Terahertz 3D bulk metamaterials with randomly dispersed split-ring resonators.

While optical systems using terahertz wave are expected to achieve beneficial applications, at present, the materials of the optical elements that compose them must be selected from limited choices. In this study, we propose a three-dimensional bulk metamaterial in which metal microstructures are dispersed in the bulk resin randomly. A bulk metamaterial was designed and fabricated, in which split-ring resonators known as typical metamaterials were dispersed in cyclo-olefin polymer.

Actively tunable THz filter based on an electromagnetically induced transparency analog hybridized with a MEMS metamaterial.

Electromagnetically induced transparency (EIT) analogs in classical oscillator systems have been investigated due to their potential in optical applications such as nonlinear devices and the slow-light field. Metamaterials are good candidates that utilize EIT-like effects to regulate optical light. Here, an actively reconfigurable EIT metamaterial for controlling THz waves, which consists of a movable bar and a fixed wire pair, is numerically and experimentally proposed.

Surface-plasmon-coupled optical force sensors based on metal-insulator-metal metamaterials with movable air gap.

We proposed surface-plasmon-coupled optical force sensors based on metal-insulator-metal (MIM) metamaterials with a movable air gap as an insulator layer. The MIM metamaterial was composed of an air gap sandwiched by a metal nanodot array and a metal diaphragm, the resonant wavelength of which was red-shifted when the air gap was narrowed by applying a normal force. We designed and fabricated a prototype of the proposed sensor and confirmed that the MIM metamaterial could be used as a force sensor with larger sensitivity than a force sensor based on Fabry-Pérot interferometer (FPI).

MEMS-based wearable eyeglasses for eye health monitoring.

A large number of world population suffer from vision related diseases/defects. Visual health supervision is important for detection, prevention and treatment of eyes disorders. Images of retina/fundus are important for diagnosis of eye diseases.

Silicon photonic microelectromechanical switch using lateral adiabatic waveguide couplers.

A silicon photonic waveguide switch using adiabatic couplers with lateral comb-drive actuators is designed, fabricated, and tested for microelectromechanical optical matrix switch. One of the waveguides of the adiabatic coupler is moved laterally by three actuators for varying the coupler gap, which enables to switch the path of the waveguides. The coupler waveguides with 250 nm thickness consist of a movable tapered waveguide from 400 nm to 500 nm in width and 50 μm in length and a straight waveguide of 400 nm in width.

Miniature Spectroscopes with Two-Dimensional Guided-Mode Resonant Metal Grating Filters Integrated on a Photodiode Array.

A small spectroscope with 25 color sensors was fabricated by combining metamaterial color filters and Si photodiodes. The metamaterial color filters consisted of guided-mode resonant metal gratings with subwavelength two-dimensional periodic structures. Transmittance characteristics of the color filters were designed to obtain peak wavelengths proportional to grating periods.

GaN microring waveguide resonators bonded to silicon substrate by a two-step polymer process.

Using a polymer bonding technique, GaN microring waveguide resonators were fabricated on a Si substrate for future hybrid integration of GaN and Si photonic devices. The designed GaN microring consisted of a rib waveguide having a core of 510 nm in thickness, 1000 nm in width, and a clad of 240 nm in thickness. A GaN crystalline layer of 1000 nm in thickness was grown on a Si(111) substrate by metal organic chemical vapor deposition using a buffer layer of 300 nm in thickness for the compensation of lattice constant mismatch between GaN and Si crystals.

Structure Shift of GaN Among Nanowall Network, Nanocolumn, and Compact Film Grown on Si (111) by MBE.

Structure shift of GaN nanowall network, nanocolumn, and compact film were successfully obtained on Si (111) by plasma-assisted molecular beam epitaxy (MBE). As is expected, growth of the GaN nanocolumns was observed in N-rich condition on bare Si, and the growth shifted to compact film when the Ga flux was improved. Interestingly, if an aluminum (Al) pre-deposition for 40 s was carried out prior to the GaN growth, GaN grows in the form of the nanowall network.

Confocal laser displacement sensor using a micro-machined varifocal mirror.

A confocal laser displacement sensor using a micro-machined varifocal mirror is reported. The focal length modulation is a key function of the confocal sensor. The mechanism of the focal length modulation determines the measurement speed and range.

Scanning Micro-Mirror with an Electrostatic Spring for Compensation of Hard-Spring Nonlinearity.

A scanning micro-mirror operated at the mechanical resonant frequency often suffer nonlinearity of the torsion-bar spring. The torsion-bar spring becomes harder than the linear spring with the increase of the rotation angle (hard-spring effect). The hard-spring effect of the torsion-bar spring generates several problems, such as hysteresis, frequency shift, and instability by oscillation jump.

25 nm Single-Crystal Silicon Nanowires Fabricated by Anisotropic Wet Etching.

We report a top-down method for fabricating ultra-high aspect ratio single-crystal silicon nanowires. The fabrication method is based on the standard photolithography technique and anisotropic wet etching of the single-crystal silicon in KOH solution. SiO₂ mask nanolines used for patterning single-crystal silicon nanowires are formed by the undercut etching of thin SiO₂ layer in buffered hydrofluoric solution.

All-optical guided resonance tuning in hybrid GaN/Si microring induced by non-radiatively trapped injected hot electrons.

Advanced Si/III-V nanophotonics that emerged in the last decade has already founded the on-chip optical interconnect technology for future integrated systems on chip. New possibilities of light-on-chip applications beside signal transmissions, such as, all-optical sensing, have been given but small attention. Here, all-optical ultraviolet (UV)-sensitive guided resonance tuning in a hybrid GaN/Si microring resonator (HMR) was studied.

Demonstration of sharp multiple Fano resonances in optical metamaterials.

We experimentally demonstrated multiple Fano resonances in optical metamaterials. By combination of two different sized asymmetric-double-bar (ADB) structures, triple Fano resonance was observed in the near-infrared region. In addition to Fano resonance due to anti-phase modes in isolated ADB structures, an anti-phase mode due to coupling among different sized ADBs was observed.

Resonant Varifocal Micromirror with Piezoresistive Focus Sensor.

This paper reports a microelectromechanical systems (MEMS) resonant varifocal mirror integrated with piezoresistive focus sensor. The varifocal mirror is driven electrostatically at a resonant frequency of a mirror plate to obtain the wide scanning range of a focal length. A piezoresistor is used to monitor the focal length of the varifocal mirror.

AlN Nanowall Structures Grown on Si (111) Substrate by Molecular Beam Epitaxy.

AlN nanowall structures were grown on Si (111) substrate using molecular beam epitaxy at substrate temperature of 700 °C with N/Al flux ratios ranging from 50 to 660. A few types of other AlN nanostructures were also grown under the nitrogen-rich conditions. The AlN nanowalls were ranged typically 60-120 nm in width and from 190 to 470 nm in length by changing N/Al flux ratio.

Reflection color filters of the three primary colors with wide viewing angles using common-thickness silicon subwavelength gratings.

We fabricated reflection color filters of the three primary colors with wide viewing angles using silicon two-dimensional subwavelength gratings on the same quartz substrate. The grating periods were 400, 340, and 300 nm for red, green, and blue filters, respectively. All of the color filters had the same grating thickness of 100 nm, which enabled simple fabrication of a color filter array.

Experimental demonstration of sharp Fano resonance in optical metamaterials composed of asymmetric double bars.

We experimentally demonstrated Fano resonance in metamaterials composed of asymmetric double bars (ADBs) in the optical region. ADB metamaterials were fabricated by a lift-off method, and the optical spectra were measured. Around a wavelength of 1100 nm, measured optical spectra clearly showed sharp Fano resonance due to weak asymmetry of the ADB structures.

Comparison of electromagnetically induced transparency between silver, gold, and aluminum metamaterials at visible wavelengths.

Electromagnetically induced transparency (EIT)-like effects in silver, gold, and aluminum metamaterials consisting of dipole resonators and quadrupole resonators were demonstrated at visible wavelengths. Optical characteristics of the metamaterials could be controlled by the gap distance between the two resonators. EIT-like effects were observed at wavelengths between 603 and 789 nm, 654 and 834 nm, and 462 and 693 nm for the silver, gold, and aluminum EIT metamaterials, respectively.

A tunable notch filter using microelectromechanical microring with gap-variable busline coupler.

A microelectromechanical tunable notch filter using silicon-photonic freestanding waveguides is proposed, and the basic characteristics are experimentally investigated. The proposed filter is composed of a wavelength-tunable silicon microring resonator and a busline switch. The tunable microring consists of freestanding single-mode waveguides and air-gap directional waveguide couplers.

Fabrication of antireflection subwavelength gratings at the tips of optical fibers using UV nanoimprint lithography.

Antireflection (AR) layers at the tips of optical fibers are indispensable in high efficiency and low noise applications. We realized the AR structures with two-dimensional binary subwavelength gratings (SWGs) at the tips of optical fibers by using a dedicated UV nanoimprint machine. Using this technique, ideal AR structures with desired refractive indices can be realized at low cost in principle.

Growth of GaN nanowall network on Si (111) substrate by molecular beam epitaxy.

GaN nanowall network was epitaxially grown on Si (111) substrate by molecular beam epitaxy. GaN nanowalls overlap and interlace with one another, together with large numbers of holes, forming a continuous porous GaN nanowall network. The width of the GaN nanowall can be controlled, ranging from 30 to 200 nm by adjusting the N/Ga ratio.

Micro fluorescent analysis system integrating GaN-light-emitting-diode on a silicon platform.

A micro fluorescent analysis system is proposed using silicon micromachining. GaN blue light-emitting diode (LED) monolithically integrated on a silicon substrate is used as a light source for the fluorescent analysis system. The blue light suits the excitation of several dyes used commonly in fluorescent analysis.

Ultra-small silicon waveguide coupler switch using gap-variable mechanism.

Submicron-wide silicon waveguide coupler with gap variable mechanism is proposed for a compact optical waveguide switch. Two freestanding silicon waveguides are placed parallel with a submicron gap. The gap is changed by electrostatic comb-drive micro-actuators to control the coupling coefficient of the coupler.

Fabrication of a high-precision spherical micromirror by bending a silicon plate with a metal pad.

We demonstrate here the fabrication of a smooth mirror surface by bending a thin silicon plate. A spherical surface is achieved by the bending moment generated in the circumference of the micromirror. Both convex and concave spherical micromirrors are realized through the anodic bonding of silicon and Pyrex glass.

III-Nitride grating grown on freestanding HfO2 gratings.

We report here the epitaxial growth of III-nitride material on freestanding HfO2 gratings by molecular beam epitaxy. Freestanding HfO2 gratings are fabricated by combining film evaporation, electron beam lithography, and fast atom beam etching of an HfO2 film by a front-side silicon process. The 60-μm long HfO2 grating beam can sustain the stress change during the epitaxial growth of a III-nitride material.