A study of improved optical sensing performances based on nanoscale porous substrates
Nanoporous dielectric and metallic materials have recently attracted a great deal of attention for chemical and biological sensing applications. Compared to conventional solid material based sensors, porous materials provide nanoscale surface morphology and an increased internal surface area for molecule binding enabling improved sensing performance. Layered structures have been demonstrated to be excellent candidates for the fabrication of optical sensors due to their ability to localize light in regions where molecules are bound. The band structures of these layered media can be systematically designed and characterized by properly choosing design parameters, including material, refractive index, dimension, and periodicity. In this work, the advantages of nanoscale porous materials were combined with layered optical structures to achieve compact, cost-effective, and highly reproducible sensing substrates with improved sensing performance. In particular, a porous silicon (PSi) membrane waveguide was first investigated. By properly choosing design parameters including pore size, porosity, and mode order, optimized PSi waveguides with significantly improved molecular detection sensitivity were achieved. Second, a dual mode sensing platform combining refractive index and surface enhanced Raman scattering (SERS) based sensing, and capable of both quantifying and identifying captured molecules, was demonstrated based on a PSi interferometer coated with gold nanoparticles (Au NPs). Enhanced SERS signal intensity was achieved by using a PSi waveguide instead of the simple interferometer design to increase the intensity of the electric field at the surface in the region of the Au NPs. Third, an easy to fabricate and effective SERS substrate based on nanoporous gold (NPG) was investigated. This SERS substrate utilizes both localized and propagating surface plasmon effects on NPG to significantly enhance the SERS signal. Ultra high enhancement factors were demonstrated by properly tuning the grating parameters and pore openings.