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Surfaces of metals, particularly silver and gold, support electromagnetic waves coupled to free electron plasma called surface plasmons (or surface plasmon polaritons). Plasmonics is a technology field that aims at fully controlling surface plasmons by engineered metallic nanostructures, transmitting light through optical nanocircuits, and confining light into a tiny cavity much smaller than the wavelength.We are developing unique plasmon nanocavities; establishment of fabrication processes, demonstration of various optical functionalities, and trial for practical applications are being carried out. We have successfully demonstrated the squeezing of light waves into nanocavities milled out from few-nanometer-thick dielectric films sandwiched between gold layers by compressing the wavelengths of visible light to those of X rays. We have also clarified that our plasmon nanocavities exhibit enhancement in various optical processes such as Raman scattering and thermal emission. Enhanced Raman scattering is promising for sensitive detection of biomolecules without fluorescent markers. Enhanced thermal emission can be applied to an infrared emitter of a specific wavelength necessary for environmental analysis such as CO2 density measurement. By assembling plasmon nanocavities, we can also realize metamaterials. Metamaterials are artificial materials that exhibit peculiar electromagnetic responses, which are never attained in nature, by controlled structures sufficiently smaller than the wavelength of the electromagnetic waves. Although metamaterials for microwaves are intensively developed, realization of metamaterials in the optical domain has been difficult even with advanced nanofabrication techniques. However, we already have long experience in the plasmon nanocavities much smaller than the optical wavelength. We expect that we can conduct unique metamaterial research by utilizing our original nanocavities as the component element (meta-atoms) of metamaterials. |
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