Cavity-enhanced sensing in a microfluidic cell

Motivation: sensing individual nanosystems
Detecting and characterizing individual non-fluorescent nanosystems with high temporal resolution can reveal their intrinsic properties and dynamics. To reach the high sensitivity needed to study single, unlabelled nanosystems in solution, we exploit signal enhancement in a high-finesse (F ~ 10⁵) fiber-based Fabry–Pérot microcavity integrated into a microfluidic channel. A nanoobject passing through the tight focus of the cavity mode (w₀ ~ 1 µm) is detected via the frequency shift and amplitude decrease of the resonant cavity transmission induced by its excess polarizability.

3D Tracking of Brownian motion
Besides determining a nanoobject's size and refractive index, we can track its position by modulating the cavity length at high frequency and recording the resonance frequency and linewidth of several transverse cavity modes. Because these modes have different radial shapes, this yields spatial information that allows us to track the particle's position with high temporal resolution in three dimensions [Kohler et al., Nature Commun. (2021)].

Accurate sizing down to 3 nm
We recently demonstrated quantitative single-particle sizing with high precision and accuracy down to particle diameters of 3 nm [Palkhivala et al., ACS Nano 2025]. We developed an analytical model of the autocorrelation function for particle diffusion in our standing-wave sensing geometry and introduced a method that accounts for the transient nature of single-particle signals. This provides a high-bandwidth, label-free cavity-based sensing tool that captures all relevant time scales of translational diffusion and delivers quantitative information about individual nanoparticles in solution.