Geunchang Choi

Plasmon Enhanced Terahertz Emission from Single Layer Graphene

Emission of terahertz radiation is observed when a single layer graphene on a glass slide is excited with femtosecond near-infrared laser pulses. Furthermore, we can enhance the amplitude of terahertz emission by surface plasmon resonance excitation on a thin gold film in the Kretschmann geometry.

Quantum dots-nanogap metamaterials fabrication by self-assembly lithography and photoluminescence studies.

We present a new and versatile technique of self-assembly lithography to fabricate a large scale Cadmium selenide quantum dots-silver nanogap metamaterials. After optical and electron microscopic characterizations of the metamaterials, we performed spatially resolved photoluminescence transmission measurements. We obtained highly quenched photoluminescence spectra compared to those from bare quantum dots film. We then quantified the quenching in terms of an average photoluminescence enhancement factor. A finite difference time domain simulation was performed to understand the role of an electric field enhancement in the nanogap over this quenching. Finally, we interpreted the mechanism of the photoluminescence quenching and proposed fabrication method of new metamaterials using our technique.

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.

Terahertz transmission through rings of quantum dots-nanogap

We report resonant funneling of terahertz (THz) waves through (9 ± 1) nm wide quantum dots-nanogap of cadmium selenide quantum dots silver nanogap metamaterials. We observed a giant THz intensity enhancement (~104) through the quantum dots-nanogap at the resonant frequency. We, further report the experimentally measured effective mode indices for these metamaterials. A finite difference time domain simulation of the nanogap enabled by the quantum dots supports the experimentally measured THz intensity enhancement across the nanogap. We propose that these low effective mode index terahertz resonators will be useful as bio/chemical sensors, gain-enhanced antennas, and wave guides.

High-power fiber lasers

We review and discuss the recent advances in high-power fiber lasers with particular attention to a multitude of laser configurations, including continuous-wave and pulsed regimes. In addition, we also discuss novel fiber technology for various specific applications.

Simple and reliable light launch from a conventional single-mode fiber into a helical-core fiber through an adiabatically tapered splice

We propose a simple and efficient light launch scheme for a helical-core fiber (HCF) by using an adiabatically tapered splice technique, through which we overcome its inherent difficulty with light launch owing to the large lateral offset and angular tilt of its core. We experimentally demonstrate single-mode excitation in the HCF in this configuration, which yields the coupling efficiency of around -5.9 dB (26%) for a ~1.1-μm light input when the splice joint is tapered down to 30 μm in diameter. To our knowledge, this is the first proof-of-principle report on the fusion-splice coupling between an HCF and a conventional single-mode fiber.

Terahertz Nanoprobing of Semiconductor Surface Dynamics

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Because of the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoprobing system to investigate the surface dynamics of bulk semiconductors, using metallic nanogap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (∼1 ps) as the gap size decreases down to nanoscale and that they return to their original values once the nanogap patterns are removed. Our THz nanoprobing system will open up pathways toward direct and nondestructive measurements of surface dynamics of bulk semiconductors.

Control of optical nanometer gap shapes made via standard lithography using atomic layer deposition

Abstract. Atomic layer deposition is an efficient method for coating a few nanometer-thick alumina over a wafer scale. This method combined with the standard photolithography process was presented to fabricate metallic nanometer gaps that optically act in terahertz regimes. However, the cross-sectional view of the gap shape of the metal–insulator–metal nanogap structure varies depending on the conditions from the stepwise procedure. In specific, selecting photoresist materials, adding ion milling and chemical etching processes, and varying metal thicknesses and substrates result in various optical gap widths and shapes. Since the cross-sectional gap shape affects the field enhancement of the funneled electromagnetic waves via the nanogap, the control of tailoring the gap shape is necessary. Thus, we present five different versions of fabricating quadrangle-ring-shaped nanometer gap arrays with varying different kinds of outcomes. We foresee the usage of the suggested category for specific applications.

Quantum dot nano gap metamaterial terahertz resonators

We present CdSe quantum dots nanogap metamaterial fabrication over large scale, resonant funneling of terahertz waves across 10 nm gap with giant terahertz intensity enhancements and quenching of photoluminescence of QDs inside the gap.

Direct amplification of a wavelength-tunable picosecond fiber laser with a large mode area erbium-doped fiber

We present the direct amplification of a wavelength-tunable picosecond fiber laser operating in the 1.5 μm region by core-pumping a large mode area erbium-doped fiber. The system can deliver peak powers and pulse energies of ~ 11 kW and 106 nJ, respectively.