A periodic array of InSb spheres on a substrate is numerically analyzed at terahertz frequencies. The incident field is shown to be coupled to the substrate due to the guided-mode resonance. The effect of the background refractive index on the transmission characteristics is investigated for sensor applications.

A grating consisting of a periodic array of InSb-coated dielectric cylinders on a substrate is analyzed at THz frequencies using the frequency-dependent finite-difference time-domain method based on the trapezoidal recursive convolution technique. The transmission characteristics of an infinite periodic array are investigated not only at normal incidence but also at oblique incidence. The incident field is shown to be coupled to the substrate due to the guided-mode resonance (GMR), indicating the practical application of a grating coupler. For the sensor application, the frequency shift of the transmission dip is investigated with attention to the variation of the background refractive index. It is found that the shift of the dip involving the surface plasmon resonance is almost ten times as large as that of the dip only from the GMR. We finally analyze a finite periodic array of the cylinders. The field radiation from the array is discussed, when the field propagates through the substrate. It is shown that the radiation direction can be controlled with the frequency of the propagating field.

Two plasmonic band-bass filters are analyzed: one is a grating-type filter and the other is a slit-type filter. The former shows a band-pass characteristic with a high transmission for a two-dimensional structure, while the latter exhibits a high transmission even for a three-dimensional structure with a thin metal layer.

“How to get the original ideas” is the fundamental and critical issue for the researchers in science and technology. In this paper, the author writes his experiences concerning how he could encounter the interesting and original ideas of three research subjects, i.e., the accelerating medium effect, the guided-mode extracted integral equation and the surface plasmon gap waveguide.

Guiding and nanofocusing of a two-dimensional (2D) optical beam in a negative-dielectric-gap waveguide is studied theoretically. An index-guiding method along the dielectric core embedded in the negative-dielectric-gap is proposed and the confinement properties of the 2D optical beam are studied by the effective-refractive-index method and FDTD simulations. We have shown that the lateral beam width of the 2D optical beam can be shrunk to zero beyond the diffraction limit. A tapered negative-dielectric-gap waveguide using adiabatic propagation achieves nano-focusing and can be applied to nano-optical couplers. This is a gateway from conventional dielectric waveguides to nano-optical integrated circuits.

The radiation properties of oscillating electric dipoles are studied theoretically for three and four layered systems including a single metallic slab based on angular spectrum representation of vector spherical waves. One of the remarkable results obtained is the transmission energy spectrum showing strong dependence on the thickness of a dielectric layer placed between oscillating electric dipole and metallic surface, which explains the experimental results of molecular fluorescence into surface plasmon modes. The theory based on angular spectrum representation and tunneling current provides us with a clear identification of plasmonic excitation transfer, transmission loss associated with plasmon transport in metallic layer, and energy dissipation or quenching of excitation due to surface plasmon excitation at the metallic surface in relation to the characteristic complex wave number of evanescent waves.

This paper describes an experimental study on resonant properties of the plasma-wave field-effect transistors (PW-FET's). The PW-FET is a new type of the electron devices, which utilizes the plasma resonance effect of highly dense two-dimensional conduction electrons in the FET channel. Frequency tunability of plasma-wave resonance in the terahertz range was experimentally investigated for sub 100-nm gate-length GaAs MESFET's by means of laser-photo-mixing terahertz excitation. The measured results, including the first observation of the third-harmonic resonance in the terahertz range, however, fairly deviate from the ideal characteristics expected for an ideal 2-D confined electron systems. The steady-state electronic charge distribution in the MESFET channel under laser illumination was analyzed to study the effect of insufficient carrier confinement on the frequency tunability. The simulated results support the measured results. It was clarified that an ideal heterostructure 2-D electron confinement is essential to assuring smooth, monotonic frequency tunability of plasma-wave resonance.

Short-circuit photocurrents (ISC) utilizing surface plasmon polariton (SPP) excitation were investigated for the merocyanine (MC) LB film photoelectric device. The device has a prism/MgF2/Al/MC LB film/Ag structure. In the attenuated total reflection (ATR) configuration, SPPs were resonantly excited at the interfaces between MgF2 and Al (MgF2/Al) and between Ag and air (Ag/air). The thickness and the dielectric constants of the layers were evaluated from the ATR measurements. Short-circuit photocurrents, ISCs, as a function of the incident angle of the laser beam were observed simultaneously during the ATR measurements. In the ISC curves, large and small peaks were observed, and the peak angles of the ISC almost corresponded to the dip angles of the ATR curves due to the SPP excitations. Electric fields and optical absorptions in the cell were calculated using the dielectric constants and the film thickness obtained from the ATR measurements. The calculated absorption in the MC layer as a function of the incident angle corresponded to the ISC curve. It was thought that the optical absorption in the MC layer affected directly to the profile of the ISC. Furthermore, the calculated absorption in the cell with the prism and the MgF2 layer exhibited much larger than that of the cell without them. It was estimated that the photocurrents were enhanced by the excitation of SPPs in the ATR configuration.

A GaAs/AlGaAs directional-coupler-type device that use polariton propagation was fabricated and its switching operation was demonstrated. The length of the switching region is as small as 300 µm. The output signal modulation under an electric field shows typical characteristics of directional-coupler type switching. The measured operation voltage is 2 V for an operation wavelength of 805 nm at 10 K. The corresponding signal extinction ratio is 8 dB. These experimental results confirm the efficient operation of the polariton devices, which can be applied to especially small optical -switching devices with low-voltage operation.

A GaAs/AlGaAs directional-coupler-type device that use polariton propagation was fabricated and its switching operation was demonstrated. The length of the switching region is as small as 300 µm. The output signal modulation under an electric field shows typical characteristics of directional-coupler type switching. The measured operation voltage is 2 V for an operation wavelength of 805 nm at 10 K. The corresponding signal extinction ratio is 8 dB. These experimental results confirm the efficient operation of the polariton devices, which can be applied to especially small optical -switching devices with low-voltage operation.

Attenuated total reflection (ATR) properties and scattered light properties were measured for Ag thin films and arachidic acid (C20) Langmuir-Blodgett (LB) ultrathin films on the Ag thin films to obtain the information about their complex dielectric constants and surface roughness utilizing an excited surface plasmon polariton. The complex dielectric constants for the Ag thin films and the C20 LB films were obtained by fitting the calculated ATR curves to the experimental ones. The surface roughnesses of these films were estimated by the angular distribution of the scattered light assuming the Gaussian function as an autocorrelation function and a linear superposition of roughness spectra. The angular spectra strongly depended on the roughness parameters: the transverse correlation length σ and the surface corrugation depth δ. The experimental angular distributions were explained by some pairs of σ and δ. It was suggested that the surface roughness of the C20 LB films changed with the number of monolayers since the angular spectra varied with the number of the C20 LB monolayers on the Ag films. It is thought that the measurement of the scattered light is useful to evaluate surface roughnesses of LB ultrathin films.