Junji Yamauchi

Surface Plasmon Resonance Waveguide Sensor in the Terahertz Regime

A surface plasmon resonance (SPR) waveguide sensor with a thin InSb layer adopted for the sensing section is proposed in the terahertz region. First, the eigenmode analysis is carried out for the sensing section with water as an analyte. The surface plasmon polariton eigenmodes are shown to exist in the sensing section, and the SPR is expected to be maximum around 1.28 THz at 300 K. Next, the performance of the sensor is investigated using the finite-difference time-domain method, with special attention to the treatment of the interface between InSb and water. As expected, the SPR absorption is found to become maximum at 1.26 THz. Furthermore, the SPR response is examined, in which an ability is shown to detect a temperature variation ranging from 280 to 320 K. As an application, final consideration is given to the possibility of detecting a mixture of an organic solvent and water.

Full-Vectorial Beam-Propagation Methods Based on a Fundamental Scheme—Design of a Short Polarization Converter

A fundamental scheme is utilized to reformulate the full-vectorial beam-propagation methods (BPMs). First, the fundamental scheme is introduced into the Fresnel equations which are discretized using the Crank-Nicolson method (FCN-BPM). The present FCN-BPM has the advantage that the total number of arithmetic operations is extremely reduced when compared with the conventional CN-BPM, while maintaining identical accuracy. Next, the fundamental scheme is applied to the alternating-direction implicit BPM (FADI-BPM). The choice of the refractive index at each split step is discussed paying attention to the commutativity of coefficient matrices. With the present method, computation time is reduced to approximately 74% for the analysis of a mode-evolution-based z-varying polarization converter. Power expression based on both electric and magnetic fields is also adopted to improve the power conservation. Finally, the present method is applied to the design of a short polarization converter. It is revealed that the use of a curvilinearly tapered core leads to an extinction ratio of more than 15 dB with a device length of 100 μm, over a wide wavelength range from 1.2 to 1.7 μm.

Numerical investigation of a three-dimensional stub-type plasmonic filter

A three-dimensional (3D) stub-type plasmonic filter is analyzed using the FDTD method based on the trapezoidal recursive convolution technique. The peak transmissivity of the 3D filter is calculated to be ≃50%, while the two-dimensional filter yields more than 90% transmissivity. To improve the transmissivity of the 3D filter, we insert a dielectric material into the stub section. It is found that the transmissivity is increased to more than 80% with the stub section being filled with SiC.

Optical nanodipole and nanospiral antennas

The antenna impedance of a nanodipole can be matched to a metal-insulator-metal (MIM) waveguide impedance by the aid of a shunt stub connected at the feed point. Generation of a circularly polarized wave is numerically demonstrated by a rectangular spiral structure fed by the MIM waveguide.

Basic studies of optical nanoantennas

A shunt stub is employed to match the antenna impedance of a nanodipole to that of a metal-insulator-metal (MIM) waveguide. A rectangular spiral fed by the MIM wave-guide radiates a circularly polarized wave, with a high radiation efficiency of 0.95 being retained.

Quarter-Wave Plates Consisting of Subwavelength Triangular Hole Arrays

Novel quarter-wave dielectric and metallic plates are proposed and the polarization characteristics are studied numerically over a wide wavelength range of 1.2 to 1.8 μm. Arrays of subwavelength triangular holes produce the polarization conversion.

An imaginary-distance YM-BPM with an enhanced amplification factor for the analysis of a periodic structure

The periodic boundary condition is applied to an imaginary-distance beam-propagation method based on Yees mesh with an enhanced amplification factor. The eigenmode analysis of a rectangular hole array is performed for various filling factors. The polarization conversion property is demonstrated using the FDTD analysis.

A waveguide configuration for reducing both pure bend and polarization dependent losses

A simple strategy for reducing both the pure bend and polarization dependent losses is newly proposed. It is based on the introduction of a partial over-cladding layer in a buried waveguide. The numerical results show that the present waveguide configuration reduces the losses over a wide spectral range.

Tapered dielectric nanorod antenna at optical wavelengths

A directive radiation element, tapered dielectric nanorod antenna, fed by a metal-insulator-metal waveguide is studied at optical wavelengths. The characteristics are numerically investigated with the frequency-dependent FDTD method. The tapered nanorod achieves a higher actual gain than that with a nanodipole, with a low reflectivity of 0.07 being retained.

Analysis of a metal disc-shaped terahertz surface wave splitter with a center rod

We have proposed a metal disc-shaped terahertz surface wave splitter with a center hole and radially placed gratings. Unfortunately, appreciable reflected waves have been observed from the input port of the center hole. To reduce the reflected waves, we insert a metal rod with a tapered end into the center hole of the disc-shaped splitter operating at two frequencies. It is found that the reflected power is reduced from 77 to 10% at 1.0 THz, and from 23 to 18% at 1.5 THz.