This invention relates to a platform (11) and a method for generating electromagnetic field distributions. The invention relates in particular to optical sensors for measuring biological or chemical substances. The platform (11) according to the invention comprises a substrate (13), a structured layer (19) and, positioned between the substrate (13) and the structured layer (19), a multilayer assembly (17), said components being so matched relative to one another that upon appropriate impingement by electromagnetic radiation an electromagnetic field distribution is generated that is at a maximum within the structured layer (19).