D-band optical modulators using electro-optic polymer waveguides and non-coplanar patch antennas.

D-band (110-170 GHz) antenna-coupled optical modulators with electro-optic (EO) polymer waveguides and non-coplanar patch antennas were fabricated using a poled EO polymer film transfer method. A carrier-to-sideband ratio (CSR) of 42.3 dB corresponding to an optical phase shift of 15.3 mrad was obtained by irradiating 150 GHz electromagnetic waves with an irradiation power density of 34.3 W/m2. Our devices and fabrication method have great potential for achieving highly efficient wireless-to-optical signal conversion in radio-over-fiber (RoF) systems.

Bayesian-based decipherment of in-depth information in bacterial chemical sensing beyond pleasant/unpleasant responses.

Chemical sensing is vital to the survival of all organisms. Bacterial chemotaxis is conducted by multiple receptors that sense chemicals to regulate a single signalling system controlling the transition between the direction (clockwise vs. counterclockwise) of flagellar rotation. Such an integrated system seems better suited to judge chemicals as either favourable or unfavourable, but not for identification purposes though differences in their affinities to the receptors may cause difference in response strength. Here, an experimental setup was developed to monitor behaviours of multiple cells stimulated simultaneously as well as a statistical framework based on Bayesian inferences. Although responses of individual cells varied substantially, ensemble averaging of the time courses seemed characteristic to attractant species, indicating we can extract information of input chemical species from responses of the bacterium. Furthermore, two similar, but distinct, beverages elicited attractant responses of cells with profiles distinguishable with the Bayesian procedure. These results provide a basis for novel bio-inspired sensors that could be used with other cell types to sense wider ranges of chemicals.

W-band optical modulators using electro-optic polymer waveguides and patch antenna arrays.

In this study, W-band (75-110 GHz) antenna-coupled optical modulators with a cyclo-olefin polymer (COP) lower cladding, an electro-optic (EO) polymer waveguide, and a gap-embedded patch antenna array were fabricated using a transfer and bonding method of a poled EO polymer film. A carrier-to-sideband ratio (CSR) of 56 dB corresponding to an optical phase shift of 3 mrad was obtained under irradiation with 100 GHz electromagnetic waves with a power of 12.8 W/m2. Our devices have the potential to achieve highly efficient wireless-optical signal conversion in radio-over-fiber (RoF) systems.

Terahertz-wave generation devices using electro-optic polymer slab waveguides and cyclo-olefin polymer clads.

We fabricated terahertz (THz) wave generation devices using electro-optic (EO) polymer slab waveguides and cyclo-olefin polymer (COP) clads with very small absorption loss of the THz waves based on a novel device fabrication procedure involving bonding of the poled EO polymer layer to the COP substrates. We demonstrated THz wave generation from the EO polymer slab devices using a 1.55 µm-band femtosecond fiber laser and evaluated the THz wave generation properties of the devices. Our results will lead to the development of compact, highly efficient, and ultrabroadband THz devices using EO polymers.

Optical Properties of Low-Loss Ag Films and Nanostructures on Transparent Substrates.

We demonstrate the fabrication of a low-loss single-crystalline Ag nanostructure deposited on transparent substrates. Our approach is based on an epitaxial growth technique in which a NaCl(001) substrate is used. The NaCl substrate is dissolved in water to allow the Ag film to be transferred onto the desired substrates. Focused ion beam milling is subsequently employed to pattern a nanoarray structure consisting of 200 nanorods. The epitaxial Ag films with nanoarray structures grown in the study exhibited very flat and smooth surfaces having excellent crystallinity and local misorientation of less than 1°. Further, spectroscopic ellipsometry measurements indicated that the imaginary part of the dielectric constant of the single-crystalline film was smaller than that of a conventional polycrystalline film. Moreover, we used the three-dimensional finite-difference time-domain method to analyze the plasmonic properties of the nanoarray structure by considering the actual processed structure. Characteristically, when the SiO2 substrate was etched by ion beam milling to a depth of 30 nm, the spectrum showed a spectral shape 20% sharper than that of the substrate with no etching (depth: 0 nm). The plasmonic performance of the single-crystalline Ag nanostructure was largely determined by its structural precision and the dielectric properties of the metal.

Infrared Spectroscopy of Ionic Liquids Consisting of Imidazolium Cations with Different Alkyl Chain Lengths and Various Halogen or Molecular Anions with and without a Small Amount of Water.

Infrared spectroscopy was performed on ionic liquids (ILs) that had imidazolium cations with different alkyl chain lengths and various halogen or molecular anions with and without a small amount of water. The molar concentration normalized absorbance due to +C-H vibrational modes in the range of 3000 to 3200 cm-1 was nearly identical for ILs that had imidazolium cations with different alkyl chain lengths and the same anions. A close correlation was found between the red-shifted +C-H vibrational modes, the chemical shift of +C(2)-H proton, and the energy stabilization of the hydrogen-bonding interaction. The vibrational modes of the water molecules interacting with anions in the range between 3300 and 3800 cm-1 was examined. The correlation between the vibrational frequencies of water, the frequencies of +C-H vibrational modes, and the center frequency of intermolecular vibrational modes due to ion pairs was discussed.

Development of UV Light Irradiation Patterning of Bacteriorhodopsin Thin Films for Biomimetic Functional Devices.

We developed a new patterning method for bacteriorhodopsin (bR) thin films using UV light irradiation. The proton pump function of bR thin films can be deactivated with UV light irradiation. Inactivation of the proton pump function of bR is related to structural changes or photo-bleaching of the retinal in bR using UV light exposure, which was confirmed with absorption and Raman spectroscopy measurements. Utilizing inactivation of the proton pump function with UV light irradiation, we prepared a bR photocell with a stripe-patterned bR thin film and measured its photocurrent response. The new patterning method is applicable to complicated patterning and patterning with a higher spatial resolution, which extends the application of bR thin films as sensor devices.

Enhanced Photocurrent Generation from Bacteriorhodopsin Photocells Using Grating-Structured Transparent Conductive Oxide Electrodes.

We fabricated a grating-structured electrode made of indium-doped zinc oxide (IZO) with a high refractive index (approximately 2) for a bacteriorhodopsin (bR) photocell. We investigated the photocurrent characteristics of the bR photocell and demonstrated that the photocurrent values from the bR/IZO electrode with the grating structure with a grating period of 340 nm were more than 3.5-4 times larger than those without the grating structure. The photocurrent enhancement was attributed to the resonance effect due to light coupling to the grating structure as well as the scattering effect based on the experimental results and analysis using the photonic band structure determined using finite-difference time-domain (FDTD) simulations. The refractive index of the bR film in electrolyte solution (1.40) used in the FDTD simulations was estimated by analyzing the extinction peak wavelength of 20-nm gold colloids in the bR film. Our results indicate that the grating- or photonic-crystal-structured transparent conductive oxide (TCO) electrodes can increase the light use efficiency of various bR devices such as artificial photosynthetic devices, solar cells, and light-sensing devices.

Duality picture of Superconductor-insulator transitions on Superconducting nanowire.

In this study, we investigated the electrical transport properties of niobium titanium nitride (NbTiN) nanowire with four-terminal geometries to clarify the superconducting phase slip phenomena and superconducting-insulator transitions (SIT) for one-dimensional superconductors. We fabricated various nanowires with different widths and lengths from epitaxial NbTiN films using the electron beam lithography method. The temperature dependence of resistance R(T) below the superconducting transition temperature Tc was analyzed using thermal activation phase slip (TAPS) and quantum phase slip (QPS) theories. Although the accuracy of experimental data at low temperatures can deviate when using the TAPS model, the QPS model thoroughly represents the R(T) characteristic with resistive tail at low temperatures. From the analyses of data on Tc, we found that NbTiN nanowires exhibit SIT because of the change in the ratio of kinetic inductance energy and QPS amplitude energy with respect to the flux-charge duality theory.

Terahertz and Infrared Spectroscopy of Room-Temperature Imidazolium-Based Ionic Liquids.

The terahertz- and infrared-frequency vibrational modes of various room-temperature imidazolium-based ionic liquids with molecular anions were examined extensively. We found that the molar-concentration-normalized absorption coefficient spectra in the low-wavenumber range for imidazolium cations with different alkyl-chain lengths were nearly identical for the same anion. Regarding the overall view of a wide range of imidazolium-based ionic liquids, we found that the reduced mass of the combination of an imidazolium-ring cation and the anion and the force constant play significant roles in determining the central frequency of the broad absorption band. In addition to these findings, we also discuss the correlation between the (+)C-H stretching vibrational modes in the 3000-3300 cm(-1) range of the infrared spectra and the intermolecular vibrational band in the low-wavenumber range. Finally, we describe some interesting characteristics of the intermolecular vibrational band observed in a wide range of imidazolium-based ionic liquids.

Vibration Modes at Terahertz and Infrared Frequencies of Ionic Liquids Consisting of an Imidazolium Cation and a Halogen Anion.

The terahertz and infrared frequency vibration modes of room-temperature ionic liquids with imidazolium cations and halogen anions were extensively investigated. There is an intermolecular vibrational mode between the imidazolium ring of an imidazolium cation, a halogen atomic anion with a large absorption coefficient and a broad bandwidth in the low THz frequency region (13-130 cm-1), the intramolecular vibrational modes of the alkyl-chain part of an imidazolium cation with a relatively small absorption coefficient in the mid THz frequency region (130-500 cm-1), the intramolecular skeletal vibrational modes of an imidazolium ring affected by the interaction between the imidazolium ring, and a halogen anion with a relatively large absorption coefficient in a high THz frequency region (500-670 cm-1). Interesting spectroscopic features on the interaction between imidazolium cations and halogen anions was also obtained from spectroscopic studies at IR frequencies (550-3300 cm-1). As far as the frequency of the intermolecular vibrational mode is concerned, we found the significance of the reduced mass in determining the intermolecular vibration frequency.