title: Material limitations on the detection limit in refractometry.
abstract: 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 {Δn} ≳ η, with n + iη 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 (λ ≳ 1100 nm) 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 the silicon absorbs strongly.
authors.affiliation: Department of Micro and Nanotechnology, Technical University of Denmark, DTU Nanotech, Building 345 East, DK-2800 Kongens Lyngby, Denmark; E-Mails:  peder.skafte-pedersen@nanotech.dtu.dk  (P.S.P.);  pedro.nunes@nanotech.dtu.dk  (P.S.N.).
authors.forename: Peder
authors.initials: P
authors.lastname: Skafte-Pedersen
authors.forename: Pedro S
authors.initials: PS
authors.lastname: Nunes
authors.forename: Sanshui
authors.initials: S
authors.lastname: Xiao
authors.forename: Niels Asger
authors.initials: NA
authors.lastname: Mortensen