Gumin Kang

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.

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.

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.

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.

Quantitative analysis of mixed hydrofluoric and nitric acids using Raman spectroscopy with partial least squares regression

Mixed hydrofluoric and nitric acids are widely used as a good etchant for the pickling process of stainless steels. The cost reduction and the procedure optimization in the manufacturing process can be facilitated by optically detecting the concentration of the mixed acids. In this work, we developed a novel method which allows us to obtain the concentrations of hydrofluoric acid (HF) and nitric acid (HNO(3)) mixture samples with high accuracy. The experiments were carried out for the mixed acids which consist of the HF (0.5-3wt%) and the HNO(3) (2-12wt%) at room temperature. Fourier Transform Raman spectroscopy has been utilized to measure the concentration of the mixed acids HF and HNO(3), because the mixture sample has several strong Raman bands caused by the vibrational mode of each acid in this spectrum. The calibration of spectral data has been performed using the partial least squares regression method which is ideal for local range data treatment. Several figures of merit (FOM) were calculated using the concept of net analyte signal (NAS) to evaluate performance of our methodology.

Porous metallic nanocone arrays for high-density SERS hot spots via solvent-assisted nanoimprint lithography of block copolymer

Porous nanostructures have been enthusiastically investigated for SERS application thanks to the internal nanogaps or protrusions acting as effective electromagnetic hotspots. In this work, we report a facile fabrication method of highly porous metallic nanocone arrays for SERS application by integrating solvent-assisted nanoimprint lithography and selective etching of block copolymer (PS-b-PMMA) film. By taking advantage of the solvent-assisted nanoimprint, we easily mould the block copolymer film under atmospheric pressure and moderate temperature below the glass transition temperature in a short time. Then, the PMMA domain of the patterned block copolymer film was selectively etched to make porous structures to form dense nanogaps and protrusions. After Ag deposition, the fabricated structure exhibited a maximum enhancement factor (EF) up to ∼3.5 × 106. In comparison to Ag coated “solid” nanocone arrays, the EF of “porous” nanocone arrays is maximum ∼8.9 times enhanced, which demonstrates the effectiveness of the internal nanogaps and protrusions as plasmonic hot spots. Our fabrication method is very time-saving and cost-effective with good SERS enhancement and also can be easily applied to conventional SERS substrates or other applications that utilize porous structures.

Improvement of Light Extraction Efficiency in Flip-Chip Light Emitting Diodes on SiC Substrate via Transparent Haze Films with Morphology-Controlled Collapsed Alumina Nanorods

We demonstrate GaN-based flip-chip light emitting diodes (FC-LEDs) on SiC substrate achieving high extraction efficiency by simply attaching the optically transparent haze films consisting of collapsed alumina nanorods. Through controlled etching time of alumina nanorods, we obtain four types of films that have different morphologies with different optical transmittance and haze properties. We show that the light output power of the FC-LEDs with film, which has 95.6% transmittance and 62.7% haze, increases by 20.4% in comparison to the bare LEDs. The angular radiation pattern of the LEDs also follows the Lambertian emission pattern without deteriorating the electrical properties of the device. The improvement of light extraction is mainly attributed to the reduced total internal reflection (TIR) via efficient out-coupling of guided light from SiC substrate to air by collapsed alumina nanorod structures in the film. The high transparency of film and reduced Fresnel reflection via graded refractive index transition between the film and SiC substrate also contribute to the extraction enhancement of the device. We systematically investigate the influence of haze films geometrical or optical properties on the extraction efficiency of FC-LEDs, and this study will provide a novel approach to enhance the performance of various optoelectronic devices.