Haesung Park

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.

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.

Robust optical measurement of water vapor in steel-making process

Based on tunable diode laser spectroscopy (TDLS), we develop a simple and efficient humidity monitoring scheme to improve reliability of measurement under randomly changing transmission condition. This measurement scheme can detect the humidity of a monitored system in either high or low scattering condition such as powder coal fired furnace in steel-making process. Distributed feed-back (DFB) diode laser is used to evaluate the relative and absolute humidity by making direct absorption measurement near 938 nm region where 2nu1 + nu3 water vapor transition band exists. This system gives reliable measurement results under either high or low scattering condition.

Permittivity manipulation of metal-dielectric composite for improved SPR sensing

We demonstrate a convenient method to improve the surface Plasmon resonance sensitivity by manipulating the permittivity of active medium using metal-dielectric (Ag-SiO2) composite monolayer. We demonstrate the successful permittivity engineering of SPR active medium in both theory and experiments. Based on the basic theory of SPR and Bruggeman effective medium theory (EMT), we theoretically confirm that the angular sensitivity enhances using manipulated permittivity of metal-dielectric composite layer.

Optical measurement for the concentrations of the pickling acid with near infrared spectroscopy in steel making industry

In the manufacturing process of stainless steel, it is essential to pickle the oxide layer of steel surface for high corrosion resistance and fine surface quality. Pickling liquor of stainless steel is commonly composed of mixed hydrofluoric and nitric acid. Real time monitoring of concentrations of each acid is crucial to optimize pickling process. It also reduces cost of production and decreases the generation of waste acid. We used non-contact near infrared spectroscopy technique and rapid analysis method, for the quantification of each acid in an on-line manner. Multivariate calibration such as partial least square regression method is employed for the better prediction results.

Tunable subwavelength focusing with slit-groove-based metamaterials in THz

We design the THz lens made of slit-groove-based metamaterials with tunable far-field focal length as well as subwavelength resolution, based on surface plasmons(SP) diffraction theory into spoof SP of THz region. In THz regime, the curved depth profile of grooves from both sides of metal slit produce directional beaming and mimic SP at the same time. By arranging the depth of grooves in traced profile, it is possible to optimize the focal position in THz region without changing the size of structure. It is performed numerical simulation of a designed structure through finite-difference time-domain (FDTD) method and shows the subwavelength imaging of the designed position. In addition, the change of focal length and the relative Ex phase are observed in the simulation and help to comprehend a subwavelength 1D slit-groove-based metamaterials in THz regime.