Ho Seok Ee

Low-power nano-optical vortex trapping via plasmonic diabolo nanoantennas

Optical vortex trapping can allow the capture and manipulation of micro- and nanometre-sized objects such as damageable biological particles or particles with a refractive index lower than the surrounding material. However, the quest for nanometric optical vortex trapping that overcomes the diffraction limit remains. Here we demonstrate the first experimental implementation of low-power nano-optical vortex trapping using plasmonic resonance in gold diabolo nanoantennas. The vortex trapping potential was formed with a minimum at 170 nm from the central local maximum, and allowed polystyrene nanoparticles in water to be trapped strongly at the boundary of the nanoantenna. Furthermore, a large radial trapping stiffness, ~0.69 pN nm(-1) W(-1), was measured at the position of the minimum potential, showing good agreement with numerical simulations. This subwavelength-scale nanoantenna system capable of low-power trapping represents a significant step toward versatile, efficient nano-optical manipulations in lab-on-a-chip devices.

Surface-plasmon-induced light absorption on a rough silver surface

We investigate light absorption in metal films, silver and aluminum, with different surface roughness. Measurements using an integrating sphere show that the reflectance in silver decreases significantly with increasing surface roughness whereas the reflectance in aluminum is almost constant. The experimental results agree well with numerical simulations in which the surface roughness of metal is described properly. In particular, the simulations demonstrate that the absorption by surface-plasmon-polaritons excited on a rough silver surface causes the surface-dependent reflectance in silver. This study suggests a convenient and feasible rule to rationally design a backside metal reflector toward high-efficiency light-emitting diodes and photovoltaics.

Nonlinear mixing in nanowire subwavelength waveguides.

The realization of nonlinear photonic circuits to achieve the control of light-by-light is contingent upon a strong nonlinear response that can be captured in a guided-wave geometry. There remains a need to further scale down waveguides while maintaining a strong nonlinear response. In this study, we report second-harmonic generation and optical parametric generation using the second-order nonlinear response in an 80 nm thick CdS nanowire subwavelength waveguide. Moreover, our three-dimensional finite-difference time-domain (FDTD) simulations demonstrate that it is possible to enhance the coherence length due to the very nature of the subwavelength geometry. Nonlinear mixing in a nanowire subwavelength waveguide represents an advance toward all-optical processing and all-optical switching in integrated photonic circuits.

A double-strip plasmonic waveguide coupled to an electrically driven nanowire LED.

We demonstrate the efficient integration of an electrically driven nanowire (NW) light source with a double-strip plasmonic waveguide. A top-down-fabricated GaAs NW light-emitting diode (LED) is placed between two straight gold strip waveguides with the gap distance decreasing to 30 nm at the end of the waveguide and operated by current injection through the p-contact electrode acting as a plasmonic waveguide. Measurements of polarization-resolved images and spectra show that the light emission from the NW LED was coupled to a plasmonic waveguide mode, propagated through the waveguide, and was focused onto a subwavelength-sized spot of surface plasmon polaritons at the tapered end of the waveguide. Numerical simulation agreed well with these experimental results, confirming that a symmetric plasmonic waveguide mode was excited on the top surface of the waveguide. Our demonstration of a plasmonic waveguide coupled to an electrically driven NW LED represents important progress toward further miniaturization and practical implementation of ultracompact photonic integrated circuits.

High-efficiency vertical GaN slab light-emitting diodes using self-coherent directional emitters

We demonstrate a highly-efficient, large-area (1x1 mm2) GaN slab light-emitting diode using a vertically directional emitter produced from constructive interference. The vertical radiation can be coupled effectively into leaky modes from the beginning and thus a high-extraction efficiency can be expected with reduced material absorption. The far-field measurements show that the desired vertical emission profiles are obtained by varying the thickness of the dielectric layer between the emitter and bottom silver mirror. With the combination of a light extractor of a randomly textured surface, the output power was increased approximately 1.4 fold compared to a non-patterned device at a standard current of 350 mA without electrical degradation.

Modal Characteristics in a Single-Nanowire Cavity with a Triangular Cross Section

In this study, the modal characteristics of a single-GaN nanowire cavity with a triangular cross section surrounded by air or located on a silicon dioxide substrate have been analyzed. Two transverse resonant modes, transverse electric-like and transverse magnetic-like modes, are dominantly excited for nanowire cavities that have a small cross-sectional size of <300 nm and length of 10 microm. Using the three-dimensional finite-difference time-domain simulation method, quality factors, confinement factors, single-mode conditions, and far-field emission patterns are investigated for a nanowire cavity as a function of one length of the triangular cross section. The results of these simulations provide information that will be vital for the design and development of efficient nanowire lasers and light sources in ultracompact nanophotonic integrated circuits.

Metal mirror assisting light extraction from patterned AlGaInP light-emitting diodes

We demonstrate light extraction from metal reflector-based AlGaInP photonic crystal (PhC) light-emitting diodes (LEDs). The photons reflected by a high-reflectivity, small-absorption, bottom Ag mirror steadily interact with the PhC, and thus enhanced light extraction is achieved. The square lattice PhC patterns are fabricated on an upper n-doped AlGaInP surface with a depth of 500 nm. An optical power measurement using an integration sphere shows that the extraction efficiency of the PhC LED is ∼1.8 times larger than that of the nonpatterned LED. A three-dimensional finite difference time domain simulation is performed to understand the output enhancement extracted by the PhC and the effect of internal absorption.We demonstrate light extraction from metal reflector-based AlGaInP photonic crystal (PhC) light-emitting diodes (LEDs). The photons reflected by a high-reflectivity, small-absorption, bottom Ag mirror steadily interact with the PhC, and thus enhanced light extraction is achieved. The square lattice PhC patterns are fabricated on an upper n-doped AlGaInP surface with a depth of 500 nm. An optical power measurement using an integration sphere shows that the extraction efficiency of the PhC LED is ∼1.8 times larger than that of the nonpatterned LED. A three-dimensional finite difference time domain simulation is performed to understand the output enhancement extracted by the PhC and the effect of internal absorption.

Ultrasmall square-lattice zero-cell photonic crystal laser

We report room-temperature lasing action in an optically pumped heterogeneous square-lattice zero-cell photonic crystal cavity. Photoluminescence spectroscopy exhibits lasing at 1511 nm with a low lasing threshold of ~130 muW. We compute an ultrasmall mode volume of 0.017 mum3 ~ 1.7 (lambda/2nslab)3 using 3D-FDTD simulation.

Design of out-coupling structures with metal-dielectric surface relief

We propose an unconventional out-coupling structure consisting of two-dimensional periodic metal-dielectric patterns. Numerical simulations show that low orders of guided modes are extracted efficiently by the metal-dielectric pattern with a pitch size of ~(λ/n) and pattern depth of <100 nm. Vertical GaN light-emitting diodes with optimized metal-ITO patterns exhibited extraction efficiencies enhanced by factors of 6.6 and 2.6 for perfect conductor and silver metals, respectively, as compared to a non-patterned structure. The plasmonic absorption loss from the corrugated silver mirror accounts for the relatively smaller enhancement in extraction efficiency with the silver-ITO pattern. Furthermore, a double-sided out-coupling structure consisting of an upper GaN-air pattern and a bottom perfect conductor-ITO pattern exhibited a 40% enhancement in extraction efficiency as compared to the structure with single GaN-air pattern. We believe that deep understandings of the interaction between light and metal-dielectric patterns will lead to improved device performances in various optoelectronic applications including high-efficiency light-emitting diodes.

Design of polarization-selective light emitters using one-dimensional metal grating mirror

This paper proposes a polarization-selective light emitter that can enhance preferentially the spontaneous emission rate of one desired polarization state using a one-dimensional metal grating mirror. Systematic numerical simulations were performed to determine the optimized structural parameters of the metal grating mirror consisting of ITO/silver, in which the two orthogonally polarized lights reflected from the grating mirror undergo completely opposite phases. This metal grating mirror was incorporated into a GaN medium, and the spontaneous emission rate of one linearly polarized light was 1.3 times higher than that of the other at a specific distance between the light source and mirror. In addition, the polarization ratio can be increased to 15:1 by considering the extracted power in a practical vertical GaN slab light-emitting diode structure. This study will be useful for demonstrating high-efficiency polarization-selective light-emitting diodes without using additional optical components, such as a polarizer.