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.

Photophysical properties of water-soluble porphyrin were studied in aqueous solutions with/without DNA and in DNA solid films. Ultrathin films were prepared from aqueous DNA solutions by a spin-coating method on glass or on gold nanoparticles (AuNPs). Remarkable enhancement of phosphorescence was observed for porphyrin immobilized in DNA films spin-coated on AuNPs, which was attributed to the electric field enhancement and the increased radiative rate by localized surface plasmon resonance of AuNPs.

A quartz-crystal-microbalance (QCM) and surface-plasmon-resonance (SPR) hybrid sensor was prepared, and the depositions of polymer electrolytes layer-by-layer (LbL) films were observed in situ. The estimated thicknesses obtained from the QCM method were different from those obtained from the SPR method. This was estimated to be caused by film swelling and water contained in the film.

We investigated optical transmission characteristics of aluminum thin films with periodic hole arrays in sub-wavelength. We divided white light into several color spectra using a color filter based on the surface plasmon resonance (SPR) utilizing aluminum showing high plasma frequency. By optimizing a hole-array period, hole shape, polarization and index difference of two surface, transmittance of 30% and full-width at half-maximum of around 100 nm were achieved.

We report on the first terahertz emission from a novel dual grating gate plasmon-resonant emitter fabricated with InAlAs/InGaAs/InP material systems. The introduction of InP based heterostructure material systems, instead of the GaAs based ones, in order to improve the quality factor, has successfully enhanced the THz emission intensity and realized the spectral narrowing at room temperature.

We have proposed a terahertz (THz) emitter utilizing two-dimensional plasmons (2DPs) in a super-grating dual-gate (SGG) high electron mobility transistor (HEMT). The plasmon under each grating gate has a unique feature that its resonant frequency is determined by the plasma-wave velocity over the gate length. Since the drain bias voltage causes a linear potential slope from the source to drain area, the sheet electron densities in periodically distributed 2DP cavities are dispersed. As a result, all the resonant frequencies are dispersed and undesirable spectral broadening occurs. A SGG structure can compensate for the sheet electron density distribution by modulating the grating dimension. The finite difference time domain simulation confirms its spectral narrowing effect. Within a wide detuning range for the gate and drain bias voltages giving a frequency shifting of 0.5 THz from an optimum condition, the SGG structure can preserve the spectral narrowing effect.

We experimentally investigated terahertz photomixing operation at room temperature in an InGaP/InGaAs/ GaAs two-dimensional plasmon-resonant photomixer incorporating grating-bicoupled dual-gate structure. Photoelectrons drifting into a high-density plasmon cavity grating from an adjacent low-density one extensively excite the plasmon resonance, resulting in emission of terahertz radiation. A vertical cavity formed between the two-dimensional plasmon grating plane and an indium-tin-oxide mirror at the back surface gains the radiation. Self-oscillation initially at around 4.5 THz excited by a dc-photo carrier component was reinforced by the photomixed differential-frequency excitation at 4.0 and 5.0 THz. This indicates a possibility of injection-locked oscillation of the photomixer in the terahertz frequency band.

A waveguide-based surface plasmon resonance (SPR) sensor with an adsorbed layer is analyzed using the beam-propagation method. For two-dimensional (2-D) models, numerical results show that the change in thickness of the adsorbed layer placed on the metal leads to a significant shift of the maximum absorption wavelength. Through eigenmode analysis, the maximum absorption wavelength is found to be consistent with the cutoff wavelength of the second-order surface plasmon mode. The designed 2-D sensor shows an absorption wavelength shift from 0.595 to 0.603 µm, when the analyte refractive index is increased from 1.330 to 1.334. After a basic investigation using the 2-D models, we next study 3-D models. When the metal with the absorbed layer is wide enough to cover the core region, the 3-D results are similar to the 2-D results. However, as the metal width is reduced, the absorption wavelength shifts toward a shorter wavelength and the sensitivity to the refractive index change degrades gradually. The degradation of the sensitivity is considerable when the metal width is narrower than the core width. As a result, the metal width of the practical SPR sensor should be slightly wider than the core width so as to maintain the sensitivity corresponding to that obtained for the 2-D model.

In this paper, we propose a new method for detecting label-free T4-DNA molecules quantitatively using a surface plasmon resonance (SPR) technique on a gold thin film. We used a solution that dissolved T4-DNA molecules in pure water, and examined the relationship between DNA concentration change and SPR angle change in the solution. As a result, it was confirmed that the SPR angle change increased with increasing DNA concentration change. Therefore, it was feasible to detect the DNA concentration change using the SPR technique. Furthermore, to examine and detect a single or a few DNA molecule, we tried to fabricate an SPR chip in which SPR area is narrowed so that it has the same effect as focusing the beam. To narrow the SPR area, we decreased the area of gold thin film in this chip, and, to reflect light from only the area of gold thin film, the area without a gold thin film was micromachined to increase its unevenness for the reduction of light reflection. By the above-mentioned method, we examined the possibility of detecting a label-free DNA molecule using the SPR technique.

We performed numerical analyses on structure sensitive field emission properties of our proposing plasmon resonant photomixer (PRX) in the terahertz range. The photomixer incorporates doubly interdigitated grating strips for gate electrodes and a vertical resonator structure for realizing highly efficient terahertz emission even at room temperature. We investigated the dependence of total field emission properties of PRX's on their material and dimension parameters. Introduction of low-conductive gate electrodes and ac-coupled 2D periodic plasmon gratings with depleted connecting portions are effective for expanding its lower cutoff frequency. The cutoff frequency, which is around 1.0 THz in standard metal-gates configuration, is expanded to less than 500 GHz. The output intensity could also be amplified more than double. On the other hand, a shorter vertical cavity is effective for expanding its upper cutoff frequency, which is expanded close to vertical resonant frequency, while maintaining the lower cutoff frequency. The combination of these design rules can realize much broader bandwidth operation.

We analytically investigated the feasibility of multiplier operation in the terahertz range for our original plasmon resonant photomixer. The photomixer features two unique structures (doubly interdigitated gate gratings and a vertical cavity) for higher radiation efficiencies. Its total field emission properties are the result of a combination of plasmon excitation dynamics and electromagnetic field dynamics. The plasmon excitation formulated by the hydrodynamic equations exhibits fundamental and harmonic resonances whose intensities monotonically decrease with the number of harmonics due to the dispersive plasma damping factors. The electromagnetic dynamics, on the other hand, formulated by the Maxwell's equations, reflect material- and structure-dependent device parameters; the grating-bi-coupled plasmonic cavity together with the vertical cavity structures produce nonlinear field emission properties. This results in extraordinary field enhancement at distinct frequencies inconsistent with the plasmon resonances. The frequency-dependent FDTD (finite difference time domain method) Maxwell's simulation revealed that the field emission peak frequency shifted upward apart from the fundamental mode of plasmon resonant frequency and approached to its second harmonic frequency with increasing the electron density in the plasmon cavity. Calculated total field emission spectra indicated that highly dense 2D-plasmon conditions enable frequency-doubler operation in the terahertz range.