Taikei Suyama

ENHANCEMENT OF TM-TE MODE CONVERSION CAUSED BY EXCITATION OF SURFACE PLASMONS ON A METAL GRATING AND ITS APPLICATION FOR REFRACTIVE INDEX MEASUREMENT

Yasuuras mode-matching method is employed in the investigation of plasmon resonance absorption on a metal grating with a gold over-coating and the results are compared with experimental data. Enhancement of TM-TE mode conversion accompanying the plasmon resonance absorption is examined. When a TM wave is incident on a metal grating, enhanced TM-TE mode conversion occurs at angles of incidence at which the surface plasmons are excited. The strength of the mode conversion depends strongly on the azimuth angle of the mounting. This is verified by experiment and an application for refractive index measurement is suggested.

Longer axial trap distance and larger radial trap stiffness using a double-ring radially polarized beam

The optical trapping forces acting on a metallic Rayleigh particle are calculated for the case where a double-ring-shaped radially polarized beam is applied. The influence of the off-focus distance and the off-axis distance of a trapping particle on the trapping force is investigated. Compared with the use of the conventional single-ring-shaped radially polarized beam, the longer axial trap distance and the larger radial trap stiffness are predicted using a double-ring-shaped radially polarized beam in an optical trap. These features are useful for improving the trapping ability of an optical trap system where a longer axial trap distance is needed.

A NUMERICAL ANALYSIS OF STOP BAND CHARACTERISTICS BY MULTILAYERED DIELECTRIC GRATINGS WITH SINUSOIDAL PROFILE

An effective computational method based on a conven- tional modal expansion approach is presented for handling a multilay- ered dielectric grating whose profiles are multilayered and sinusoidally modulated. This structure fabricated by dielectric material is one of the useful photonic crystals. The method is based on Yasuuras modal expansion, which is known as a least-squares boundary residual method or a modified Rayleigh method. In the extended method, each layer is divided into shallow horizontal layers. The Floquet modal func- tions and approximate solutions are defined in each shallow layer, and the latter are matched with boundary conditions in the least-squares sense. A huge-sized least-squares problem that appears in finding the modal coefficients is solved by the QR decomposition accompanied by sequential accumulation. This procedure makes it possible to treat the case where the groove depths are the same as or a little more than the grating period. As numerical example, we calculate a diffractive char- acteristic by a multilayered deep dielectric grating and confirm that a common band gap occurs for both polarizations.

A COMBINATION OF UP- AND DOWN-GOING FLOQUET MODAL FUNCTIONS USED TO DESCRIBE THE FIELD INSIDE GROOVES OF A DEEP GRATING

An effective computational method based on a conven- tional modal-expansion approach is presented for solving the problem of diffraction by a deep grating. The groove depth can be the same as or a little more than the grating period. The material can be a perfect conductor, a dielectric, or a metal. The method is based on Yasuuras modal expansion, which is known as a least-squares boundary residual method or a modified Rayleigh method. The feature of the present method is that: (1) The semi-infinite region U over the grating sur- face is divided into an upper half plane U0 and a groove region UG by a fictitious boundary (a horizontal line); (2) The latter is further di- vided into shallow horizontal layers U1 ,U 2, ··· ,U Q again by fictitious boundaries; (3) An approximate solution in U0 is defined in a usual manner, i.e., a finite summation of up-going Floquet modal functions with unknown coefficients, while the solutions in Uq (q =1 , 2, ··· ,Q ) include not only the up-going but also the down-going modal functions; (4) If the grating is made of a dielectric or a metal, the semi-infinite region L below the surface is partitioned similarly into L0 ,L 1, ··· ,L Q, and approximate solutions are defined in each region; (5) A huge-sized least squares problem that appears in finding the modal coefficients is solved by the QR decomposition accompanied by sequential accu- mulation. The method of solution for a grating made of a perfect conductor is described in the text. The method for dielectric gratings can be found in an appendix. Numerical examples include the results for perfectly conducting and dielectric gratings.

Surface Plasmon Resonance Absorption in a Multilayered Thin-Film Grating

We numerically analyze surface plasmon resonance absorption of incident light energy by a multilayered grating, which consists of a stack of periodically corrugated metal and dielectric films. We apply Yasuuras modal expansion method for solving the problem of plane-wave diffraction by a multilayered grating. Employing the numerical algorithm, we evaluate the resonance absorption which is observed as dips of diffraction efficiency curves. We investigate field distributions in the vicinity of a multilayered grating when the absorption occurs. We thus show that the surface plasmons are excited on a thin metal film embedded in a multilayered grating. In view of the design of a multilayered thin-film grating, we investigate the influence of the corrugation depth and the refractive index of a substrate on characteristics of plasmon resonance absorption.

A GRATING-BASED PLASMON BIOSENSOR WITH HIGH RESOLUTION

We present an idea of grating-based plasmon biosensor utilizing phase detection to realize high resolution in flnding a refractive index of a material put on the surface of a metal grating. Considering a trade-ofi between high resolution and experimental practicability, we show a table of recommended setup that covers a wide range of the index. Keeping the difiraction e-ciency no less than 10 i3 and assuming the resolution in phase detection to be 2:5 £ 10 i2 degrees, we estimate the resolution of the biosensor as 7:5£10 i7 refractive index units. We also discuss the possible improvement to realize a predicted superior limit of resolution around 10 i8 refractive index units.

Plasmon resonance-absorption in a metal grating and its application for refractive-index measurement

Yasuuras mode-matching method is employed in the investigation of plasmon resonance-absorption in a metal grating with a gold over-coating and the results are compared with experimental data. Enhancement of TM-TE mode conversion accompanying the plasmon resonance-absorption is examined. When a TM wave is incident on a metal grating, enhanced TM-TE mode conversion occurs at angles of incidence at which the surface plasmons are excited. The strength of the mode conversion depends strongly on the azimuth angle of the mounting. This is verified by experiment and an application for refractive-index measurement is suggested.

3D Super-Resolution Fluorescence Microscopy Using Cylindrical Vector Beams

We propose a method to obtain nano-scale 3D super- resolution in STED ∞uorescence microscopy. A double-ring-shaped cylindrical vector vortex beam, with an appropriate vortex angle and a proper truncation parameter of the beam, is used to generate a 3D dark spot as the erase spot. A single-ring-shaped radially polarized beam is used as a pump beam, which can generate a sharper 3D bright spot. The volume of the generated 3D dark spot is small and the uniformity of the light wall surrounding the spot is quite high. Consequently, the 3D super-resolution ability of a STED microscope is improved and nano-scale three-dimensional resolutions are obtained.

Light scattering from two-dimensional periodic arrays of noble-metal disks and complementary circular apertures

Numerical solution is presented for light scattering from two kinds of free-standing periodic arrays, that is, disks made of noble-metal and circular apertures perforated in a thin noble-metal sheet. The shapes of them are complementary to each other, and the circular areas are allocated along two orthogonal coordinates with the same periodicity. Using the generalized boundary conditions of the surface impedance type, we formulate the boundary value problem into a set of integral equations for unknown electric and magnetic current densities defined over the circular area. Employment of the method of moments allows us to solve the integral equations and give the expansion coefficients of the current densities, from which we can find reflected, transmitted, and absorbed powers. Dependence of the powers on the array parameters and wavelength is discussed in detail from the viewpoint of grating resonance. Special attention is paid to the extraordinary transmission which occurs in the arrays of apertures of sub-wavelength size by analytical derivation of the quasi-static solutions.

Surface Plasmon Resonance Absorption in a Multilayered Bigrating

We numerically analyze surface plasmon resonance absorption of incident light en- ergy by a multilayered bigrating, which consists of dielectric and metallic thin-fllms corrugated periodically in two directions. We apply Yasuuras modal expansion method for solving the prob- lem of plane-wave difiraction by a multilayered bigrating and evaluating the absorption, which is observed as dips in difiraction e-ciency curves. Employing the numerical algorithm, we numer- ically examine characteristics of the surface plasmon resonance absorption in the multilayered bigrating.