Y. U. Lee

Double Fano resonances in a composite metamaterial possessing tripod plasmonic resonances

Double Fano resonances are observed in a planar composite metamaterial possessing tripod plasmonic resonances, where a common subradiant driven oscillator is coupled with two superradiant oscillators. As a classical analogue of four-level tripod atomic system, the extinction spectrum of the composite metamaterial exhibits a coherent effect based on double Fano resonances. It is shown that a transfer of the absorbed power between two orthogonal superradiant oscillators is mediated by a common subradiant oscillator.

Fabrication of polarization-dependent reflective metamaterial by focused ion beam milling.

By focused ion beam milling, we fabricated near-IR reflective metamaterials consisting of nano-aperture arrays. Optimum parameters of ion beam current and accelerating voltage in the fabrication process are obtained. Nano-apertures constituting reflective metamaterial are successfully milled, and possess a reflective resonance in the near-IR spectral range. With a double-split-ring resonator structure for the nano-aperture, the intensity reflection at resonance is rendered polarization dependent. It is found that the point group symmetry of the nano-aperture array determines the amount of anisotropy in the intensity reflection. Finite-difference time-domain simulation was adopted to identify details of nano-aperture metastructures transferred from nano-aperture patterns by the focused ion beam milling.

Anisotropic change in THz resonance of planar metamaterials by liquid crystal and carbon nanotube

THz metamaterials are employed to examine changes in the meta-resonances when two anisotropic organic materials, liquid crystal and carbon nanotubes, are placed on top of metamaterials. In both anisotropic double split-ring resonators and isotropic four-fold symmetric split-ring resonators, anisotropic interactions between the electric field and organic materials are enhanced in the vicinity of meta-resonances. In liquid crystal, meta-resonance frequency shift is observed with the magneto-optical coupling giving rise to the largest anisotropic shift. In carbon nanotube, meta-resonance absorptions, parallel and perpendicular to nanotube direction, experience different amount of broadening of Lorentzian oscillator of meta-resonance. Investigation reported here opens the application of metamaterials as a sensor for anisotropic materials.

Electro-optic switching in phase-discontinuity complementary metasurface twisted nematic cell

Electro-optic switching of refraction is experimentally demonstrated in a phase-discontinuity complementary metasurface twisted nematic cell. The phase-discontinuity complementary metasurface is fabricated by focused-ion-beam milling, and a twisted nematic cell is constructed with complementary V-shape slot antenna metasurface. By application of an external voltage, switching is achieved between ordinary refraction and extraordinary refraction satisfying the generalized Snells law. It has a strong implication for applications in spatial light modulation and wavelength division multiplexer/demultiplexer in a near-IR spectral range.

Polarization-independent light emission enhancement of ZnO/Ag nanograting via surface plasmon polariton excitation and cavity resonance.

In this study, we observed that the photoluminescence (PL) intensity of ZnO/Ag nanogratings was significantly enhanced compared with that of a planar counterpart under illumination of both transverse magnetic (TM) and transverse electric (TE)-mode light. In the TM mode, angle-resolved reflectance spectra exhibited dispersive dips, indicating cavity resonance as well as grating-coupled surface plasmon polariton (SPP) excitation. In the TE mode, cavity resonance only was allowed, and broad dips appeared in the reflectance spectra. Strong optical field confinement in the ZnO layers, with the help of SPP and cavity modes, facilitated polarization-insensitive PL enhancement. Optical simulation results were in good agreement with the experimental results, supporting the suggested scenario.

Reflection resonance switching in metamaterial twisted nematics cell.

Electric switching of reflection resonances at near-IR spectral range is experimentally demonstrated in a reflective metamaterial twisted nematic liquid crystal cell. Reflective metamaterial composed of nano-sized double-split ring resonator aperture is fabricated by a focused ion beam milling. Two-fold rotational symmetry of double-split ring resonators allows for two orthogonal polarization-dependent reflection resonances in the reflective metamaterial. With an external voltage of 10V across 12μm cell gap, a full switching is achieved between two reflection resonances. Dynamic measurements show the time constants of switch-on and switch-off are in the order of 100ms and 10ms, respectively.

Mie Resonance-Modulated Spatial Distributions of Photogenerated Carriers in Poly(3-hexylthiophene-2,5-diyl)/Silicon Nanopillars

Organic/silicon hybrid solar cells have great potential as low-cost, high-efficiency photovoltaic devices. The superior light trapping capability, mediated by the optical resonances, of the organic/silicon hybrid nanostructure-based cells enhances their optical performance. In this work, we fabricated Si nanopillar (NP) arrays coated with organic semiconductor, poly(3-hexylthiophene-2,5-diyl), layers. Experimental and calculated optical properties of the samples showed that Mie-resonance strongly concentrated incoming light in the NPs. Spatial mapping of surface photovoltage, i.e., changes in the surface potential under illumination, using Kelvin probe force microscopy enabled us to visualize the local behavior of the photogenerated carriers in our samples. Under red light, surface photovoltage was much larger (63 meV) on the top surface of a NP than on a planar sample (13 meV), which demonstrated that the confined light in the NPs produced numerous carriers within the NPs. Since the silicon NPs provide pathways for efficient carrier transportation, high collection probability of the photogenerated carriers near the NPs can be expected. This suggests that the optical resonance in organic/silicon hybrid nanostructures benefits not only broad-band light trapping but also efficient carrier collection.

Cryogenic temperature measurement of THz meta-resonance in symmetric metamaterial superlattice

We investigated a change in the Q-factor of THz meta-resonance as a function of temperature in a symmetric metamaterial superlattice composed by double-split ring resonators (DSRR). THz time-domain spectroscopy is carried out to measure the transmission spectra. At low temperatures, we could analyze the impact factor for determining the Q-factor of the trapped mode compared with open-mode meta-resonance where radiative damping is still dominant.

Optical resonance-mediated spatial modulation of photo-excited carrier concentrations in P3HT/Si nanopillars(Conference Presentation)

We fabricated Si nanopillar (NP) arrays using e-beam lithography and coated them with poly(3-hexylthiophene-2,5-diyl) (P3HT) organic semiconductor layers. Optical reflection spectra showed that Mie resonance significantly increased the scattering cross-sections of the NPs and strongly concentrated incident light in the NPs. Such concentrated light should produce numerous charge carriers and affect the subsequent drift/diffusion of the carriers. Surface photovoltage (SPV), defined as the difference of the surface potential in dark and under light, could reveal the formation and separation of the photo-generated carriers. Especially, Kelvin probe force microscopy technique allowed us to obtain real space SPV maps with nanoscopic spatial resolution. The SPV values at the NP tops were much larger than those at the flat regions around the NPs. This study would provide us insights into improving performance of organic/inorganic hybrid nanostructure-based devices.

Weak measurement of optical spin Hall effect in phase-discontinuity metasurface

Recently it has been demonstrated that a rapid phase-change at a phase-discontinuity metasurface (PDM) leads to an additional momentum gradient enabling a direct observation of optical spin Hall (OSH) shift. We show that the helicity-dependent OSH shift depends on incidence and refraction angles at PDM, and construct a weak value measurement to control OSH shift by a variable phase retardance in the post-selection.