic detection from enemy marine force. So far, deperm has been accomplished by applying an alternating magnetic field of which amplitude decreases linearly. However, the reduction of the residual flux density in the direction of magnetic field is not linear in the case of the linear protocol, since the ferromagnetic material used to construct a surface vessel, mainly Fe-C, shows a nonlinear behavior in an alternating magnetic field. This is one of main reasons to make an ordinary deperm protocol inefficient. In this paper, we propose the exponential deperming protocol and compare the exponential protocol to conventional linear protocol within the framework of deperm performance. We found out that step number could be reduced in the exponential protocol compare with in the linear protocol, because the larger numbers of deperm steps are dedicated in the irreversible domain process region on the magnetic hysteresis.