Kwangchil Lee

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.

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.

Super-lensing effect of TiO2 NFSL in the mid IR regime

Recently, the near-field superlens (NFSL) has been used as a suitable material for the super-resolution beyond the diffraction limits. These NFSL materials in the nature, such as metals and polar dielectric crystals, usually have intrinsic absorption loss in the Re(ε) < 0 region. In the imaging system, such absorption loss decreases the retardation effects by softening the singularity of transmission resonances, but it does not remove the phase singularity that severely deteriorates the ideal image restoration. Because of this problem, TiO2 thin film cannot still have sufficient band of spatial frequency for the super-resolution in the mid IR regime. In this research, we report the achievement super-resolution in TiO2 NFSL by elimination of the phase singularity based on the phase correction method.

Elimination of phase singularity to achieve superresolution in lossy metamaterials.

The presence of absorption losses softens the singular behavior of transmission resonances and leads to a good image in spite of limited effective spatial frequency range. Nonetheless, we found that the phase singularity does not disappear despite the considerably reduced retardation effects by softening the transmission resonances. Because the phase singularity severely deteriorates the ideal image reconstruction, broad transmission bandwidth in spatial frequency domain is not sufficient enough to achieve superresolution in TiO(2) thin film lens. The present work predicts successful elimination of the phase singularity and the achievement of approximately lambda /12.9 superresolution in TiO(2) thin film lens through the phase correction method.

Enhanced image quality of a lossy near-field superlens using index mismatch approach

Over the years a near-field superlens (NFSL) has received much attention because of its super-resolution beyond the diffraction limits. Till now the index matched NFSL is widely applied to the near-field imaging system under the loss-free assumption. However, the natural materials for a NFSL have intrinsic absorption loss, which prevents ideal image reconstruction and blurs the near-field image. In connection with this issue, we wish to address the compensation of blurred image in the lossy NFSL system through the analysis of optical transfer function and the full-wave numerical approach. The results to be drawn from this research are that we obtained a high-quality image of lossy NFSL through active phase correction of OTF using the index mismatch approach by tuning the wavelength of incident light. And this approach can be applied to a lossy NFSL image system in the optical, IR and THz regime.

Reliable optical measurement of water vapor in highly scattering environment

Based on tunable diode laser spectroscopy with direct absorption measurement, we developed a reliable measurement scheme which can precisely detect water vapor concentration independent of the scattering (attenuation) caused by the non-water dust particles inside the observed humidity chamber. The relative water vapor concentration was measured at 2nu(1)+nu(3) water vapor transition band by 938 nm distributed feed-back diode laser at different temperatures using the tunable diode laser spectroscopy technique. These relative water vapor concentrations are converted into the absolute water vapor concentrations and we confirmed that the experimental results at different temperatures are consistently independent of the attenuation. Our measurement system gives the consistent water vapor concentrations without regard to either high or low scattering caused by the non-water dust particles inside the observed humidity chamber.