Silicon photonic evanescent field molecular sensor using resonant grating interrogation

A silicon waveguide evanescent field molecular sensor interrogated by reflection from a surface grating is demonstrated. The sensor is probed by a beam of light incident at θ = 45° through the backside of the silicon-on-insulator (SOI) wafer. The reflected power shows a strong resonant feature near λ = 1532 nm, arising from resonant coupling of incident beam, guided mode, and the reflected beam. The resonant wavelength is very sensitive to molecular surface coverage. We have measured a Δλ = 1.0 nm resonance shift when a monolayer of streptavidin protein is bound to the sensor surface, or a 60% reflectivity change when measured at a fixed wavelength.