Mitsuo Wada Takeda

Fabrication of electromagnetic crystals with a complete diamond structure by stereolithography

Three-dimensional electromagnetic or photonic crystals with periodic variations of the dielectric constants were fabricated by a rapid prototyping method called stereolithography. Millimeter-order epoxy lattices with a diamond structure were designed to reflect electromagnetic waves by forming an electromagnetic bandgap in the GHz range. Titanium oxide-based ceramic particles were dispersed into the lattice to control the dielectric constant, resulting in the formation of narrow and deep bandgaps. Crystals of the inverse form that have a network of holes with the diamond structure in a dielectric matrix were successfully fabricated as well. These inverse diamond structures formed the perfect bandgap reflecting electromagnetic waves for all directions. The location of the bandgap agreed with the band calculation using the plane wave propagation method.

Control of microwave emission from electromagnetic crystals by lattice modifications

Millimeter order electromagnetic crystals composed of epoxy lattices containing titania-based particles and having a three-dimensional diamond structure were fabricated by using a stereolithograpy process. The diamond lattice structure formed a perfect bandgap to microwave transmission, while the directional diamond structures with stretched lattice spacing showed the directional transmission of microwaves. The stretching ratio of the lattice spacing was changed according to the band calculation using a plane wave propagation method. A microwave antenna head composed of normal and stretched diamond structures was fabricated which achieved unidirectional transmission.

Ultrafast optical control of group delay of narrow-band terahertz waves

We experimentally demonstrate control over the group delay of narrow-band (quasi continuous wave) terahertz (THz) pulses with constant amplitude based on optical switching of a metasurface characteristic. The near-field coupling between resonant modes of a complementary split ring resonator pair and a rectangular slit show an electromagnetically induced transparency-like (EIT-like) spectral shape in the reflection spectrum of a metasurface. This coupling induces group delay of a narrow-band THz pulse around the resonant frequency of the EIT-like spectrum. By irradiating the metasurface with an optical excitation pulse, the metasurface becomes mirror-like and thus the incident narrow-band THz pulse is reflected without a delay. Remarkably, if we select the appropriate excitation power, only the group delay of the narrow-band THz pulse can be switched while the amplitude is maintained before and after optical excitation.

Characterization of Terahertz Metamaterials Fabricated on Flexible Plastic Films: Toward Fabrication of Bulk Metamaterials in Terahertz Region

Electromagnetic characteristics of a split-ring resonator (SRR) fabricated on flexible thin plastic films are investigated for the purpose of developing three-dimensional (3D) metamaterials in the terahertz region. Both electric and magnetic resonances are observed at frequencies determined by the structural parameters of the SRR. Since the optical path length of the plastic film is smaller than the wavelength of the resonant frequency of metamaterials, the diffraction effect is avoided in a 3D metamaterial. We also observe that each layer functions independently without interlayer interaction, indicating that the properties of the metamaterial are determined only by the unit cell design.

Three-dimensional bulk metamaterials operating in the terahertz range

Three-dimensional bulk metamaterials that operate in the terahertz (THz) frequency range were fabricated by stacking 100 two-dimensional sheets containing metallic split-ring resonators (SRR) on thin polyethylene terephthalate film substrates. The THz magnetic resonance for the incident magnetic field perpendicular to the plane of the SRR structure was measured. We also investigated the dependence of the magnetic resonant strength on the metal thickness.

Fano effect of metamaterial resonance in terahertz extraordinary transmission

We show that the terahertz resonant transmission through metal hole array can be tailored by filling the holes with metamaterials. Experiment and finite difference time domain simulations show this type of resonant transmission to be induced by locally resonant modes, instead of the usual lateral surface grating mode. As the metamaterial’s local resonances can be manipulated by varying their geometric configurations, this type of resonant transmission can be tuned over a broad frequency regime that is subwavelength to the array periodicity, with a transmission profile that can also be tailored by the frequency location of the resonance. Such tunability of resonant transmission, with its attendant enhanced local field intensity in the vicinity of the aperture, may provide some potential applications.

Analysis of derivative spectrum of indirect exciton absorption in silicon

Abstract It is shown that an analysis of the wavelength modulated absorption spectrum enables us to estimate the transition matrix elements in indirect absorption. The transition matrix elements in silicon are determined to be 0.110 h A for indirect absorption with the TO phonon, 0.0367 h A for the LO phonon and 0.0178 h A for the TA phonon.

Direct observation of split-off exciton and phonon structures in absorption spectrum of silicon

Abstract The split-off band exciton of silicon has been observed in the absorption spectrum by using a wavelength modulation technique. The spin-orbit splitting of the valence band is determined to be 44.1 ± 0.3 meV at 1.8 °K. The structures associated with some two-phonon indirect transitions have also been observed in the absorption spectrum.

Highly sensitive surface plasmon terahertz imaging with planar plasmonic crystals

We report on the operation of a highly sensitive terahertz imaging system relying on a planar metallic plasmonic crystal as a terahertz surface plasmon resonant (THz-SPR) sensor. The terahertz imaging is based on the resonantly enhanced transmission phenomenon of a periodically perforated metal film. The detection sensitivity and the imaging contrast for small amounts of substance are considerably better than those of the conventional terahertz transmission imaging without a THz-SPR sensor. As a demonstration, a high contrast image of a fingerprint recorded on a thin film can be achieved by using this system.

Exploiting scattering media for exploring 3D objects

Scattering media, such as diffused glass and biological tissue, are usually treated as obstacles in imaging. To cope with the random phase introduced by a turbid medium, most existing imaging techniques recourse to either phase compensation by optical means or phase recovery using iterative algorithms, and their applications are often limited to two-dimensional imaging. In contrast, we utilize the scattering medium as an unconventional imaging lens and exploit its lens-like properties for lensless three-dimensional (3D) imaging with diffraction-limited resolution. Our spatially incoherent lensless imaging technique is simple and capable of variable focusing with adjustable depths of focus that enables depth sensing of 3D objects that are concealed by the diffusing medium. Wide-field imaging with diffraction-limited resolution is verified experimentally by a single-shot recording of the 1951 USAF resolution test chart, and 3D imaging and depth sensing are demonstrated by shifting focus over axially separated objects.