Biosensor comprising waveguide

The invention relates to an optical device (110) and a corresponding detection apparatus (100) that may for example be used for monitoring the replication of nucleotide sequences at a surface. In a preferred embodiment, the optical device (110) comprises a waveguide substrate (130) with a wiregrid (140) on a bottom surface (132), wherein apertures (141) of the wiregrid are in at least one direction (x) smaller than a characteristic wavelength (λ) of input light (IL). Moreover, a diffractive structure (120) is disposed on the opposite surface (131) of the substrate (130) for coupling input light (IL) into the substrate (130) such that constructive interference occurs at the apertures (141). Thus evanescent waves can be generated with high efficiency in these apertures, allowing for example for a surface-specific excitation of fluorescence (FL) that can be sensed by a detector (160).