A system for energy upconversion and/or down conversion and a system for producing a photostimulated reaction in a medium. These systems include 1) a nanoparticle configured, upon exposure to a first wavelength λ1 of radiation, to generate a second wavelength λ2 of radiation having a higher energy than the first wavelength λ1 and 2) a metallic structure disposed in relation to the nanoparticle. A physical characteristic of the metallic structure is set to a value where a surface plasmon resonance in the metallic structure resonates at a frequency which provides a spectral overlap with either the first wavelength λ1 or the second wavelength λ2, or with both λ1 and λ2. The system for producing a photostimulated reaction in a medium includes a receptor disposed in the medium in proximity to the nanoparticle which, upon activation by the second wavelength λ2, generates the photostimulated reaction.