We have studied characteristics of an optical resonator with two stubs as functional devices in a plasmon waveguide in
order to realize compact integrated optical circuits. The resonator is consisting of two stubs set separately in the
waveguide where the stub works as low loss mirror. Numerical simulation has clearly shown that the resonator process
high Q value as a plasmonic resonator. We have also fabricated stub-type resonators in gap plasmonic waveguides, of
which the gap width is around 150 nm, with stubs embedded in a silver thin film on a substrate by using FIB direct
processing techniques. The characteristics of these structures have been observed experimentally from visible to near-infrared
light. And we have successfully observed the resonance of the resonator in the transmission spectrum.
We have performed the three dimensional (3D) numerical analysis for a gap plasmon waveguide with two stubs in a
silver film by using the 3D finite-difference time-domain (FDTD) method. The simulated transmittance shows that such
the 3D structure works as a wavelength selective device with submicron size as already predicted in the 2D simulations.
We have also fabricated such a structure on a glass substrate by using the focused ion beam method. The width of the
gap is around 150 nm. The observed transmittance spectra of the structure have clearly indicated the wavelength
dependence and agree well with those obtained by a numerical simulation. Our results show that the structure proposed
by us is promising for a compact wavelength selective device.
We have analyzed the characteristics of three types of gap plasmon waveguides having wavelength
selective functions: one is structured by a Fabry-Perot resonator with reflectors, one is structured by a
slot resonator and the other is the waveguide with a single or two stubs. We have presented the
numerical results of the transmittance spectra for these structures calculated by using the finite-difference
time-domain method. The numerical results have clearly indicated that all structures of a sub-micron
size work properly as wavelength selective devices. The advantage of a stub type of waveguide
is on easiness in fabricating, while that for a Fabry-Perot type of waveguide is to lead to making the size
decrease considerably and relatively high Q-factor.
We have measured the propagation distances of wedge plasmons and two-dimensionally localized gap plasmons (GPW)
at a vacuum wavelength of 632.8nm. The measured propagation distances of the wedge plasmons increased from
2.2μm to 3.1μm with increasing the wedge tip radius from 20nm to 125nm. The GPW has the measured propagation
distance of 8.2μm for a gap width of 100nm and 900nm height.
We have developed a finite-difference time-domain (FDTD) method represented by the spherical coordinates which is
applicable for numerical calculations of nonlinear optical responses. This FDTD technique gives information about
time-dependent spatial distributions of light intensity in nonlinear metallic particles and we can deeply understand
nonlinear optical phenomena related with localized surface plasmons in a spherical particle.
We have numerically investigated characteristics of plasmonic waveguides for coupled wedge plasmons (CWPs)
consisting two silver wedges separated by a nano gap all on a glass substrate. Three types of waveguides for CWPs on
a glass substrate are considered: (1) two metallic wedges on a planar substrate, (2) two metallic wedges built into the
substrate and (3) two-folded free-standing metallic wedges. For numerical calculation, we have employed the Drude
model for the dielectric constant of silver and the excitation light with the vacuum wavelength of 632.8 nm. The
refractive index of the glass n<sub>s</sub> is fixed at n<sub>s</sub> = 1.5. We have calculated field distributions in the waveguide as well as
dependence on changing the gap w between wedges and the wedge angle θ. CWPs eigenmodes of such structures are
shown to exist and propagate along waveguides structures employed here. The propagation constant k<sub>//</sub>, propagation
distance L and the beam area of a CWP depends on w and θ. L and the beam area size for waveguide employed here are
in the order of 10 μm and in the range from 10<sup>-4</sup> μm<sup>2</sup> to 10<sup>-1</sup> μm<sup>2</sup>, respectively. These values mean that waveguides for
CWPs have a potential to be utilized for the nano optical waveguides in future.
We have performed numerical analysis of localized surface plasmons (LSP) at a nano silver particle on a glass substrate using the finite-difference time-domain method, taking into account the size dependence of the dielectric constants of silver. It was found that the characteristics of LSP at a nano metal particle depend on both shapes of the particles and a dielectric constant of the substrate. In calculations, we employed the geometry in which a nano-particle was located on a glass substrate, the area of calculations. The three types of particles were assumed: a sphere, a spheroid and a hemisphere. In spheroid, the aspect ratio of particles R, is changed from 1.0 to 2.0. For the normal incidence to the spheroid, i.e., the long-axis, the characteristics of LSP are insensitive to R compared. It was observed the red-shift of LSP resonance wavelength and the field enhancement due to the mirror image in the substrate. For spheres and spheroid, the strong enhancement of the z-component field was observed on the substrate. For the hemisphere, we have found the strong localization of the field along the edge of the hemisphere and the strong enhancement of the x-component field was observed on the surface of the substrate.