A size selective porous silicon grating-coupled Bloch surface and sub-surface wave biosensor

• We report a novel mode called the Bloch sub-surface wave (BSSW).
• The BSSW enables enhanced detection of small molecules in porous silicon.
• The BSSW wave has detection sensitivity > 2000 nm/RIU (> 70°/RIU).
• The Bloch surface wave mode supports detection of large surface-bound species.
• The two modes are characterized through simulation and experiment.

A porous silicon (PSi) grating-coupled Bloch surface and sub-surface wave (BSW/BSSW) biosensor is demonstrated to size selectively detect the presence of both large and small molecules. The BSW is used to sense large immobilized analytes at the surface of the structure while the BSSW that is confined inside but near the top of the structure is used to sensitively detect small molecules. Functionality of the BSW and BSSW modes is theoretically described by dispersion relations, field confinements, and simulated refractive index shifts within the structure. The theoretical results are experimentally verified by detecting two different small chemical molecules and one large 40 base DNA oligonucleotide. The PSi-BSW/BSSW structure is benchmarked against current porous silicon technology and is shown to have a 6-fold higher sensitivity in detecting large molecules and a 33% improvement in detecting small molecules. This is the first report of a grating-coupled BSW biosensor and the first report of a BSSW propagating mode.