Material Limitations on the Detection Limit in Refractometry

TL;DR

This paper analyzes the fundamental physical limits of refractometric sensors using high-Q optical cavities, showing how material properties and device parameters constrain the minimum detectable refractive index change.

Contribution

It derives the ultimate classical detection limit based on the complex refractive index and examines how finite Q factors and filling fractions affect this limit, especially in silicon resonators.

Findings

Detection limit is fundamentally constrained by the material's complex refractive index.

In silicon, the detection limit is nearly independent of filling fraction in the transparency window.

In the visible spectrum, silicon's absorption causes the detection limit to strongly depend on filling fraction.

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{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.