Grating-based evanescent field molecular sensor using a thin silicon waveguide layer

A method of sensing a target molecule if present, within a fluid flow, the method comprising:

• providing a silicon on insulator (SOI) wafer comprising a single crystal silicon layer waveguide over a SiO2 insulator layer, the waveguide having a diffraction grating on a sensor surface that is functionalized with receptors for a specific target molecule;
• applying the fluid flow to the sensor surface so that if the target molecule is present, it will preferentially attach to the receptors;
• applying a polarized beam of light incident the sensor surface so that the diffraction grating couples the beam to excite a single mode in the waveguide and equally couples the single mode from the waveguide to form a sensed output signal, the waveguide having a thickness of 10-400 nm, selected so that art overlap integral of the waveguide's mode intensity over the functionalized sensor surface is greater than 50% of the overlap integral over any other region of the same thickness within the waveguide;
• and
• determining a property of the sensed output that varies with interaction of the evanescent field with the target molecules if present.