**4. Waveguide structures**

In waveguide structures, light is guided in the plane of the thin film, facilitating the integration of such a sensor, along with source and detector components, into a single integrated chip device [37]. This is a significant advantage in the development of on-chip micro- and nano-optical biosensors. Secondly, for porous silicon waveguides, the active sensing layer is the top porous layer, eliminating the infiltration difficulties that can plague Bragg mirrors, rugate filters, and micro-cavities, where biomolecules must filter through many layers of both high and low porosity. As a result of both advantages mentioned above, waveguide sensors display electric field confinement, sharp resonance peaks, and a thin sensing layer, all qualities that facilitate a fast response and a high sensitivity detection of molecules.
