NOVELTY - Photocatalyst comprises a substrate; a reflective layer adjacent to the substrate, where the reflective layer is configured to reflect light; a spacer layer adjacent to the reflective layer, where the spacer layer is at least partially transparent to light; and a nanocomposite layer adjacent to the spacer layer, where nanocomposite layer is formed of a matrix and particles, and where upon exposure to electromagnetic radiation, the particles absorb far field electromagnetic radiation and excite plasmon resonances that interact with the reflective layer to form electromagnetic resonances.    USE - The photocatalyst, system, or method is useful for catalyzing reaction (claimed), e.g. waste water treatment, hydrogen fuel production, and hydrocarbon fuel production from sequestered carbon dioxide (CO2).    ADVANTAGE - The photocatalyst can utilize plasmon resonance based, near-perfect optical absorption for performing and enhancing photo catalytic reactions. The heterogeneous photocatalyst, devices and systems can be considered a platform for enhancing the activity of various, existing photocatalytic semiconductor materials.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a photoelectrochemical system, comprising a first electrode, comprising a nanocomposite layer adjacent to a spacer layer, where the spacer layer is adjacent to a reflective layer, where the nanocomposite layer is formed of a matrix and particles that, upon exposure to electromagnetic radiation, absorb far field electromagnetic radiation and excite plasmon resonances that interact with the reflective layer to form electromagnetic resonances; and a second electrode comprising a metallic material adjacent to the first electrode, where the first electrode and/or second electrode are adapted such that, upon exposure of the first electrode to electromagnetic radiation, the first electrode and/or the second electrode generate one or more reaction products from at least one reactant species; and    (2) a method for catalyzing a reaction, which involves: (a) providing a photoelectrochemical system, comprising: a first electrode comprising a nanocomposite layer adjacent to a spacer layer, where the spacer layer is adjacent to a reflective layer, where the nanocomposite layer comprises a matrix and particles that, upon exposure to light, absorb far field electromagnetic radiation and excite plasmon resonances that interact with the reflective layer to form magnetic resonances; a second electrode comprising a metallic material coupled to the first electrode; and a reactant species in contact with the first electrode and the second electrode;(b) exposing the first electrode to electromagnetic radiation; and (c) generating one or more reaction products from at least one reactant species at the first electrode and/or the second electrode.