Label-Free Biosensing With a Slot-Waveguide-Based Ring Resonator in Silicon on Insulator

We present a slot-waveguide-based ring resonator in silicon on insulator (SOI) with a footprint of only $13\ \mu\hbox{m} \times 10\ \mu\hbox{m}$, fabricated with optical lithography. Experiments show that it has 298 nm/RIU sensitivity and a detection limit of $4.2\cdot 10^{-5}\ \hbox{RIU}$ for changes in the refractive index of the top cladding. We prove for the first time that surface chemistry for selective label-free sensing of proteins can be applied inside a 100 nm-wide slot region and demonstrate that the application of a slot waveguide instead of a normal waveguide increases the sensitivity of an SOI ring resonator with a factor 3.5 for the detection of proteins.