Molecular layer detection on a diffractive optical balance.

Diffraction-based molecular detection is achieved by etching optical gratings into thermal oxide on silicon. The gratings perform as a stable common-path diffractive optical balance (DOB) designed to operate near a missing diffraction order. The biosensor is operated in an off-null condition with a phase bias to produce a high-contrast responsivity that is linear in accumulated molecules but with a low background. The DOB linear responsivity is a factor of 20 larger than the reflectometric responsivity of planar thermal oxide.