Mun-Cheol Paek

Holographic polarization-selective module based on a small Dove prism coupler for magneto-optical pickup heads

A new structure for polarization-selective elements, consisting of two holographic gratings and a Dove prism coupler, is proposed. The absence of a multistage waveguide and the benefits of compact size and lightweight volume are the outstanding features of the new structure. Based on the coupled-wave theory, the analysis and design of the structure are discussed in detail to calculate the required index modulation. Several parameters, such as the recording intensity, the exposure time, and the recording angles for the fabrication of the proposed element, are determined. Under these conditions, the element is fabricated in Dupont photopolymer HRF-150-38 material and with an operating wavelength of 532 nm. A simplified pickup head is constructed to evaluate the performance of the fabricated element.

Characterization of Aluminum Nitride Thin Films on Silicon Substrates Grown by Plasma Assisted Molecular Beam Epitaxy

The growth characteristics and microstructure of AlN thin films grown by plasma assisted molecular beam epitaxy on Si substrates were investigated by comparing structure differences between AlN/Si(100) and AlN/Si(111). A model for the growth of AlN crystals on Si(100) and Si(111) substrate is proposed and the growth behavior of AlN film on single crystalline silicon substrate has been characterized. The difference in morphologies and microstructures between AlN/Si(100) and AlN/Si(111) can be explained by crystalline coherency at the interface between the AlN film and the Si substrate. While the c-axis direction of the wurtzite AlN [0001] on silicon substrates are the same, the coincidence of the radial direction of each of the AlN crystallites is dependent on the coherency of the interface of AlN/Si(100) and AlN/Si(111). Thus, although each AlN column on the Si(100) has a (0002) basal plane as a bottom of the column, their radial directions do not coincide and show a columnar growth. However on Si(111), each AlN column has directional coherency and therefore are able to combine together to form a single crystallite, and show epitaxial growth. The difference in surface morphology between AlN/Si(100) and AlN/Si(111) is clearly the result of the difference of the interfacial structures.

A model for the growth of AlN films on silicon substrates by plasma-assisted molecular beam epitaxy

A model for the growth of aluminum nitride (AIN) films on Si(100) and Si(111) substrates by plasma-assisted molecular beam epitaxy is proposed. AIN thin films were grown in a c-axis preferred orientation irrespective of Si substrates. AIN films grown on Si(100) substrates, having no lattice coherency with AlN, showed a columnar structure and each column was rotated by a uniform angle in a direction lateral to the substrate. AIN films grown on Si(111) having lattice coherency showed an epitaxial relationship for AlN[0001]//Si[111] in a lateral direction to the substrate. Based on a comparison between experimental results and a schematic lattice diagram of AlN/Si, the model for the growth of AlN on Si is proposed.

FORMATION AND EPITAXIAL-GROWTH OF TITANIUM-DISILICIDE ON SI (111)

Titanium-disilicide was formed by deposition of high purity titanium (99.99%) on a silicon (111) wafer in ultra-high vacuum and in-situ annealing. The deposition was monitered using reflection high energy electron diffraction as a function of deposited titanium thickness. The epitaxial growth of TiSi2 on Si (111) by deposition of 200 monolayer thick titanium on the clean surface of silicon and annealing at 750°C for 1 h was identified using a transmission electron microscope. Almost the whole area of the silicide layer was revealed as epitaxial TiSi2 with C54 structure. Cross-sectional high resolution images of the sample showed that the TiSi2/Si interface is incoherent. Three kinds of orientation relationships were identified as (111)TiSi2|(220)Si, [123]TiSi2|[112]Si, (311)TiSi2|(002)Si, [136] TiSi2|[110]Si and (022)TiSi2|(200)Si, [233]TiSi2|[011]Si with misorientations of 6°, 2° and 14.74°, respectively.

Structural evolution of Fe80C20 alloy with alloying times

The structural evolution of mechanically alloyed Fe-C alloys was studied as a function of alloying times. The effect of alloying time on local structural changes of Fe-C has been investigated by means of Fe57 Mossbauer spectrometry, extended x-ray-absorption fine structure (EXAFS), and x-ray diffraction (XRD). XRD pattern from 24h alloyed Fe-C powder indicates at least the mixture of bcc-Fe and Fe3C phases. Mossbauer spectra analysis reveals that bcc-Fe decreases to the detriment of Fe3C phase with increasing alloying time, while both carbon-containing bcc-Fe and amorphouslike phase assigned to Fe located in grain boundaries (estimated at two atomic layers) remain alloying time independent. The variation of Fe3C phase content is in a good agreement with that observed by EXAFS analysis.

Synthesis and Characterization of Sodium Tungsten Oxide (Na x WO3, 0 < x < 1) Nanobundles

Sodium tungsten oxide nanobundles are synthesized by acid precipitation method with the ammonium tungstate and sodium hydroxide. The crystal structure, morphology and optical properties of the sodium tungsten oxide (NaxWO3, x = 0.33, 0.5, 0.75, and 1.0) nanobundles are analyzed by X-ray diffraction, field-emission scanning electron microscopy, and UV-visible spectroscopy, respectively.

A Phased Array Antenna Using Ferroelectric CPW Phase Shifter Active Modules

A phased array antenna using ferroelectric coplanar waveguide (CPW) phase shifter active modules is presented based on a Ba0.6Sr0.4TiO3 (BST) thin film. The ferroelectric BST thin film was deposited on an MgO substrate by using pulsed laser deposition. The active module amplifies an RF signal with low noise and changes the phase of a received RF signal to control the main beam of the array antenna. It is composed of low noise amplifier (LNA), dc block, and ferroelectric CPW phase shifter. The prototype of 6×12 phased array patch antenna with the active modules using a ferroelectric phase shifter is proposed. The proposed array antenna continuously scanned its main beam in elevation plane to 32° from its broadside direction at 11.85 GHz by applying the continuous dc voltage to the ferroelectric phase shifter.

A DIELECTRIC PROPERTY ANALYSIS OF FERROELECTRIC THIN FILM USING TERAHERTZ TIME-DOMAIN SPECTROSCOPY

ABSTRACT This paper presents complex dielectric properties of ferroelectric BSTO thin film, deposited on MgO substrate by pulsed laser deposition, in the frequency range of 0.5 ∼ 3.0 THz using terahertz time-domain spectroscopy (THz-TDS). In order to extract complex dielectric properties of the thin film, multiple reflections within the thin film were considered and an error function between the calculated and measured data was introduced. The real part of the dielectric constant of the BSTO thin film was less than 170 and the dielectric loss tangent of the BSTO thin film varied between 0.9 and 3.0. All values of an error function were less than 7 × 10− 4 in the measured frequency ranges. The analysis method using THz-TDS enables us to find out complex dielectric properties of ferroelectric thin film exactly.

Diffraction limit of the focusing waveguide grating coupler for optical probe information storage

A focusing waveguide grating coupler (FWGC) has been applied to an optical information storage element. To achieve minimum diffraction, focusing and high-coupling efficiency with high power, several fabrication and design difficulties have to be overcome. In this work, we report some new strategies to improve the fabrication and design, and greatly improve the pattern quality in defining curved gratings in the nanometre range using single-step electron-beam lithography with a conventional exposure mode. We also present the optimum fabrication conditions for the FWGC and the minimum diffraction limit (1.1 µm) obtained for the structure, which is optically the smallest diffraction spot size obtainable from diffraction gratings focusing beams into a spot in air.

Raman scattering and terahertz-wave parametric generation with nonlinear optical crystal MgO:LiNbO 3

Polariton dispersion and THz-wave generation in MgO:LiNbO3 single crystal have been investigated by using Raman scattering and THz parametric generation system. The polariton dispersion curve was obtained on the basis of A1(z) mode of Raman scattering experiment. Wavelengths of the idler beam and the THz-wave were calculated using the phase matching condition with the dispersion curve. We have made a TPG system and directly measured the wavelength of the idler beam. The range of the phase matching angle in the TPG system was 0.35∼0.80°, and the frequency of the THz-wave corresponding to the pump and idler beam was measured to be in the range of 0.95∼2.30THz. They agreed well with those calculated by the polariton dispersion curve.