The invention relates to the treatment of a medium, particularly to the purification of water, air, or surfaces. A photoactive layer (120) is disposed on an energy-transfer surface (111) of a substrate (110). Thus light energy transfer from said substrate (110) to the photoactive layer (120) is directly achieved without an intermediate passage through the medium. The substrate (110) may preferably be a waveguide from which light energy is transferred into the photoactive layer (120) via evanescent waves. Moreover, the optical coupling between the substrate (110) and the photoactive layer (120) may spatially vary.
