[in Japanese]

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Extraordinary optical transmission through Penrose metal hole arrays (PMHA) in the terahertz region is investigated numerically by using coupled-mode analysis. Spectral change of the transmittance is studied at resonant frequencies by increasing the number of holes and comparing the spectra of PMHA and a periodic metal hole arrays (MHA). While the resonant mode of MHA is excited in periodic order along one direction, the resonant mode on PMHA has a complicated shape. It seems that the propagating length of surface wave of PMHA is shorter than that of the MHA. Therefore, when the number of hole is increased, increasing rate of the resonant transmittance through PMHA is smaller than that through MHA. In addition, the transmission spectra for three kinds of generalized PMHA are analyzed numerically. Although the distributions of holes in these PMHA are different from each other in real space, the transmission spectra have almost the same shape. Consequently, the structure given by the aperture configuration of PMHA in reciprocal lattice space has stronger effects on extraordinary optical transmission through metal hole arrays, rather than that in real space.

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We have experimentally investigated photoluminescence from thin film of organic semiconductor deposited on hyperbolic metamaterials (HMMs). The HMMs composed of alternating multilayer of subwavelength-thick silver and titanium dioxide with various fill fractions were fabricated utilizing magnetron sputtering. Their optical properties were evaluated from transmittance and reflectance spectra and also compared with theoretical effective dielectric characteristics obtained by effective medium theory. Thin film of organic semiconductor, Tris-(8-hydroxyquinoline)aluminum, Alq3, was deposited by thermal evaporation on the fabricated HMMs. The samples were excited using ultraviolet light emitting diode (UV-LED) and photoluminescence (PL) spectra were measured using multichannel spectrometer. A 2.6-fold enhancement of PL was achieved from Alq3 on the optimized HMM using as a substrate. Our findings may be utilized to develop various types of efficient optical devices based on organic semiconductors such as organic light emitting diodes (OLEDs) and organic solid state lasers.

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We have numerically analyzed the interaction of electron-beam (e-beam) with split-ring resonator (SRR) meta-atoms based on the simplified particle-in-cell finite-difference time-domain (PIC-FDTD) method. We have shown that narrow-band magnetic resonance can be initiated with e-beam excitation, whose resonant frequency can be tuned by structure parameters of SRRs. We have also presented that strength of magnetic resonance varies with changing the acceleration energy of e-beam in a nonmonotonous way. We have shown that there are two important factors, the interaction time and the frequency content of the magnetic field around the e-beam, which should be taken into account for the e-beam excitation. This is very different situation from that in the photo-excitation with varying the excitation intensity. This unique property for the e-beam excitation may play a crucial role in developing the terahertz (THz) devices. Our findings may open the way for the development of e-beam driven THz optical devices.

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An analytic theory for the band structure of surface plasmon polaritons in a one-dimensional corrugated metal surface is constructed. We construct a perturbation theory by regarding the surface corrugation as a perturbation. A hermitian eigenvalue equation is obtained from the matching conditions of electromagnetic fields. Hermiticity of the eigenvalue equation determines the condition for radiation of surface plasmon polaritons. Solutions of the equation are derived, and dependences on the perturbation coincide with those in the electronic band theory in solids.

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We investigated characteristics of Fano resonance in asymmetric-double-bar (ADB) metamaterials in the optical region. An ADB structure consisted of a gold nanowire pair with slightly different bar lengths. ADB metamaterials with different sizes were fabricated by a lift-off method, and optical spectra were measured. For three kinds of fabricated ADB metamaterials with different dimensions, experimental spectra clearly showed Fano resonance at around wavelengths of 1500, 1100, and 750 nm, respectively. Characteristics of Fano resonance were estimated from optical spectra, and dependence of the degree of asymmetry was studied. The quality of Fano resonance increased with decreased in asymmetry due to a small radiative loss. In the visible region, quality of resonance was relatively small compared with that in the near-infrared region. Peak values and contrast depended on structural size of the metamaterials. The results will be useful for designing optical metamaterials with high-quality resonance.

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This paper focuses on silver nanoparticles “dimer” with a molecular bridge for highly sensitive trace detection by Surface-Enhanced Raman Spectroscopy (SERS). We clarified Raman intensity was determined from a concentration of particle dimers in mixing of nanoparticle and molecule solutions by characterization of SERS with the experimental and the analytical studies. The particle dimer gives us higher enhancement factor of SERS intensity than the conventionally used larger cluster of nanoparticles. We proposed a multilayered mixing of nanoparticle and molecule solutions to achieve a higher concentration of the particle dimer in an entire laser spot of a portable Raman spectroscope.

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In this study, we propose electrically controlled active plasmon devices that consist of metallic nanoslits modulated by a nano electro mechanical systems (NEMS) actuator. The proposed device can shift the plasmon and anomaly resonance wavelength with a bias voltage. To the best of our knowledge, this is the first demonstration of a nano-optical filter, sensor, shutter, lens, and waveguide that allows the plasmon and anomaly resonance wavelength to be actively and electrically shifted in a single sample.

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We have established a fabrication method of Split Ring Resonator (SRR) for optical region on a peelable polymer film and developed a stacking technique for the film with SRR. The film with SRR (SRR film) fabricated by nanosphere lithography can be separated from the glass substrate by melting a sacrificial layer formed between the SRR film and the substrate, and stacked on other substrate by scooping the SRR film floating in water. We have successfully fabricated the double-layered SRR film by the stacking technique. The experimental scattered light spectra of the double-layered SRR film possess the peak related to the LC resonance mode of the SRR. The vertical SRR to the substrate was also fabricated on a micro-size cubic on a silica substrate by the stacking technique.

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We propose an approach to build tunable terahertz (THz) devices by using the concept of reconfigurable metamaterials developed by surface MEMS (micro electro mechanical system) technique. The implemented metamaterial is composed of an array of unit cells based on the inductor-capacitor (LC) resonators. By using the MEMS technique to reconfigure the structure of the unit cell, thus the capacitance of the resonator, we demonstrated the tunable transmission of the metamaterial device controlled by applied voltage (around 40 V). The device can be used as a tunable THz band-stop filter and an amplitude modulator with double frequency bands. By engineering the arrayed configuration for the unit cells, we propose a method to constitute a programmable THz spatial modulator, such as a THz lens, which has wide range of applications in THz spectroscopy and THz communication.

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Electromagnetically induced transparency (EIT)-like effects in gold metamaterials consisting of dipole resonators and quadrupole resonators were experimentally demonstrated in terahertz frequency region, and their transmittance characteristics were evaluated. Transmittance characteristics of the metamaterials could be controlled by the gap distance between the two resonators. At frequencies around 1.5 THz, transmittance between 82.0% and 0.9% were observed for fabricated EIT metamaterials with gap distances between 3.1 and 23.1 µm.

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Desire for terahertz sophisticated optical devices is rapidly increasing in order to prepare the utilization of higher frequencies, such as terahertz frequencies and optical ranges. However, conventional techniques cannot solve the limitation that devices are designed only by naturally-occurring materials with positive refractive indices. Metamaterials with a negative refractive index can overcome the limitation and provide potential solutions to the demands for novel devices. We design a negative refractive index by a metal-slit array with split-ring resonators in a terahertz frequency. A negative permittivity is designed by a parallel-plate waveguide under a cutoff frequency, and a negative permeability is by the resonance of sprit-ring resonators. An approximate analysis with periodic boundary walls designs an effective negative refractive index with n_eff = -2.5 + j0.40 at 0.30 THz, and a full model analysis visually confirms performance with n = -2.6 at 0.30 THz. The approximate model can drastically reduce a time-consuming design procedure and simultaneously prepare initial parameters for the full model analysis. The proposed design model with the negative refractive index would lead to potential applications, such as bandpass filters with a high extinction ratio in undesired frequency bands and superlenses overcoming a diffraction limit.

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The current understanding is that the progress of terahertz technology demands highly-functional and high-performance optical devices. Materials are limited in naturally-occurring materials with positive refractive indices when conventional techniques design the devices. However, metamaterial techniques can shed light on a variety of potential applications because a refractive index including a negative one is arbitrarily controlled by various subwavelength structures. Here, inspired by novel fabrication techniques of micron-scale structures, we design a negative refractive index by a metal-slit array with three-dimensional metal microcoils in a terahertz frequency. The parallel-plate waveguide under a cutoff frequency can produce a negative permittivity, and the resonance of the sprit-ring resonators can also produce a negative permeability. An effective negative refractive index with n_eff = -6.9 + j1.1 is designed at 0.26 THz by an approximate analysis with periodic boundary walls, and performance with n = -3.2 at 0.30 THz is visually confirmed by a full model analysis. A time-consuming procedure in the iterative design of the full model can be drastically escaped by initial parameters which is obtained by the approximate model. The structure with the negative refractive index would be applied to many progressive devices, such as bandpass filters, superlenses, and antennas.

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This paper reports the fabrication technique for chain-like arrangements of gold nanoparticles on a substrate and its optical characterization. We successfully fabricated the directional chain-like arrangements and controlled the particle number by the self-assembly using nanotrench templates. We fabricated the dense arrangement in the nanotrenches with high yield reducing an electrostatic repulsive force between particles in water suspension by an addition of electrolytes. Extinction spectra showed the shift of spectrum peaks depending on the particle number, which was consistent with the analytical results.

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A metamaterial can produce a negative refractive index and apply to a superlens overcoming a diffraction limit. We have previously reported that the measurement of asymmetric paired metal cut wires confirms a negative refractive index with a low loss at 0.42 THz. However, it has yet to be shown whether the structure could have a parameter with a negative refractive index at the other frequencies. We confirm that the structure can design a negative refractive index from 0.5 THz to 0.9 THz. This proposed structure with asymmetric paired metal cut wires would provide superlenses and lead to drastic progress of terahertz imaging.

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The polarization control of a terahertz wave can provide fruitful technology for communication, imaging, and exploration of novel phenomena in terahertz frequencies. However, it is not so easy to realize a high-performance wave plate with a large aperture, flexibility, and thinness at low cost and by easy fabrication. We design and fabricate a terahertz quarter-wave plate on a cyclo-olefin polymer film with a copper layer by etching. Measurements by terahertz time-domain spectroscopy confirm a transmission power with 82% and an ellipticity with -1.0 at 0.43 THz. The quarter-wave plate would lead to compact transmission devices and many progressive applications.

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