Doo Jae Park

Terahertz Wave Applications of Single-Walled Carbon Nanotube Films with High Shielding Effectiveness

We demonstrate that a filtration method is efficient for the fabrication of thick single-walled nanotube films and is capable of shielding terahertz waves. Shielding effectiveness can be engineered by controlling the film thickness and we achieved 38 dB for a 950-nm-thick film. In addition, we found that the films exhibit a dispersion of dielectric constant obeying the Drude free-electron model, whereas the plasma frequency decreases with increasing film thickness. Based on the nanotube films with a thickness greater than the skin depth, we fabricated grid polarizers by laser-machining process, which enable us to achieve a large polarization extinction ratio.

Suppressing spontaneous polarization of p-GaN by graphene oxide passivation: Augmented light output of GaN UV-LED

GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. Therefore, improving hole injection rate is a key step towards high performance UV-LEDs. Here, we report a new method of suppressing spontaneous polarization in p-type region to augment light output of UV-LEDs. This was achieved by simply passivating graphene oxide (GO) on top of the fully fabricated LED. The dipole layer formed by the passivated GO enhanced hole injection rate by suppressing spontaneous polarization in p-type region. The homogeneity of electroluminescence intensity in active layers was improved due to band filling effect. As a consequence, the light output was enhanced by 60% in linear current region. Our simple approach of suppressing spontaneous polarization of p-GaN using GO passivation disrupts the current state of the art technology and will be useful for high-efficiency UV-LED technology.

Epsilon-Near-Zero meta-lens for high resolution wide-field imaging

Herein, we will propose a new application possibility of epsilon-near-zero (ENZ) materials: high resolution wide-field imaging. We show that the resolution can be dramatically enhanced by simply inserting a thin epsilon-near-zero (ENZ) material between the sample and substrate. By performing metal half-plane imaging, we experimentally demonstrate that the resolution could be enhanced by about 47% with a 300-nm-thick SiO2 interlayer, an ENZ material at 8-μm-wavelength (1250 cm-1). The physical origin of the resolution enhancement is the strong conversion of diffracted near fields to quasi-zeroth order far fields enabled by the directive emission of ENZ materials.

Tip-Enhanced Raman Scattering Imaging of Two-Dimensional Tungsten Disulfide with Optimized Tip Fabrication Process

We successfully achieve the tip-enhanced nano Raman scattering images of a tungsten disulfide monolayer with optimizing a fabrication method of gold nanotip by controlling the concentration of etchant in an electrochemical etching process. By applying a square-wave voltage supplied from an arbitrary waveform generator to a gold wire, which is immersed in a hydrochloric acid solution diluted with ethanol at various ratios, we find that both the conical angle and radius of curvature of the tip apex can be varied by changing the ratio of hydrochloric acid and ethanol. We also suggest a model to explain the origin of these variations in the tip shape. From the systematic study, we find an optimal condition for achieving the yield of ~60% with the radius of ~34u2009nm and the cone angle of ~35°. Using representative tips fabricated under the optimal etching condition, we demonstrate the tip-enhanced Raman scattering experiment of tungsten disulfide monolayer grown by a chemical vapor deposition method with a spatial resolution of ~40u2009nm and a Raman enhancement factor of ~4,760.

Operation of a wet near-field scanning optical microscope in stable zones by minimizing the resonance change of tuning forks

A resonant shift and a decrease of resonance quality of a tuning fork attached to a conventional fiber optic probe in the vicinity of liquid is monitored systematically while varying the protrusion length and immersion depth of the probe. Stable zones where the resonance modification as a function of immersion depth is minimized are observed. A wet near-field scanning optical microscope (wet-NSOM) is operated for a sample within water by using such a stable zone.

Surface plasmon resonance extension through two-block metal-conducting polymer nanorods

Research on surface plasmon resonance coupling of metallic nanostructures is an important area in the field of plasmonics because distinctive collective optical properties can be realized that are different from the individual constituents. Here we report the localized surface plasmon resonance of hybrid metal-organic nanorods. Colloidal-dispersed Au-PPy nanorods were synthesized as a representative material using a modified electrochemical method, and the collective oscillation properties were systematically investigated by comparing these materials with pure Au nanorods. We observed the extended surface plasmon resonance of a hybrid system. The presence of doped-PPy segments on Au segments induced an enhanced coherent electric field due to the partial contribution of π-electrons on the PPy segment, which led to a red-shifted plasmon feature. Additionally, we demonstrated that surface plasmon resonance extension can be tuned by dopant anions, which demonstrates a way of tuning a dopant-induced plasmonic system.Localized surface plasmon resonance can be exploited for a range of applications, but remains difficult to tailor in metal-organic nanostructures. Here the authors synthesize gold-polypyrrole nanorods and observe a unique extended surface plasmon resonance, which they find to be tunable through doping.

Thickness-dependent enhancement of the optical resolution in the vicinity of an epsilon-near-zero slab

Recent studies have reported that an epsilon-near-zero (ENZ) thin slab between a specimen and a substrate contributes to enhancing the spatial resolution of the optical system. Here, we investigate the ENZ thickness dependence of the resolution enhancement. By employing the edge response function, we directly measure the resolution of an optical system when imaging the sharp edge of a metal film. We found that the optimum ENZ slab thickness was 700 nm and the achieved resolution was 11 μm at a wavelength of 8 μm. Owing to the enhanced resolution by ENZ slab, we successfully imaged subwavelength slit arrays.

Fabrication of plasmonic nanopore by using electron beam irradiation for optical bio-sensor

The Au nano-hole surrounded by the periodic nano-patterns would provide the enhanced optical intensity. Hence, the nano-hole surrounded with periodic groove patterns can be utilized as single molecule nanobio optical sensor device. In this report, the nano-hole on the electron beam induced membrane surrounded by periodic groove patterns were fabricated by focused ion beam technique (FIB), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Initially, the Au films with three different thickness of 40 nm, 60 nm, and 200 nm were deposited on the SiN film by using an electron beam sputter-deposition technique, followed by removal of the supporting SiN film. The nanopore was formed on the electron beam induced membrane under the FESEM electron beam irradiation. Nanopore formation inside the Au aperture was controlled down to a few nanometer, by electron beam irradiations. The optical intensities from the biomolecules on the surfaces including Au coated pyramid with periodic groove patterns were investigated via surface enhanced Raman spectroscopy (SERS). The fabricated nanopore surrounded by periodic patterns can be utilized as a next generation single molecule bio optical sensor.

Au cluster formation on a pore containing membrane under the various surface treatments

In this report, the authors will investigate the formation of Au clusters on the nanoscale membrane formed during various surface treatments such as electron beam irradiations, Ga ion focused ion beam (FIB) technique, and thermal treatment. Nanoapertures on the freestanding Au film were fabricated by using FIB technique, and a nanometer scale membrane created in the aperture by various surface treatments. Transmission electron microscopy reveals that Au clusters has formed on the membrane after the sample storage at room temperature for several months. In addition, Au clusters on the carbon-containing membrane were also observed after surface treatments of Ga ion beam etching, and thermal heating of freestanding 40u2009nm thick Au film at temperatures ranging from 400 to 800u2009°C. Spinodal decomposition, spinodal dewetting, and coalescence of the Au particles on the carbon-containing membrane were also observed.In this report, the authors will investigate the formation of Au clusters on the nanoscale membrane formed during various surface treatments such as electron beam irradiations, Ga ion focused ion beam (FIB) technique, and thermal treatment. Nanoapertures on the freestanding Au film were fabricated by using FIB technique, and a nanometer scale membrane created in the aperture by various surface treatments. Transmission electron microscopy reveals that Au clusters has formed on the membrane after the sample storage at room temperature for several months. In addition, Au clusters on the carbon-containing membrane were also observed after surface treatments of Ga ion beam etching, and thermal heating of freestanding 40u2009nm thick Au film at temperatures ranging from 400 to 800u2009°C. Spinodal decomposition, spinodal dewetting, and coalescence of the Au particles on the carbon-containing membrane were also observed.

Effects of Organic Thin Films on Local Resonance of Metamaterials under Photoexcitation

We demonstrate that the local resonance of metamaterials can be tuned by the effects of organic thin films under photoexcitation. Tris (8-hydroxyquinolinato) aluminum (Alq3) layers are deposited on metamaterial/silicon hybrid structures. By varying the thickness of the Alq3 layer on the subwavelength scale, the resonant peak of the metamaterial becomes very adjustable, due to the effect of a thin dielectric substrate. In addition, under photoexcitation all the spectral peaks of the resonance shift to higher frequencies. This originates from the reduction of the capacitive response generated inside the gaps of split-ring resonators. The adjustability of the electromagnetic spectrum may be useful for developing optical systems requiring refractive-index engineering and active optical devices.