Seung Beom Kang

A terahertz metamaterial with unnaturally high refractive index

Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial ‘atoms’ has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.

Fiber link loss monitoring scheme in bidirectional WDM transmission using ASE-injected FP-LD

We propose and demonstrate a novel fiber link loss monitoring method for a wavelength-division-multiplexed (WDM) passive optical network. This method is based on the optical signal comparison between a reference amplified spontaneous emission signal and its reflection from an optical reflector. Therefore, we can calculate the link loss exactly and also measure the characteristics of upstream optical signals without suspending the in-service channels. Since this scheme does not use narrow-band wavelength-dependent components such as fiber Bragg gratings, it is cost-effective in implementation and maintenance within a real optical network. We experimentally demonstrate that the link quality of upstream signals as well as fiber link faults on all WDM channels can be monitored in real time

Terahertz dielectric response of ferroelectric Ba x Sr 1¿x TiO 3 thin films

Terahertz time-domain spectroscopy has been used to investigate the dielectric and optical properties of ferroelectric Ba_xSr_1-xTiO₃ thin films for nominal x-values of 0.4, 0.6, and 0.8 in the frequency range of 0.3 to 2.5 THz. The ferroelectric thin films were deposited at approximately 700 nm thickness on [001] MgO substrate by pulsed laser deposition. The measured complex dielectric and optical constants were compared with the Cole-Cole relaxation model. The results show that the Cole-Cole relaxation model fits well with the data throughout the frequency range and the dielectric relaxation behavior of ferroelectric Ba_xSr_1-xTiO₃ thin films varies with the films compositions. Among the compositions of Ba_xSr_1-xTiO₃ films with different Ba/Sr ratios, Ba_0.6Sr_0.4TiO₃ has the highest dielectric constants and the shortest dielectric relaxation time.

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.

Dielectric Characteristics of Pb(Zr, Ti)O3 Films on MgO Single Crystal Substrate by Terahertz Time Domain Spectroscopy

Lead zirconium titanate (PbZr0.52Ti0.48 O3 and PbZr0.30Ti0.70O3) films were prepared on MgO (100) substrate by the sol-gel method. Film Thickness, microstructure and crystalline structure of the films were investigated by scanning electron microscope (SEM) and X-ray diffraction analysis, respectively. Terahertz time-domain spectroscopy has been used to investigate the dielectric properties of ferroelectric Pb(Zr, Ti)O3 thin films in the frequency range of 0.2 to 2.0 THz.

Guided Wave THz Spectroscopy of Explosive Materials

One of the important applications of ㎔ time-domain spectroscopy (TDS) is the detection of explosive materials through identification of vibrational fingerprint spectra. Most recent ㎔ spectroscopic measurements have been made using pellet samples, where disorder effects contribute to line broadening, which results in the merging of individual resonances into relatively broad absorption features. To address this issue, we used the technique of parallel plate waveguide (PPWG) ㎔-TDS to achieve sensitive characterization of three explosive materials: TNT, RDX, and HMX. The measurement method for PPWG ㎔-TDS used well-established ultrafast optoelectronic techniques to generate and detect sub-picosecond THz pulses. All materials were characterized as powder layers in 112 ㎛ gaps in metal PPWG. To illustrate the PPWG ㎔-TDS method, we described our measurement by comparing the vibrational spectra of the materials, TNT, RDX, and HMX, applied as thin powder layers to a PPWG, or in conventional sample cell form, where all materials were placed in Teflon sample cells. The thin layer mass was estimated to be about 700 ㎍, whereas the mass in the sample cell was ～100 ㎎. In a laboratory environment, the absorption coefficient of an explosive material is essentially based on the mass of the material, which is given as: a(w)=[ln(I R (w)I S (w))]/m. In this paper, we show spectra of 3 different explosives from 0.2 to 2.4 ㎔ measured using the PPWG THz-TDS.

Increase of Current Limiting Capacity of SFCLs by Using Matrix-Type SFCL Module

We report the increase of the current limiting capacity of SFCLs by using the basic 1 × 3 matrix module with 3 superconducting elements. To do this, we fabricated the basic superconducting module with the 1 × 3 matrix structure which consists of a trigger part and current limiting parts. Firstly, we analysed the current limiting characteristics of the basic superconducting module with the 1 × 3 matrix structure. Secondly, we investigated characteristics about the increase of the current limiting capacity of 2 × 3 matrix-type SFCLs and 3 × 3 matrix-type SFCLs which were connected in parallel with 2 or 3 basic 1 × 3 modules. From this procedure, we approved that the easy increase of the current limiting capacity of SFCLs could be realized throughout the simply parallel connection of basic 1 × 3 modules without complex wiring. We used YBCO superconducting thin films and constructed matrix-type SFCLs by using 9 superconductors and 3 reactors. Experimental results were reported in terms of the increase characteristics of the limiting capacity for fault currents and fault voltages.

Dielectric Characteristics of Co Doped ZnO Thin Films at Terahertz Frequencies

We report the complex dielectric characteristics of the Co doped ZnO thin films by terahertz time-domain spectroscopy. The Co doped ZnO thin films are prepared by sol-gel spin coating process on glass substrate. The crystal structure and morphology of the Zn1-xCoxO films are characterized by high resolution X-ray diffraction and scanning electron microscopy, respectively. The Zn1-xCoxO thin film with 10 at.% Co concentration exhibits highly c-axis orientation and the lowest electrical resistivity. The measured THz pulse and the complex dielectric constant of Zn0.8Co0.2O film exhibit different behavior to others due to the decrease of the crystallinity of the film.

All-optical wavelength conversion for 20-Gb/s RZ format data

We have successfully demonstrated an all-optical wavelength conversion scheme for high-speed return-to-zero (RZ) format data input by using a semiconductor-optical-amplifier (SOA) converter incorporated with a stabilized asymmetric Mach-Zehnder interferometer (AMZI). A bit-error-rate penalty of the converted signals less than 1.2 dB compared to that of the back-to-back signals was achieved for the entire wavelength conversion range from 1530 to 1560 nm with an AMZI. In this scheme, the conversion speed faster than the carrier recombination rate of the SOAs was possible. Noninverted wavelength conversion of 20-Gb/s RZ format data signals has been achieved.

Detection of Explosive RDX using Parallel Plate Waveguide THz-TDS

In this paper we presented the detection of the explosive material RDX using a parallel plate waveguide (PPWG) THz time domain spectroscopy (TDS). Normally the explosive materials have been characterized through identification of vibrational fingerprint spectra. Until now, most of all THz spectroscopic measurements have been made using pellet samples where disorder effects contribute to line broadening such that individual resonances merge into relatively broad absorption features. In order to avoid such disadvantages we used the technique of PPWG THz-TDS to achieve sensitive characterization of explosive material RDX. The PPWG THz-TDS used in this work well established ultrafast optoelectronic techniques to generate and detect sub-picosecond THz pulses. The explosive material was analyzed as powder layers in gap of metal PPWG. The thin later mass was estimated to be about . Finally, we showed spectra of explosives from 0.2 to 2.4 THz measured using PPWG THz-TDS.