Carrier conversion from terahertz wave to dual-wavelength near-infrared light for photonic terahertz detection in wireless communication.

THz waves are promising wireless carriers for next-generation wireless communications, where a seamless connection from wireless to optical communication is required. In this study, we demonstrate carrier conversion from THz waves to dual-wavelength NIR light injection-locking to an optical frequency comb using asynchronous nonpolarimetric electro-optic downconversion with an electro-optic polymer modulator. THz wave in the W band was detected as a stable photonic RF beat signal of 1 GHz with a signal-to-noise ratio of 20 dB via the proposed THz-to-NIR carrier conversion. In addition, the results imply the potential of the photonic detection of THz waves for wireless-to-optical seamless communication.

Ultra-thin deep ultraviolet perfect absorber using an Al/TiO2/AlN system.

An ultra-thin perfect absorber for deep ultraviolet light was realized using an Al/TiO2/AlN system. The TiO2 thickness was optimized using the Fresnel phasor diagram in complex space to achieve perfect light absorption. As a result of the calculation almost perfect absorption into the TiO2 film was found, despite the film being much thinner than the wavelength. An optimized Al/TiO2/AlN system was fabricated, and an average absorption greater than 97% was experimentally demonstrated at wavelengths of approximately 255-280 nm at normal light incidence. Our structure does not require nanopatterning processes, and this is advantageous for low-cost and large-area manufacturing.

Experimental confirmation of self-imaging effect between guided light and surface plasmon polaritons in hybrid plasmonic waveguides.

We fabricated a hybrid plasmonic device using self-imaging effect between guided light and surface plasmon polaritons in the hybrid plasmonic waveguide. The hybrid plasmonic device was fabricated by evaporating gold on the part of the silicon waveguide. Self-imaging was generated at the gold-covered section in the waveguide. Self-imaging of guided light and surface plasmon polaritons in hybrid plasmonic waveguides affect the output intensity of the hybrid plasmonic waveguide. The length of the hybrid plasmonic waveguide changes self-imaging conditions. We confirmed that the output intensity was affected by the length of the hybrid plasmonic waveguide. These findings contribute to the development of hybrid plasmonic devices and potentially improve integration density of hybrid photonic integrated circuits.

Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe.

Tip-enhanced Raman spectroscopy (TERS) is a nano-optical approach to extract spatially resolved chemical information with nanometer precision. However, in the case of direct-illumination TERS, which is often employed in commercial TERS instruments, strong fluorescence or far-field Raman signals from the illuminated areas may be excited as a background. They may overwhelm the near-field TERS signal and dramatically decrease the near-field to far-field signal contrast of TERS spectra. It is still challenging for TERS to study the surface of fluorescent materials or a bulk sample that cannot be placed on an Au/Ag substrate. In this study, we developed an indirect-illumination TERS probe that allows a laser to be focused on a flat interface of a thin-film waveguide located far away from the region generating the TERS signal. Surface plasmon polaritons are generated stably on the waveguide and eventually accumulated at the tip apex, thereby producing a spatially and energetically confined hotspot to ensure stable and high-resolution TERS measurements with a low background. With this thin-film waveguide probe, TERS spectra with obvious contrast from a diamond plate can be acquired. Furthermore, the TERS technique based on this probe exhibits excellent TERS signal stability, a long lifetime, and good spatial resolution. This technique is expected to have commercial potential and enable further popularization and development of TERS technology as a powerful analytical method.

Optical detection for magnetic field using Ni-subwavelength grating on SiO2/thin-film Ag/glass structure.

An optical sensor for magnetic field detection using Ni-subwavelength grating (SWG) on SiO2/Ag-thin-film/glass substrates was experimentally developed on the basis of the re-radiation condition of surface-plasmon-polaritons (SPPs) at Ag surfaces. The fabricated sample showed two dips in the reflection spectra associated with SPP excitation, and the optical response exhibited good agreement with that simulated by the finite-difference time-domain method. The reflectivity at one of the dip wavelengths varied minimally with the application of the magnetic field, whereas that at the other dip wavelength significantly decreased owing to the large electric field overlap of SPP with the magnetized Ni-SWG. As a result, a magnetic field on the order of a few mT could be detected with a simple normal-incidence optical system.

GaN-Based High-Contrast Grating for Refractive Index Sensor Operating Blue-Violet Wavelength Region.

Owing to its versatility, optical refractive index (RI) sensors with compact size and high chemical stability are very suitable for a wide range of the applications in the internet of things (IoT), such as immunosensor, disease detection, and blood mapping. In this study, a RI sensor with very simple system and high chemical stability was developed using GaN-based high-contrast grating (HCG). The designed HCG pattern was fabricated on GaN-film grown on c-plane sapphire substrate. The fabricated GaN-HCG sensor can detect minuscule RI change of 1.71 × 10-3 with extreme simple surface normal irradiation system. The light behavior inside the GaN-HCG was discussed using numerical electromagnetic field calculation, and the deep understand of the sensing mechanism was provided. The simple system and very high chemical stability of our sensor exploit RI sensing applications in IoT society.

Femtosecond laser self-assembly for silver vanadium oxide flower structures.

Flower-like silver vanadium oxide (SVO) micropatterns were realized by femtosecond laser in situ writing from its precursor. Self-assembled petals irradiated by a femtosecond laser were observed standing on the substrate along the scanned routine assisted by the formation of silver seeds and plasmonic-mediated effects. By controlling the concentration of ammonium monovanadate and the laser exposure time, a different thickness of petals was manipulated from ∼100 nm to micrometers. The SVO products were confirmed Ag4V2O7, AgVO3, and part of Ag3VO4 by x-ray diffraction (XRD) measurement. Photon-driven self-assembly for in situ fabrication of microstructures looks to be an effective and facile technique for SVO and other functional compounds.

Ultraviolet polarizer with a Ge subwavelength grating.

A polarizer with high extinction ratio (ER) was developed in the ultraviolet wavelength region using a germanium subwavelength grating (Ge-SWG). By utilizing an eigenmode with the Ge-SWG, a high ER was numerically predicted to exist without requiring a high structural aspect ratio. After using lithography to fabricate the proposed Ge-SWG, an experimentally high ER value of 17.4 dB was reached at a wavelength of 360 nm in the developed Ge-SWG, which had a very low structural aspect ratio of 1.67.

Design optimization and fabrication of Mach- Zehnder interferometer based on MIM plasmonic waveguides.

We proposed and designed a compact unbalanced Mach-Zehnder interferometer (MZI) based on metal/insulator/metal (MIM) plasmonic waveguides for ultrafast optical signal processing. The MZI was fabricated by a lithography technique and we provide, for the first time experimental evaluation of the transmission performance of the MZI using MIM PWGs. The experimental results were in good agreement with the numerical simulations. The proposed structure could be considered as a key device for on-chip optical integrated circuits.

Optical characterization of the antigen-antibody thin layer using the whispering gallery mode.

We immobilized an antibody (anti-ß-Galactosidase) on a polystyrene microsphere by using a covalent bond, and observed the resonance peaks in the scattered light intensity spectra related to the whispering gallery mode (WGM) excitation of the microsphere. The amount and the optical parameters, i.e., thickness and refractive index, of anti-ß-Galactosidase on the sphere surface were evaluated based on an absorbance measurement and a resonance peak shift measurement, respectively. Moreover, we measured the variation of the WGM spectra depending on the concentration of the enzyme solution (ß-Galactosidase), which allowed us to optically evaluate the thickness and the refractive index of the antigen-antibody layer from the shift of the WGM spectra peak.

Fabrication of microelectrodes based on precursor doped with metal seeds by femtosecond laser direct writing.

Silver (Ag) seeds for assisting femtosecond laser direct writing (FsLDW) were employed in the fabrication of microelectrodes (MEs). Pattern-controllable and size-tunable MEs can be easily constructed by introducing Ag seeds to the ion precursor solution in the process of laser-induced photoreduction of the Ag ions. The fabrication process is stable under sufficient material supply, and the applied laser power is reduced to one-tenth of that without Ag seeds. Finally, as a representative application, an organic field effect transistor (OFET) was fabricated, based on this laser-fabricated Ag ME. The OFET exhibited good photoelectric properties, and achieved an on-off ratio of 200.

Dependence of LC resonance wavelength on size of silver split-ring resonator fabricated by nanosphere lithography.

We fabricated silver split-ring resonators (SRRs) using nanosphere lithography and measured the LC resonance wavelength of single isolated SRRs in optical wavelength range. The SRRs' sizes decreased when smaller polystyrene spheres were used as templates, and their LC resonance wavelength decreased to 721 nm. The LC resonance wavelength corresponding to the observed properties of the SRRs was calculated using the LC circuit model; we confirmed that the observational and calculated results agreed well. The LC resonance frequency of a miniaturized SRR with a constant shape was also calculated. For SRRs of the shape that we fabricated, the estimated short-wavelength limit was 426 nm.

Sterilization effect of UV light on Bacillus spores using TiO(2) films depends on wavelength.

UV light and photocatalysts such as titanium dioxide (TiO(2)) and silver (Ag) are useful for disinfection of water and surfaces. However, the effect of UV wavelength on photocatalytic disinfection of spores is not well understood. Inactivation of Bacillus spores has been examined using different UV wavelengths and TiO(2) or TiO(2)/Ag composite materials. The level of UVA disinfection of Bacillus anthracis and Bacillus brevis vegetative cells increased with the presence of the TiO(2) and Ag photocatalysts, but had little effect on their spores. B. brevis spores were slightly more sensitive to UVB and UVC than the spores of B. atrophaeus. Photocatalytic sterilization against spores was strongest in UVC and UVB and weakest in UVA. The rate of inactivation of Bacillus spores was significantly increased by the presence of TiO(2), but was not markedly different from that induced by the presence of Ag. Therefore, TiO(2)/Ag plus UVA can be used for the sterilization of vegetative cells, while TiO(2) and UVC are effective against spores.

Additional effects of silver nanoparticles on bactericidal efficiency depend on calcination temperature and dip-coating speed.

There is an increasing interest in the application of photocatalytic properties for disinfection of surfaces, air, and water. Titanium dioxide is widely used as a photocatalyst, and the addition of silver reportedly enhances its bactericidal action. However, the synergy of silver nanoparticles and TiO(2) is not well understood. The photocatalytic elimination of Bacillus atrophaeus was examined under different calcination temperatures, dip-coating speeds, and ratios of TiO(2), SiO(2), and Ag to identify optimal production conditions for the production of TiO(2)- and/or TiO(2)/Ag-coated glass for surface disinfection. Photocatalytic disinfection of pure TiO(2) or TiO(2) plus Ag nanoparticles was dependent primarily on the calcination temperature. The antibacterial activity of TiO(2) films was optimal with a high dip-coating speed and high calcination temperature (600°C). Maximal bacterial inactivation using TiO(2)/Ag-coated glass was also observed following high-speed dip coating but with a low calcination temperature (250°C). Scanning electron microscopy (SEM) showed that the Ag nanoparticles combined together at a high calcination temperature, leading to decreased antibacterial activity of TiO(2)/Ag films due to a smaller surface area of Ag nanoparticles. The presence of Ag enhanced the photocatalytic inactivation rate of TiO(2), producing a more pronounced effect with increasing levels of catalyst loading.

Visible near-infrared light scattering of single silver split-ring structure made by nanosphere lithography.

We succeeded in making a silver split-ring (SR) structure of approximately 130 nm in diameter on a glass substrate using a nanosphere lithography technique. The light scattering spectrum in visible near-infrared region of a single, isolated SR was measured using a microscope spectroscopy optical system. The electromagnetic field enhancement spectrum and distribution of the SR structure were simulated by the finite-difference time-domain method, and the excitation modes were clarified. The long wavelength peak in the light scattering spectra corresponded to a fundamental LC resonance mode excited by an incident electric field. It was shown that a single SR structure fabricated as abovementioned can operate as a resonator and generate a magnetic dipole.

Nonlinear trimer resonators for compact ultra-fast switching.

We propose and numerically verify a scheme for compact optical modulation which can enable complex directional switching of signals in integrated micro-optical circuits within hundreds of femtoseconds. The scheme is based on a trimer comprised of two identical silica whispering gallery mode (WGM) microresonators spaced by a central non-linear WGM resonator. The non-linear resonator is in the form of a silica cylinder with a thin coating of an ultrafast Kerr nonlinear material (a J-aggregate of cyanine dye). Using a two-dimensional finite-difference time-domain method and realistic material and structural parameters, we investigated the near-field coupling from a waveguide to the trimer and the subsequent switching process. In our scheme the sandwiched central control resonator has a resonant frequency that is mismatched to that of the input and output resonators. Therefore the optical energy is coupled from the waveguide into only the primary resonator in linear operation. However, for control light intensities of more than approximately 10(-2) W/microm the effective index and hence eigenfrequency of the central resonator can be shifted to match that of its neighbors and hence the optical energy can be redirected.

Characteristics of gap plasmon waveguide with stub structures.

We found that metal-dielectric-metal plasmon waveguides with a stub structure, i.e. a branch of the waveguide with a finite length, can function as wavelength selective filters of a submicron size. It was found that the transmission characteristics of such structures depend on the phase relationship between the plasmon wave passing through the stub and the one returning to the waveguide from the stub. We also propose structures with a lossless 90 degrees bend in a plasmon waveguide, utilizing a stub structure. Furthermore, we present a functional stub structure, e.g., a 1:1 demultiplexer and a wavelength selective demultiplexer.

Composite domain control for surface plasmon resonance.

We developed a method of composite domain control (CDC) for plasmon resonance as an application of multilayered domain control (MLDC). A distinctive characteristic of CDC is the utilization of dielectric and metal particles. Its structure is similar to that fabricated by the sol-gel method. It is considerably thinner than that prepared by MLDC. In addition, it is possible to conveniently and exactly adjust the plasmon resonance utilizing CDC because it combines the characteristics of MLDC. Accordingly, CDC is a conventional method that is more effective than MLDC. Moreover, CDC is suitable in manufacturing with regard to stress reduction, miniaturization, and cost of the products.