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A sensor and method for surface plasmon resonance sensing, wherein a small variation of the refractive index of an ambient medium results in a large variation of loss of a sensing mode. The surface plasmon resonance sensor comprises an antiguiding waveguide including a core characterized by a refrac

A sensor and method for surface plasmon resonance sensing, wherein a small variation of the refractive index of an ambient medium results in a large variation of loss of a sensing mode. The surface plasmon resonance sensor comprises an antiguiding waveguide including a core characterized by a refractive index and a reflector surrounding the core. The reflector has an external surface and is characterized by a band gap and a refractive index higher than the refractive index of the core. A coating is deposited on the external surface of the core, the coating defining with the ambient medium a coating/ambient medium interface. In operation, the coating is in contact with the ambient medium, and the antiguiding waveguide is supplied with an electromagnetic radiation to (a) propagate a mode for sensing having an effective refractive index lower than the refractive index of the core and higher than a refractive index of an ambient medium and (b) produce surface plasmons at the coating/ambient medium interface. The mode for sensing is phase-matched with the surface plasmons at a wavelength within the band gap and a variation of the refractive index of the ambient medium results in a variation of loss of the sensing mode to detect a characteristic of the ambient medium.

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What is claimed is: 1. A surface plasmon resonance sensor for an ambient medium characterized by a refractive index, the surface plasmon resonance sensor comprising: an antiguiding waveguide comprising: a core characterized by a refractive index; and a reflector surrounding the core, the reflector

What is claimed is: 1. A surface plasmon resonance sensor for an ambient medium characterized by a refractive index, the surface plasmon resonance sensor comprising: an antiguiding waveguide comprising: a core characterized by a refractive index; and a reflector surrounding the core, the reflector having an external surface and being characterized by a band gap and a refractive index higher than the refractive index of the core; and a coating deposited on the external surface of the core, the coating defining with the ambient medium a coating/ambient medium interface suitable to support surface plasmons; wherein, in operation: the coating is in contact with the ambient medium; the antiguiding waveguide is supplied with an electromagnetic radiation to (a) propagate a mode for sensing having an effective refractive index lower than the refractive index of the core and higher than the refractive index of the ambient medium and (b) produce surface plasmons at the coating/ambient medium interface; the mode for sensing is phase-matched with the surface plasmons at a wavelength within the band gap; and a variation of the refractive index of the ambient medium results in a variation of loss of the sensing mode to detect a feature of the ambient medium. 2. A surface plasmon resonance sensor as recited in claim 1, wherein: the reflector is a multilayer reflector comprising a plurality of individual layers, each individual layer being characterized by a refractive index, the effective refractive index of the mode for sensing being lower than the refractive index of each of the individual layers. 3. A surface plasmon resonance sensor as recited in claim 2, wherein: the refractive index of each individual layer equals either to a lower refractive index or to a higher refractive index, the lower refractive index being lower than the higher refractive index, and the lower refractive index being higher than or equal to the refractive index of the core. 4. A surface plasmon resonance sensor as recited in claim 3, wherein: the individual layers and the core are substantially planar; the core is interposed between a first and a second individual layers; the first and second layers have both the higher refractive index; and refractive indices of adjacent individual layers alternate from the higher refractive index to the lower refractive index. 5. A surface plasmon resonance sensor as recited in claim 1, wherein the mode for sensing is independent from the size of the core. 6. A surface plasmon resonance sensor as recited in claim 1, wherein the coating comprises a metal film. 7. A surface plasmon resonance sensor as recited in claim 6, wherein the film is made of a metal selected from the group consisting of gold, silver, copper, nickel, titanium, tantalum and chromium. 8. A surface plasmon resonance sensor as recited in claim 1, wherein the coating comprises a semiconductor. 9. A surface plasmon resonance sensor as recited in claim 1, wherein the waveguide comprises an element selected from the group consisting of a photonic crystal, a hollow core Bragg fiber and a gas-filled capillary. 10. A surface plasmon resonance sensor as recited in claim 1, wherein the electromagnetic radiation comprises a laser beam. 11. A surface plasmon resonance sensor as recited in claim 10, wherein the laser beam is Gaussian-like. 12. A surface plasmon resonance sensor as recited in claim 2, wherein a number of individual layers of the multilayer reflector is chosen according to a desired coupling strength between modes of the plasmon and modes of the core. 13. A surface plasmon resonance sensor as recited in claim 1, wherein a thickness of the core is significantly larger than a wavelength of operation. 14. A surface plasmon resonance sensor as recited in claim 1, wherein the mode for sensing comprises a leaky mode having an effective refractive index lower than the refractive index of the core. 15. A surface plasmon resonance sensor as recited in claim 1, wherein, in operation, when the mode for sensing is produced over a sufficient length, the antiguiding waveguide behaves substantially like a single-mode waveguide for any frequency within the band gap, enabling the mode for sensing to phase-match with the surface plasmons at any desirable wavelength within the band gap. 16. A method for surface plasmon resonance sensing a feature of an ambient medium characterized by a refractive index, comprising: providing an antiguiding waveguide comprising: a core characterized by a refractive index; and a reflector surrounding the core, the reflector having an external surface and being characterized by a band gap and a refractive index higher than the refractive index of the core; and depositing a coating on the external surface of the core, the coating defining with the ambient medium a coating/ambient medium interface suitable to support surface plasmons; bringing the coating in contact with the ambient medium; supplying the antiguiding waveguide with an electromagnetic radiation to (a) propagate a mode for sensing having an effective refractive index lower than the refractive index of the core and higher than the refractive index of the ambient medium and (b) produce surface plasmons at coating/ambient medium interface; phase-matching the mode for sensing with the surface plasmons at a wavelength within the band gap; and sensing a variation of the refractive index of the ambient medium through a variation of loss of the sensing mode to detect a characteristic of the ambient medium. 17. A method for surface plasmon resonance sensing as recited in claim 16, wherein, in operation, when the mode for sensing is produced over a sufficient length, the antiguiding waveguide behaves substantially like a single-mode waveguide for any frequency within the band gap, enabling the mode for sensing to phase-match with the surface plasmons at any desirable wavelength within the band gap. 18. A method for surface plasmon resonance sensing as recited in claim 16, wherein: the reflector is a multilayer reflector comprising a plurality of individual layers, each individual layer being characterized by a refractive index, the effective refractive index of the mode for sensing being lower than the refractive index of each of the individual layers. 19. A method for surface plasmon resonance sensing as recited in claim 18, wherein: the refractive index of each individual layer equals either to a lower refractive index or to a higher refractive index, the lower refractive index being lower than the higher refractive index, and the lower refractive index being higher than or equal to the refractive index of the core. 20. A method for surface plasmon resonance sensing as recited in claim 19, wherein: the individual layers and the core are substantially planar; the core index is interposed between a first and a second individual layers; the first and second layers have both the higher refractive index; and refractive indices of adjacent individual layers alternate from the higher refractive index to the lower refractive index. 21. A method for surface plasmon resonance sensing as recited in claim 16, wherein the mode for sensing is independent from the size of the core. 22. A method for surface plasmon resonance sensing as recited in claim 16, wherein the coating comprises a metal film. 23. A method for surface plasmon resonance sensing as recited in claim 16, wherein the coating comprises a semiconductor. 24. A method for surface plasmon resonance sensing as recited in claim 18, comprising choosing a number of individual layers of the multilayer reflector according to a desired coupling strength between modes of the plasmon and modes of the core. 25. A method for surface plasmon resonance sensing as recited in claim 16, comprising making a thickness of the core significantly larger than a wavelength of operation. 26. A method for surface plasmon resonance sensing as recited in claim 16, wherein the mode for sensing comprises a leaky mode having an effective refractive index smaller than the refractive index of the core.