Min Hyung Cho

Single- and double-negative refractive indices of combined metamaterial structure

We experimentally and numerically studied the transmission spectra of a combined metamaterial structure whose negative refractive index was supposed to be achievable only with simultaneously negative permittivity and permeability. However, it was found that the negativity of the refractive index of such a structure could be obtained not only when both parameters were negative but also when only one parameter, more specifically the permittivity was negative. These characteristics of combined structure were analyzed in detail by using the standard retrieval effective-medium method. According to the analyses, it can be concluded that the negativity originates from the complex permittivity and permeability. The interplay among the real and the imaginary parts of those parameters is the key to the negative behavior of refractive index.

Polarization-independent extraordinary optical transmission in one-dimensional metallic gratings with broad slits

Extraordinary optical transmission (EOT) is achievable for transverse-magnetic polarization in one-dimensional metallic gratings with very narrow slits due to the excitation of coupled surface plasmon polaritons (SPPs). In contrast, SPP-produced EOT for transverse-electric (TE) polarized light is impossible because of the absence of SPPs for this polarization. However, TE-polarized EOT produced by trapped modes has been demonstrated. In this work, we reanalyze this phenomenon and apply it to gratings with broad slits (still in subwavelength) without the need for a specific dielectric filler in the grooves. The design proposed in this work simplifies and makes more practical the realization of gratings that possess polarization-independent EOT.

Magneto-optical enhancement through gyrotropic gratings

Diffracted magneto-optical (MO) effects are numerically investigated for one-dimensional lossy gyrotropic gratings in the zeroth and the first orders for the polar magnetization by utilizing the rigorous coupled-wave approach implemented as an Airy-like internal-reflection series. The simulated Kerr spectra agree well with the experimental ones. The dependence of the MO Kerr enhancement on the grating depth in the first-order diffraction, compared with that in the zeroth one, is illustrated, and the diffracted MO Faraday effect is theoretically investigated as well. Such a MO enhancement through the gyrotropic gratings is superior to the conventional MO devices and magneto-photonic crystals. The potential applications are also suggested.

Robust fast direct integral equation solver for quasi-periodic scattering problems with a large number of layers.

We present a new boundary integral formulation for time-harmonic wave diffraction from two-dimensional structures with many layers of arbitrary periodic shape, such as multilayer dielectric gratings in TM polarization. Our scheme is robust at all scattering parameters, unlike the conventional quasi-periodic Greens function method which fails whenever any of the layers approaches a Wood anomaly. We achieve this by a decomposition into near- and far-field contributions. The former uses the free-space Greens function in a second-kind integral equation on one period of the material interfaces and their immediate left and right neighbors; the latter uses proxy point sources and small least-squares solves (Schur complements) to represent the remaining contribution from distant copies. By using high-order discretization on interfaces (including those with corners), the number of unknowns per layer is kept small. We achieve overall linear complexity in the number of layers, by direct solution of the resulting block tridiagonal system. For device characterization we present an efficient method to sweep over multiple incident angles, and show a 25× speedup over solving each angle independently. We solve the scattering from a 1000-layer structure with 3 × 105 unknowns to 9-digit accuracy in 2.5 minutes on a desktop workstation.

Influence of lattice parameters on the resonance frequencies of a cut-wire-pair medium

We studied both theoretically and experimentally the influence of the lattice constant on the resonance frequencies of cut-wire-pair structures, which are essential components in assembling left-handed materials. These structures were designed and fabricated, and the transmission spectra were measured in the microwave-frequency regime. Numerical simulation was performed using the transfer-matrix method. All the numerical results were in good agreement with the experimental data. It was found that the lattice parameters of the cut-wire-pair structure provide sensitive effects on the electric resonance, especially, in the propagation k and the electric field E directions, while the magnetic resonance frequency is nearly unchanged, except for its bandwidth and depth. In addition, we also studied the effect of dielectric layer thickness on the resonance frequencies of the cut-wire-pair structure. These sensitive changes in the electric resonance might have remarkable effects on the effective plasma frequency ...

A Wideband Fast Multipole Method for the two-dimensional complex Helmholtz equation

A Wideband Fast Multipole Method (FMM) for the 2D Helmholtz equation is presented. It can evaluate the interactions between N particles governed by the fundamental solution of 2D complex Helmholtz equation in a fast manner for a wide range of complex wave number k, which was not easy with the original FMM due to the instability of the diagonalized conversion operator. This paper includes the description of theoretical backgrounds, the FMM algorithm, software structures, and some test runs.

Effect of the dielectric layer thickness on the electromagnetic response of cut-wire-pair and combined structures

This report investigates the effect of the dielectric layer thickness on both magnetic and electric resonances of cut-wire-pair (CWP) structures in the microwave frequency regime. It was found that the resonances are sensitive to the thickness of the dielectric layer. As the thickness increases, the bandwidth of the magnetic resonance is slightly extended to a higher frequency, while the low-frequency edge of the electric-resonance band is remarkably shifted to a lower frequency. It was also found that the dependence of the magnetic resonance frequency on the dielectric layer thickness follows the trend of the closed formula based on the cavity model for the coupled metallic elements (Cai et al 2007 Opt. Express 15 3333). In addition, we also studied the effect of the dielectric layer thickness on the left-handed behaviour of a combined structure consisting of CWP and continuous wire. The actual measurements are compared with the numerical simulation values to show a good coincidence.

Rigorous approach on diffracted magneto-optical effects from polar and longitudinal gyrotropic gratings.

The rigorous coupled-wave analysis with Airy-like internal-reflection series and Fourier-factorization for the calculation of the diffracted magneto-optical (MO) effects from polar and longitudinally magnetized gyrotropic gratings are fully described. For both gratings the numerical and experimental results are in good agreement, and the enhancement of Kerr rotation in higher orders compared to that of the 0th order diffraction is calculated as a function of grating depth. At last, this numerical method can be applied to many other applications such as extraordinary optical transmission from metallic gratings either through surface plasmon or cavity mode, and MO hysteresis loops.

A Boundary Integral Equation Method for Photonic Crystal Fibers

A boundary integral equation for the eigenmode of photonic crystal fibers is formulated and numerically solved using the Nyström method. The real and imaginary parts of the propagation constant, which are related to the dispersion and the confinement loss of fibers, are obtained using a secant method. This formulation is very flexible to handle the fiber geometry, and therefore can be applied to photonic crystal fibers with novel refractive index profile and hole geometry.

Diffracted magneto-optical Kerr effect of a Ni magnetic grating

We report the results of a joint experimental and theoretical investigation focused on the magneto-optical (MO) properties of one-dimensional magnetic grating structure made of Ni. It was found that the longitudinal Kerr rotation of the second-order diffracted beam is nearly three times larger than that of the zeroth-order beam. The calculational results further confirmed the experimental ones, and almost perfectly reproduced the measured hysteresis loops of the longitudinal MO Kerr rotation, elucidating the origin of the enhanced MO rotation.