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We study the optical properties of fine structures (nanostructures) of metals and dielectrics, metamaterials, and the development of optical devices.

Metal nanostructures can exhibit various optical responses depending on their shapes and structures. Gold nanoparticles have long been used as a coloring material for stained glass in churches. Gold nanoparticles do not look golden but red or orange in color. Various colors can also be achieved by changing the particles' shape. They are stable and will not fade for long time. In addition to metals, dielectrics and semiconductors are now known to exhibit various optical responses in microscopic structures such as nanoparticles.

Metamaterials that exhibit peculiar optical responses that do not exist in nature are realized through the nanostructures of metals and dielectrics. Examples of exotic optical responses include materials that exhibit negative refraction and invisibility (cloaking) of objects. Metamaterials are characterized by a high degree of freedom in structural design. Therefore, we theoretically predict the optical response by numerical calculations, such as the finite difference time domain (FDTD) or finite element method (FEM). We design the structure, create the structure, and measure the optical response. Recently, structural design using machine learning has also been used, and metamaterials exhibiting various optical functions will continue to emerge in the future.

Our group designs, performs computational experiments, fabricates, and evaluates (measures) the nanostructures and metamaterials of metals and dielectrics. For example, we realized black bodies absorbing light over a wide range of wavelengths using metallic films thinner than the wavelength of light, we have experimentally made objects invisible, and we have designed objects to be invisible using machine learning and genetic algorithms. We also work on cooling objects using blackbody radiation. The applications include devices that can cool objects without using power or generate electricity at night, which can contribute to solving energy and environmental problems.

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Correspondance
Prof. Kotaro KAJIKAWA
Dept. Electrical and Electronic Engineering,
School of Engineering,
Tokyo Institute of Technology,
Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
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