Ruey-Lin Chern

Polarization-independent broad-band nearly perfect absorbers in the visible regime.

Polarization-independent broad-band absorbers in the visible regime are theoretically investigated. The absorbers are three-layered structures consisting of a lossy dielectric grating on top of a low-loss dielectric layer and a substrate of the same lossy dielectric placed at the bottom. Enhanced absorption in the underlying structure is attained over a broad range of frequency for both TE and TM polarizations. In particular, a nearly perfect absorbance (over 99.6%) is achieved at λ ≈ 600 nm, around which the absorption spectra show a substantial overlap between two polarizations. The enhanced absorption is attributed to cavity resonance and its hybridization with a weakly bound surface wave. This feature is illustrated with the electric field patterns and time-averaged power loss density associated with the resonances.

Nearly perfect absorption in intrinsically low-loss grating structures

The feature of enhanced absorption in two-layered grating structures is theoretically investigated. The underlying structures make the most use of resonance mechanism to achieve a nearly perfect absorption in an intrinsically low-loss medium. For standalone gratings, the maximum absorption efficiency is shown to be 50%, which is attributed to the coupling of short range (bonding) or long range (antibonding) surface plasmons with cavity resonances. By attaching a dielectric slab on top or bottom to the metallic grating, the maximum absorption efficiency can be raised to nearly 100%. The presence of guided waves in the dielectric slab causes the strong concentration of fields and reinforces the absorption to its extreme value. The efficient absorption mechanism is illustrated with the pattern of resonance fields and the distribution of power loss density. A phenomenological theory is also used to characterize the absorption anomaly in terms of complex pole and zero.

Connected hexagonal photonic crystals with largest full band gap

A two-dimensional photonic crystal with a large full band gap has been designed, fabricated, and characterized. The photonic crystal design was based on a calculation using inverse iteration with multigrid acceleration. The fabrication of the photonic crystal on silicon was realized by the processes of electron-beam lithography and inductively coupled plasma reactive ion etching. It was found that the hexagonal array of circular columns and rods has an optimal full photonic band gap. In addition, we show that a larger extraction of light from our designed photonic crystal can be obtained when compared with the frequently used photonic crystals reported previously. Our designed PC structure therefore should be very useful for creating highly efficient optoelectronic devices.

Surface plasmon-like modes on structured perfectly conducting surfaces

Surface plasmon-like (SPL) modes are the electromagnetic surface eigenmodes supported by the structured perfectly conducting surfaces. The standard eigenvalue-solving method is adopted to solve these SPL modes. The field patterns of the SPL modes in the square holes for in-plane wavevectors k(x) = 2pi / 2d and k(x) = 2pi /d are TE(10)-like and TE(11), respectively. However, the field patterns can no longer be identified as any particular waveguide mode for other in-plane wavevectors. The dispersion relations of the SPL modes are obtained numerically. The change in mode character with wavevector prevents the dispersion relation from being derived by assuming only the fundamental mode in the holes. On a thin perfect conductor perforated with structures, the SPL mode splits into a high-frequency anti-symmetric mode and a low-frequency symmetric mode, which is caused by the mutual interaction of the electromagnetic evanescent fields on both sides.

Anomalous optical absorption in metallic gratings with subwavelength slits

The absorption in metallic gratings with subwavelength slits is theoretically investigated. Anomalous optical absorption occurs over a wide range of incident angles for both polarizations. In particular, a nearly perfect absorbance up to 99.5% with a significant bandwidth is attained for TM polarization with compound slits. Enhanced absorption is associated with extreme concentration of fields inside the structure. The respective field pattern depicts a special feature of surface plasmons excited on single interface only, which are identified as semibonding modes. The anomalous absorption is also achieved for TE polarization, when the compound grating is reduced to a simple grating. For this polarization, the nomalous absorption is attributed to the occurrence of trapped modes, with a slightly smaller absorbance (98.4%).

Multiple Fano resonances in metallic arrays of asymmetric dual stripes.

Characteristics of Fano resonances in metallic arrays of asymmetric dual stripes are theoretically investigated. The structure consists of two perfect metal stripes with unequal sizes in a unit cell. In addition to the total reflection that usually occurs in single-stripe arrays, the dual-stripe arrays exhibit two extra pairs of reflection peaks and dips, which are identified as the Fano resonance with the reflection line shape characterized by a Fano formula. In particular, the peak-dip pair on the high-frequency side is recognized as the high-order Fano resonance, as compared to its counterpart on the low-frequency side. Features of the Fano resonances are further illustrated with the electric field and surface current patterns at the corresponding frequencies. The underlying mechanism of multiple Fano resonances in the dual-stripe arrays is also discussed.

Nonlocal optical properties in periodic lattice of graphene layers

Based on the effective medium model, nonlocal optical properties in periodic lattice of graphene layers with the period much less than the wavelength are investigated. Strong nonlocal effects are found in a broad frequency range for TM polarization, where the effective permittivity tensor exhibits the Lorentzian resonance. The resonance frequency varies with the wave vector and coincides well with the polaritonic mode. Nonlocal features are manifest on the emergence of additional wave and the occurrence of negative refraction. By examining the characters of the eigenmode, the nonlocal optical properties are attributed to the excitation of plasmons on the graphene surfaces.

Spatial dispersion and nonlocal effective permittivity for periodic layered metamaterials

The feature of spatial dispersion in periodic layered metamaterials is theoretically investigated. An effective medium model is proposed to derive the nonlocal effective permittivity tensor, which exhibits drastic variations in the wave vector domain. Strong spatial dispersion is found in the frequency range where surface plasmon polaritons are excited. In particular, the nonlocal effect gives rise to additional waves that are identified as the bonding or antibonding modes with symmetric or antisymmetric surface charge alignments. Spatial dispersion is also manifest on the parabolic-like dispersion, a non-standard type of dispersion in the medium. The associated negative refraction and backward wave occur even when the effective permittivity components are all positive, which is considered a property not available in the local medium.

Dispersion mechanism of surface magnetoplasmons in periodic layered structures

We investigate the dispersion mechanism of surface magnetoplasmons for periodic layered structures in the Voigt configuration. An analytical dispersion relation that retains a similar form with ordinary surface plasmons is obtained. The splitting of surface plasma frequency is accompanied with unequal field strengths of surface modes at the two interfaces and is characterized by a simple dynamic model that recasts the role of magnetic force on to the effective mass. The underlying mechanism is illustrated with the transverse currents induced by the cyclotron motion of electrons, which appears as the typical feature of the dynamic Hall effect. In particular, the acoustical and optical branches exhibit an anticrossing scheme for small filling fractions, due to the like symmetry of modes in the two branches. As the parallel wave number changes, the two interaction branches experience a transition of mode pattern from symmetry to antisymmetry, or vice versa.

Effective parameters and quasi-static resonances for periodic arrays of dielectric spheres

We investigate the effective parameters and quasi-static resonances for periodic arrays of dielectric spheres. The effective medium model based on the homogenization of normal modes for spherical particles is used to determine the effective permittivity ϵeff and permeability μeff in the quasi-static regime. Major features of ϵeff and μeff are characterized by the Lorentz-type anomalous dispersion around the frequencies pertaining to the leading-order electric and magnetic resonances, respectively. In particular, the anomalous dispersion is depicted by a resonance function associated with the spherical cavity. The underlying mechanism of quasi-static resonance is illustrated with the localized and dipole-like field patterns at the resonant frequencies. A comparison with the effective parameters for periodic arrays of dielectric circular cylinders is also discussed.