Kyoungsik Kim

Flexible thin-film black gold membranes with ultrabroadband plasmonic nanofocusing for efficient solar vapour generation.

Solar steam generation has been achieved by surface plasmon heating with metallic nanoshells or nanoparticles, which have inherently narrow absorption bandwidth. For efficient light-to-heat conversion from a wider solar spectrum, we employ adiabatic plasmonic nanofocusing to attain both polarization-independent ultrabroadband light absorption and high plasmon dissipation loss. Here we demonstrate large area, flexible thin-film black gold membranes, which have multiscale structures of varying metallic nanoscale gaps (0–200 nm) as well as microscale funnel structures. The adiabatic nanofocusing of self-aggregated metallic nanowire bundle arrays produces average absorption of 91% at 400–2,500 nm and the microscale funnel structures lead to average reflection of 7% at 2.5–17 μm. This membrane allows heat localization within the few micrometre-thick layer and continuous water provision through micropores. We efficiently generate water vapour with solar thermal conversion efficiency up to 57% at 20 kW m−2. This new structure has a variety of applications in solar energy harvesting, thermoplasmonics and related technologies.

Broadband electromagnetic cloaking with smart metamaterials

The ability to render objects invisible with a cloak that fits all objects and sizes is a long-standing goal for optical devices. Invisibility devices demonstrated so far typically comprise a rigid structure wrapped around an object to which it is fitted. Here we demonstrate smart metamaterial cloaking, wherein the metamaterial device not only transforms electromagnetic fields to make an object invisible, but also acquires its properties automatically from its own elastic deformation. The demonstrated device is a ground-plane microwave cloak composed of an elastic metamaterial with a broad operational band (10-12 GHz) and nearly lossless electromagnetic properties. The metamaterial is uniform, or perfectly periodic, in its undeformed state and acquires the necessary gradient-index profile, mimicking a quasi-conformal transformation, naturally from a boundary load. This easy-to-fabricate hybrid elasto-electromagnetic metamaterial opens the door to implementations of a variety of transformation optics devices based on quasi-conformal maps.

Broadband Optical Antireflection Enhancement by Integrating Antireflective Nanoislands with Silicon Nanoconical‐Frustum Arrays

Based on conventional colloidal nanosphere lithography, we experimentally demonstrate novel graded-index nanostructures for broadband optical antireflection enhancement including the near-ultraviolet (NUV) region by integrating residual polystyrene antireflective (AR) nanoislands coating arrays with silicon nano-conical-frustum arrays. This is a feasible optimized integration method of two major approaches for antireflective surfaces: quarter-wavelength AR coating and biomimetic moths eye structure.

Observation of theS01→P03Clock Transition inAl+27

We report, for the first time, laser spectroscopy of the 1S0-->3P0 clock transition in 27Al+. A single aluminum ion and a single beryllium ion are simultaneously confined in a linear Paul trap, coupled by their mutual Coulomb repulsion. This coupling allows the beryllium ion to sympathetically cool the aluminum ion and also enables transfer of the aluminums electronic state to the berylliums hyperfine state, which can be measured with high fidelity. These techniques are applied to measure the clock transition frequency nu=1,121,015,393,207,851(6) Hz. They are also used to measure the lifetime of the metastable clock state tau=20.6+/-1.4 s, the ground state 1S0 g factor gS=-0.000,792,48(14), and the excited state 3P0 g factor gP=-0.001,976,86(21), in units of the Bohr magneton.

Broadband light-trapping enhancement in an ultrathin film a-si absorber using whispering gallery modes and guided wave modes with dielectric surface-textured structures

An embedded nanosphere dielectric structure on an a-Si ultrathin film improves weighted absorption from 23.8% to 39.9%. The PMMA embedding layer offers a guided wave mode as well as mechanical robustness, in addition to the resonant whispering gallery modes coupling. Broadband light-trapping enhancements are observed by dielectric surface textured structures of hemispheres, nanocones, nanospheres, or embedded nanospheres.

Effect of human mesenchymal stem cell transplantation combined with growth factor infusion in the repair of injured spinal cord

Recently, bone marrow stromal cells have been shown to have the capacity to differentiate into neural cell under experimental cell culture conditions. Some investigators suppose that these cells, when placed into an environment of injury, express factors that promote repair or active compensatory mechanisms and endogeneous stem cells within the injured tissue. Rats were subjected to a weight driven implant spinal cord injury. After one week, the rats were treated with cultured human mesenchymal stem cells (MSCs) transplantation and basic fibroblast growth factor (bFGF) infusion into the CSF space. Functional outcome and histologic evaluation were performed. The data showed improved functional outcome in the group treated with MSCs transplantation and bFGF administration compared with the group of MSCs transplantation and control, which means bFGF might take an additional role to improve functional outcome. Glial differentiation of MSCs was noted but neuronal differentiation was doubtful. In this study, we did not demonstrate the mechanism of the neurotrophic factor affecting neural repair. However, this study is consistent with growing literature that MSCs and neurotrophic factor promote tissue repair and functional recovery after spinal cord injury and suggests that MSCs transplantation and bFGF warrants investigation as a therapeutic intervention after spinal cord injury.

Broadband and ultrahigh optical haze thin films with self-aggregated alumina nanowire bundles for photovoltaic applications

Optical haze is one of the most important factors to be considered in the development of efficient photonic and optoelectronic devices by manipulating the light behavior. In this work, we fabricated a high optical haze film composed of self-aggregated alumina nanowire arrays and applied this novel structure to improve the energy conversion efficiency of organic photovoltaic (OPV) devices. We controlled the optical properties of films, such as total/diffuse transmittance and haze value, by changing the packing density of nanowires during the wet etching process. By optimizing the etching conditions, the nanowire bundle arrays enabled us to obtain an ultrahigh optical haze value up to ∼98% or high transmittance up to ∼96%. By simply attaching the haze film onto the front glass surface of the OPV device, we significantly elongated the optical path length in the active layer, thereby achieving an overall efficiency of 9.01% out of the bare device of 8.17%, resulting in 10.28% enhancement.

A versatile smart transformation optics device with auxetic elasto-electromagnetic metamaterials.

Synergistic integration of electromagnetic (EM) and mechanical properties of metamaterials, a concept known as smart metamaterials, promises new applications across the spectrum, from flexible waveguides to shape-conforming cloaks. These applications became possible thanks to smart transformation optics (STO), a design methodology that utilizes coordinate transformations to control both EM wave propagation and mechanical deformation of the device. Here, we demonstrate several STO devices based on extremely auxetic (Poisson ratio −1) elasto-electromagnetic metamaterials, both of which exhibit enormous flexibility and sustain efficient operation upon a wide range of deformations. Spatial maps of microwave electric fields across these devices confirm our ability to deform carpet cloaks, bent waveguides, and potentially other quasi-conformal TO-based devices operating at 7 ~ 8 GHz. These devices are each fabricated from a single sheet of initially uniform (double-periodic) square-lattice metamaterial, which acquires the necessary distribution of effective permittivity entirely from the mechanical deformation of its boundary. By integrating transformation optics and continuum mechanics theory, we provide analytical derivations for the design of STO devices. Additionally, we clarify an important point relating to two-dimensional STO devices: the difference between plane stress and plane strain assumptions, which lead to elastic metamaterials with Poisson ratio −1 and −∞, respectively.

Active phase control of a Ag near-field superlens via the index mismatch approach

We recognize that the phase control of optical transfer function is profoundly important in realizing nanoimaging beyond the diffraction limit. The difficulty of the optical phase measurement in the near field, required for the conventional adaptive control method, motivates us to achieve active phase control in the superlens imaging system. The visibility and resolving capabilities are significantly enhanced through the index mismatch approach by tuning the wavelength of the incident light.

Tunable subwavelength focusing with dispersion-engineered metamaterials in the terahertz regime

We develop a terahertz lens with both subwavelength resolution and tunable far-field focal length by extending the surface plasmon (SP) diffraction theory into spoof SPs of slit-groove-structure terahertz metamaterials. The dispersion properties of terahertz groove structures are engineered in the curved depth profile to produce a directional beaming feature and mimic SPs at the same time. The finite-difference time-domain simulation results confirm that the far-field focal position can be tuned by controlling the curvature of the relative electric field phase distribution profile on the output surface.