Bidiffractive grating coupler: universal transducer for optical interface analytics

We report on a novel transducer devised for sensitive detection of the binding of molecules in the immediate vicinity of an optical surface. The transducer consists of a chip with a surface coated by an extremely thin waveguide film of amorphous TiO2 structured with a submicron grating relief. This microrelief is composed of two superimposed, uniform diffraction gratings of different periodicities, forming a bidiffractive grating with a frequency spectrum composed of two fundamental spatial harmonics. This bidiffractive grating serves as both an input and an output port for coupling and decoupling light beams to and from the planar waveguide. The bidiffractive coupler has translation-invariant coupling efficiency and allows background-free detection of the light decoupled from the waveguide. We outline the fabrication of the transducer and discuss the material properties of the waveguide film, which are very important for achieving optimum detection sensitivity. We assess the key factors affecting the detection limits of the transducer and comment on the resolution and the dynamic range for the measurement of changes in interfacial mass loading.