Takashi Sato

Bio: Takashi Sato is an academic researcher from Tohoku University. The author has contributed to research in topics: Photonic crystal & Polarizer. The author has an hindex of 18, co-authored 73 publications receiving 1977 citations.

Superprism Phenomena in Photonic Crystals

Abstract: Extraordinary angle-sensitive light propagation, which we call a superprism phenomenon, was demonstrated at optical wavelength in photonic crystals with three-dimensional-periodic structure fabricated on Si substrate. The propagation beam was swung from $\ensuremath{-}90\ifmmode^\circ\else\textdegree\fi{}$ to $+90\ifmmode^\circ\else\textdegree\fi{}$ with a slight change in the incident angle within $\ifmmode\pm\else\textpm\fi{}12\ifmmode^\circ\else\textdegree\fi{}.$ This effect together with wavelength sensitivity is at least two orders of magnitude stronger than that of the conventional prism. The incident-angle dependence including negative refraction and multiple beam branching was interpreted from highly anisotropic dispersion surfaces derived by photonic band calculation. These phenomena will be available to fabricate microscale light circuits on Si with LSI-compatible lithography techniques.

Mechanism of shape formation of three-dimensional periodic nanostructures by bias sputtering

Abstract: We previously demonstrated a process for fabricating three-dimensional (3D) periodic nanostructures composed of corrugated a-Si/SiO2 multilayers, which behave as 3D photonic crystals. In this process, bias sputtering is a key technique by which the pattern is self-forming. This letter clarifies the mechanism of the self-shaping effect of bias sputtering by comparing deposition simulation and experiments. The mechanism is decomposed into three main effects: diffuse incidence of neutral particles of film material, sputter etching by normally incident rare-gas ions, and subsequent redeposition of sputtered film material. Specifically, redeposition has a self-adjusting effect on the depth of holes or valleys, and is the key of formation of stable patterns.

Photonic crystal polarisation splitters

Abstract: The design, fabrication, and measurement of an a-Si/SiO/sub 2/ photonic crystal polarisation splitter are reported. The device consists of a 10-period corrugated multilayer film and is made by a combination of sputter-deposition and sputter-etching processes. The measured insertion loss and extinction ratio at /spl lambda/=1.55 /spl mu/m are 0.4 dB and >40 dB, respectively.

Photonic crystals for the visible range fabricated by autocloning technique and their application

Abstract: We fabricate photonic crystals for the visible range by the ‘autocloning’ technique, in which multilayers are stacked by an appropriate combination of sputter deposition and sputter etching. TiO2/SiO2 and Ta2O5/SiO2 are chosen as materials since they are transparent in the range and give a high contrast of refractive indices. The fabrication technique has flexibility regarding materials and size and is very reliable and reproducible even if the pitch is less than 0.2 μm. We also study the application of photonic crystals to birefringent elements such as waveplates and polarization selective gratings and experimentally verify that they are useful for optical pick-up systems.

Compact ellipsometer employing a static polarimeter module with arrayed polarizer and wave-plate elements.

Abstract: A portable ellipsometer with a compact static polarimeter using an arrayed polarizer, an arrayed wave plate, and a CCD image sensor is developed. A high level of repeatability at a measurement speed of 0.3 s is demonstrated by measurement of SiO2 films ranging from 2 to 300 nm in thickness deposited on an Si wafer. There is the potential to realize an ultracompact ellipsometer module by integrating the optical source and receiver, suitable for deployment in a variety of manufacturing equipment and measurement instruments.

Photonic crystals

Abstract: The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

Optical negative-index metamaterials

Abstract: Artificially engineered metamaterials are now demonstrating unprecedented electromagnetic properties that cannot be obtained with naturally occurring materials. In particular, they provide a route to creating materials that possess a negative refractive index and offer exciting new prospects for manipulating light. This review describes the recent progress made in creating nanostructured metamaterials with a negative index at optical wavelengths, and discusses some of the devices that could result from these new materials.

Silicon photonics

Abstract: The silicon chip has been the mainstay of the electronics industry for the last 40 years and has revolutionized the way the world operates. Today, a silicon chip the size of a fingernail contains nearly 1 billion transistors and has the computing power that only a decade ago would take up an entire room of servers. As the relentless pursuit of Moore's law continues, and Internet-based communication continues to grow, the bandwidth demands needed to feed these devices will continue to increase and push the limits of copper-based signaling technologies. These signaling limitations will necessitate optical-based solutions. However, any optical solution must be based on low-cost technologies if it is to be applied to the mass market. Silicon photonics, mainly based on SOI technology, has recently attracted a great deal of attention. Recent advances and breakthroughs in silicon photonic device performance have shown that silicon can be considered a material onto which one can build optical devices. While significant efforts are needed to improve device performance and commercialize these technologies, progress is moving at a rapid rate. More research in the area of integration, both photonic and electronic, is needed. The future is looking bright. Silicon photonics could provide low-cost opto-electronic solutions for applications ranging from telecommunications down to chip-to-chip interconnects, as well as emerging areas such as optical sensing technology and biomedical applications. The ability to utilize existing CMOS infrastructure and manufacture these silicon photonic devices in the same facilities that today produce electronics could enable low-cost optical devices, and in the future, revolutionize optical communications

Electrospun nanofibers in energy and environmental applications

Abstract: Nanotechnology is providing new solutions and opportunities to ensure sustainable energy and environments for the future. Materials of nanofiberous morphology are attractive to solve numerous energy and environmental issues. Nanofibers can be effectively produced by electrospinning, which is a simple and low cost technique. In addition, electrospinning allows the production of nanofibers from various materials e.g. organics and inorganics in different configurations and assemblies. This is highly beneficial for energy devices, where inorganic materials especially metal oxides can be synthesized and electrospun, improving conducting and ceramic properties. Excitonic solar cells fabricated with aligned nanofiberous metal oxide electrodes provide higher solar–electric energy conversion efficiency, whereas fuel cells made with nanofiberous electrodes enable uniform dispersion of catalysts, and thus increase electrocatalytic activity to obtain higher chemical–electric energy conversion efficiency. The nanofibers used in filtration membranes for environmental remediation, minimize the pressure drop and provide better efficiency than conventional fiber mats. The large surface area-to-volume ratio of nanofiber membranes allows greater surface adsorption of contaminants from air and water, and increases the life-time of the filtration media. This review highlights the potential and application of electrospun nanofiberous materials for solving critical energy and environmental issues.