FLOWER: a frequency-locked optical whispering evanescent resonator for label-free molecular detection.

The ability to detect trace amounts of biomolecules, chemicals, and environmental contaminants is critical for advancing diagnostics, drug discovery, and public health. Most high-sensitivity detection methods rely on molecular labels, such as radioactive or fluorescent tags which can perturb the system or may not be feasible in all contexts. In contrast, optical microcavities provide a powerful platform for label-free sensing by confining light and enhancing light-matter interactions. FLOWER (frequency locked optical whispering evanescent resonator) is a versatile sensing technique that can be applied to any optical microcavity to enable real-time, quantitative detection with high sensitivity and stability. In our work, we pair FLOWER with silica microtoroid resonators that offer ultra-high quality factors for prolonged light confinement, along with a small footprint and compatibility with on-chip fabrication. By combining these features, FLOWER bridges the gap between high performance and portability, making it particularly well suited for precision sensing. FLOWER has enabled label-free detection at the single-molecule and zeptomolar levels. In this review, we place FLOWER within the broader optical sensing landscape, highlighting how it addresses longstanding challenges in sensitivity, noise suppression, and molecular specificity across a wide range of chemical and biological applications.