Yoji Jimba

Light diffraction from a bilayer lattice of microspheres enhanced by specular resonance

Anomalously strong diffraction from a bilayer lattice of dielectric microspheres, previously reported by the authors, has been attributed to the enhancement of diffraction by specular resonance in constituent bispheres. On the basis of rigorous calculations and experiments, specular resonance from bispheres is found to be dominant even in the scattering from a cluster of several microspheres. As a consequence, a diffraction model, in which a bilayer lattice of microspheres is viewed as a two-dimensional array of bispheres, is constructed. By incorporating the specular resonance into a diffraction theory of two-dimensional lattices as a structure factor, observed diffraction behavior is explained. In the diffraction from a bilayer lattice of microspheres, the specular resonance functions as a blazing mechanism. Possible configurations and suitable parameters for efficient gratings are discussed, and a self-assembled bilayer lattice of dielectric microspheres with a diameter 1.6–3.2 times as large as the wa...

Fabrication and Performance of

A two-dimensional dielectric metamaterial composed of a TiO2 cube array is fabricated, and its performance is characterized in the terahertz region. The effective electromagnetic parameters are retrieved from the complex transmission and reflection coefficients of the sample, obtained using terahertz time-domain spectroscopy. The retrieved parameters show negative permeability and permittivity at around 0.28 and 0.38 THz, respectively. We also show via simulations that negative refractive index metamaterials can be realized by combining two sets of TiO2 cubes of appropriate sizes. We also propose and fabricate composite structures made of TiO2 spheres and a metal mesh to obtain a negative refractive index. The observed transmission band is attributed to the negative refractive index. TiO2 is a promising material for fabricating negative refractive index metamaterials for the terahertz frequency region.

\hbox{TiO}_{2}

The concept of local photonic lattice (PL) is introduced in 1D photonic lattices (PLs) to interpret the defects as donor-like or acceptor-like within any band gap of the host PL. Based on the concept of the local PL, an analysis is presented on the tunability of a movable defect which would be used as a tunable frequency filter. By further extending the local PL, we introduce a 1D photonic well composed of the well-like 1D PL sandwiched by the barrier-like 1D PLs. Similarity is investigated between the 1D photonic wells and the semiconductor quantum wells (QWs). It is shown that the bound states are well described by the effective “mass” approximation in analogy to the QWs. These bound states appear as sharp peaks in the transmission spectrum of the finite size 1D photonic well.

-Ceramic-Based Metamaterials for Terahertz Frequency Range

By using coupled-mode analysis, we investigate numerically the extraordinary optical transmission (EOT) of incommensurate metal hole arrays (IMHAs) in the terahertz region to study how the degree of long-range and short-range orders affect EOT. In IMHAs, the holes of one set of metal hole arrays (MHAs) are regarded to work as impurities to the holes of another set of MHAs. Therefore, the transmittance spectra are expected to be significantly modified from those of the constituent MHAs. It is found that the resonance transmission frequencies of IMHAs almost coincide with those of the composite MHAs despite the fact that the surface waves of each MHA are mutually perturbed strongly due to the presence of short-range disorder. This indicates that the presence of long-range order is essential to establish EOT. These observations allow more flexible design of narrow bandpass filters in the terahertz region.

Defects and Photonic Wells in One-Dimensional Photonic Lattices

We have investigated the effect of a semi-infinite substrate on the eigenstates of two-dimensional periodically arrayed dielectric spheres on the basis of the internal electric field intensity distributions. The internal field intensity distributions clearly represent the features of each eigenstate, compared with transmission spectra and external near-field intensity distributions. A substrate with low dielectric constant hardly affects the optical properties of monolayer spheres. However, when the dielectric constant of the substrate exceeds a certain threshold, the field intensity of eigenstates drastically reduces. This behavior can be explained by energy dissipation into the substrate by opening of diffraction channels. Therefore, dips in transmission spectra become broader by interaction with the monolayer spheres and the substrate.

Extraordinary optical transmission through incommensurate metal hole arrays in the terahertz region

In self-assembled multilayer arrays of micrometer-sized spheres that include small amounts of fluorescent particles, unique six-dot-triangular and seven-dot-hexagonal patterns have been known to appear in the fluorescence microscopic images. Although it has been suggested that these two types of patterns correspond to local domain structures, i.e., face centered cubic (fcc) or hexagonal closed packed (hcp), no conclusive evidence has been provided to support this claim. In this study, we systematically investigated the relationship between the propagation patterns and the arrangement of the particles. Through a cross-check between an experiment using well-defined clusters fabricated by a micromanipulation technique and a rigorous calculation based on the expansion of vector spherical harmonics, we confirmed that the six-dot-triangular and seven-dot-hexagonal patterns correspond to the fcc and hcp domains, respectively. Further, we also found that the propagation patterns depend on the size of the clusters. As a result of a quantitative discussion on the light propagation in clusters with various sizes, it was clarified that a sufficient domain size is necessary for the appearance of clear triangular or hexagonal patterns.