Akio Higo

A transparent sheet display by plastic MEMS

— A new type of color-image display pixel based on MEMS (microelectromechanical systems) technology of plastic materials is presented. The mechanism for making color is optical interference using a Fabry-Perot interferometer. A thin sheet of PEN (polyethylene naphthalate) with a metal half-mirror was laminated over a glass or PEN substrate with an optical cavity inside. The electrostatically controlled deformation of the PEN film changes the color of the transmitting light by interference within the optical cavity. Color pixels of three primary colors (red, green, and blue) were successfully developed and demonstrated, with a driving voltage ranging from 80 to 120 Vdc. Thanks to the mechanical flexibility of the PEN films, the display could operate even when placed on a curved surface.

Quantum size effects in GaAs nanodisks fabricated using a combination of the bio-template technique and neutral beam etching.

We successfully fabricated defect-free, distributed and sub-20-nm GaAs quantum dots (named GaAs nanodisks (NDs)) by using a novel top-down technique that combines a new bio-template (PEGylated ferritin) and defect-free neutral beam etching (NBE). Greater flexibility was achieved when engineering the quantum levels of ND structures resulted in greater flexibility than that for a conventional quantum dot structure because structures enabled independent control of thickness and diameter parameters. The ND height was controlled by adjusting the deposition thickness, while the ND diameter was controlled by adjusting the hydrogen-radical treatment conditions prior to NBE. Photoluminescence emission due to carrier recombination between the ground states of GaAs NDs was observed, which showed that the emission energy shift depended on the ND diameters. Quantum level engineering due to both diameter and thickness was verified from the good agreement between the PL emission energy and the calculated quantum confinement energy.

Observation of micromechanically controlled tuning of photonic crystal line-defect waveguide

We fabricated a photonic crystal (PC) line-defect waveguide integrated with a microelectromechanical actuator and demonstrated the optical switching operation. The device consisted of a PC line-defect waveguide fabricated in a silicon-on-insulator substrate and a polycrystalline-Si dielectric plate located above the PC waveguide. An applied voltage moved the dielectric plate towards the PC surface due to the electrostatic force. This motion increased out-of-plane scattering of the guided light through the evanescent interaction with the dielectric plate, and modulated the transmittance of the PC waveguide. With only a 5μm interaction length, an extinction ratio of ∼10dB was obtained at a wavelength of 1568nm under an applied voltage of 60V. The response time of the switching operation was approximately 1ms.

Experimental demonstration of self-aligned InP/InGaAsP polarization converter for polarization multiplexed photonic integrated circuits

Highly efficient, low-loss, and compact InP/InGaAsP polarization converter based on a half-ridge waveguide structure is fabricated and demonstrated experimentally. The device is fabricated by a simple self-aligned process and integrated with a ridge InP waveguide. Using a 150-μm-long device, we obtain the mode conversion of more than 96% and the on-chip loss of less than 1.0 dB over the broad wavelength range from 1510 to 1575 nm. The experimental results are explained quantitatively using the full-vector eigenmode calculation, which also reveals large fabrication tolerance of the demonstrated device.

Light-emitting devices based on top-down fabricated GaAs quantum nanodisks.

Quantum dots photonic devices based on the III–V compound semiconductor technology offer low power consumption, temperature stability, and high-speed modulation. We fabricated GaAs nanodisks (NDs) of sub-20-nm diameters by a top-down process using a biotemplate and neutral beam etching (NBE). The GaAs NDs were embedded in an AlGaAs barrier regrown by metalorganic vapor phase epitaxy (MOVPE). The temperature dependence of photoluminescence emission energies and the transient behavior were strongly affected by the quantum confinement effects of the embedded NDs. Therefore, the quantum levels of the NDs may be tuned by controlling their dimensions. We combined NBE and MOVPE in a high-throughput process compatible with industrial production systems to produce GaAs NDs with tunable optical characteristics. ND light emitting diode exhibited a narrow spectral width of 38 nm of high-intensity emission as a result of small deviation of ND sizes and superior crystallographic quality of the etched GaAs/AlGaAs layer.

Monolithic InP strictly non-blocking 8×8 switch for high-speed WDM optical interconnection

A strictly non-blocking 8 × 8 switch for high-speed WDM optical interconnection is realized on InP by using the phased-array scheme for the first time. The matrix switch architecture consists of over 200 functional devices such as star couplers, phase-shifters and so on without any waveguide cross-section. We demonstrate ultra-broad optical bandwidth covering the entire C-band through several Input/Output ports combination with extinction ratio performance of more than 20dB. Also, nanoseconds reconfiguration time was successfully achieved by dynamic switching experiment. Error-free transmission was verified for 40-Gbps (10-Gbps × 4ch) WDM signal.

Development of Multi-User Multi-Chip SOI CMOS-MEMS Processes

This paper presents a new method of integrating multiple MEMS designs with 40V class CMOS driver circuits in a multi-user-multi-chip manner. The multi-chip multi-user CMOS-MEMS process was done at 35 mm × 35 mm SOI chip. More than six different designs of SOI-bulk micromachined actuators including the pitch-tunable gratings were monolithically integrated onto the pre-fabricated high-voltage level-shifter circuits. We measured electro mechanical characteristics of the grating light valve integrated with high-voltage level-shifter and successfully demonstrated 1MHz operation.

Simple and compact inp polarization converter for polarization-multiplexed photonic integrated circuits

We propose and investigate a novel type of waveguide polarization converter, which is particularly suited for monolithic integration in InP photonic integrated circuits. Efficient mode conversion with 0.4-dB loss and 16.6-dB extinction is demonstrated numerically.

Transparent color pixels using plastic MEMS technology for electronic papers

We present a new type of image display device that could be used as, for instance, over-sized rewritable color posters by using a simple MEMS fabrication technique. The mechanism of making colors is based upon the optical interference of the Fabry-Perot interferometer. A thin PEN (polyethylene naphthalate) film with metal half-mirror was laminated over a glass substrate with an optical cavity, and the electrostatic deformation of the film controlled the color of the transmitting light. Color pixels of three primary colors (red, green and blue) were successfully demonstrated with driving voltage ranging from 80 to 120Vdc.

Oxidation states of GaAs surface and their effects on neutral beam etching during nanopillar fabrication

We have investigated a new process for fabricating GaAs sub-20 nm nanopillars that uses a top–down combination of a bio-template and damage-free neutral beam etching. A two-dimensional array of nanoparticles composed of a protein shell embedded with a metal oxide core was formed on the top of a GaAs surface treated by neutral beam oxidation. Because of device requirements, three low-temperature oxygen techniques were investigated for removing the protein shell prior to the etching process: oxygen radical, oxygen neutral beam, and low-temperature oxygen annealing in vacuum (LT-OAV). X-ray photoelectron spectroscopy was used to monitor the effects of the different treatments on the GaAs surface. While the three processes could efficiently remove a protein shell, subsequent oxidation of the GaAs surface showed some differences in the oxide layer composition. Therefore, LT-OAV was selected considering its lower gallium oxide formation. A hydrogen radical process was then performed at temperatures lower than 400 °C to remove the oxide layer prior to etching. This process completely removed arsenide oxide and only residual gallium oxide was found on the surface afterwards. Etching was performed using a pure chlorine neutral beam of GaAs samples with metal oxide core etching masks. We found that control of the Ga-oxide amount on the surface is the key parameter for controlling the diameter and the density of nanopillars. Finally, high-aspect ratio nanopillars using stacked layers of GaAs and AlGaAs were obtained and showed no damage layer.