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A localized plasmon resonance sensor for detecting a change in optical constant uses a structure including metal. In a response spectrum with respect to light incident on the structure, there are at least two resonance peaks including at least one resonance peak shifted to a longer wavelength side a

A localized plasmon resonance sensor for detecting a change in optical constant uses a structure including metal. In a response spectrum with respect to light incident on the structure, there are at least two resonance peaks including at least one resonance peak shifted to a longer wavelength side and at least another peak shifted to a shorter wavelength side, by the change in optical constant.

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What is claimed is: 1. A sensing method for detecting a change in an optical constant in an ambient solvent of a nanostructure comprising metal and being capable of localized plasmon resonance, said method comprising: detecting the change in the optical constant by detecting a change in each of a p

What is claimed is: 1. A sensing method for detecting a change in an optical constant in an ambient solvent of a nanostructure comprising metal and being capable of localized plasmon resonance, said method comprising: detecting the change in the optical constant by detecting a change in each of a pair of plasmon resonance peaks in a response spectrum with respect to light incident on said nanostructure, wherein one of said pair of plasmon resonance peaks is shifted to a longer wavelength side by the change in the optical constant in the ambient solvent and the other of said pair of plasmon resonance peaks is shifted to a shorter wavelength side by the change in the optical constant in the ambient solvent, wherein at least one of said pair of plasmon resonance peaks is shifted to the shorter wavelength side when a dielectric constant of the ambient solvent is increased and is shifted to the longer wavelength side when the dielectric constant of the ambient solvent is decreased. 2. A method according to claim 1, wherein said nanostructure has a localized plasmon resonance frequency ωLPR which, in quasi-electrostatic field approximation, is represented by the following quadratic equation with respect to the complex dielectric constant ∈metal (ωLPR) of said metal contained in said nanostructure: ∈2metal(ωLPR)+2α ∈metal(ωLPR)+β=0 and the coefficients α and β satisfy the following relationship: 0 <√{square root over (α2-β)}<<1. 3. The method according to claim 2, wherein said nanostructure is a multi-layer shell structure comprising metal. 4. The method according to any one of claims 1 to 3, wherein the optical constant is any one of a refractive index, a dielectric constant, a magnet permeability, and combinations of these. 5. The method according to any one of claims 1 to 3, wherein the change in optical constant is caused by any one of a chemical reaction, a physical interaction, an antigen-antibody reaction, a change in temperature, and combinations of these. 6. The method according to any one of claims 1 to 3, wherein the response spectrum is any one of a transmission spectrum, a reflection spectrum, a quenching spectrum, a scattering spectrum, and an absorption spectrum. 7. The method according to any one of claims 1 to 3, wherein said nanostructure is disposed in a path in which a fluid is moved. 8. A method according to claim 7, wherein said nanostructure is disposed in a neighborhood of an area in which a width of the path is narrowed in the path. 9. The method according to any one of claims 1 to 3, wherein the ambient of said nanostructure is modified with an antigen or an antibody at a peripheral portion of said nanostructure. 10. A localized plasmon resonance sensor for detecting a change in optical constant of an ambient solvent by using a structure comprising a dielectric material as a core coated with metal as a shell, wherein in a response spectrum with respect to light incident on said structure, there are at least two resonance peaks including at least one resonance peak shifted to a longer wavelength side by the change in optical constant of the ambient solvent and at least another peak shifted to a shorter wavelength side by the change in optical constant of the ambient solvent, and wherein at least one of said at least two resonance peaks is shifted to the shorter wavelength side when a dielectric constant of the ambient solvent is increased and is shifted to the longer wavelength side when the dielectric constant of the ambient solvent is decreased. 11. A sensor according to claim 10, wherein said structure having at least two interfaces between the metal and the dielectric material and localized plasmon resonances at respective interfaces are correlated with each other. 12. A sensor according to claim 10, wherein said structure has a localized plasmon resonance frequency which includes two coefficients α and β and is determined by a quadratic equation, with respect to complex dielectric constant of metal contained in said structure, represented by the following equation: ∈2metal+2α∈metal +β=0 and the coefficients α and β satisfying the following relationship: 0<√{square root over (α2-β)}<<1. 13. A method according to claim 1, wherein said pair of plasmon resonance peaks includes a shorter wavelength side peak shifted to the longer wavelength side and includes a longer wavelength side peak shifted to the shorter wavelength side. 14. A sensor according to claim 10, wherein the at least two resonance peaks includes a shorter wavelength side peak shifted to the longer wavelength side and includes a longer wavelength side peak shifted to the shorter wavelength side.