A miniature optical biosensor and biosensor array where high sensitivity for detection of biomolecular interaction does not require a fluorescent label. Non-linear frequency-shifts of optical resonators (‘nanobeams’) provide a digital all-or-nothing response to equilibrium binding of a biomarker to surface-immobilized bio-recognition elements, a signal suitable to identify active components in genetic and proteomic circuits, as well as toxic substances. The threshold level for the digital response is adjustable to accommodate for varying receptor affinities. A bistable cavity sensing (BCS) method can be used to track the shift of the resonance induced by the analyte more precisely than the conventional cavity sensing method, where the resolution is limited by the cavity linewidth. BCS method can be used to quantitate the concentration of the analyte, and their binding kinetics, affinities and etc.