  We discuss the detection limit for refractometric sensors relying on high-Q
optical cavities and show that the ultimate classical detection limit is given
by min{Dn} > eta with n+i*eta being the complex refractive index of the
material under refractometric investigation. Taking finite Q factors and
filling fractions into account, the detection limit declines. As an example we
discuss the fundamental limits of silicon-based high-Q resonators, such as
photonic crystal resonators, for sensing in a bio-liquid environment, such as a
water buffer. In the transparency window of silicon the detection limit becomes
almost independent on the filling fraction, while in the visible, the detection
limit depends strongly on the filling fraction because silicon absorbs
strongly.
