Noise-Limited Nanophotonic Sensing: Fundamental Fluctuations Setting the Detection Floor.

Nanophotonic sensing has traditionally been driven by maximizing optical sensitivity through stronger field confinement and higher quality factors. However, as devices approach regimes of extreme light-matter interaction, detection performance is increasingly limited not by signal strength but by fluctuations. This review presents a unified framework for noise-limited detection in nanophotonic sensors, spanning intrinsic thermodynamic noise, stochastic fluctuations at sensing interfaces, instrumental noise, and environmental perturbations. We highlight how these contributions set practical limits that depend on mode volume, material properties, analyte concentration, and measurement bandwidth. We further argue that conventional metrics based on sensitivity and quality factor are insufficient to describe detection capability and instead require noise-aware figures of merit. Finally, we discuss design strategies, including mode engineering, differential architectures, and system-level co-design, as pathways toward reducing noise and approaching fundamental limits. This perspective establishes a framework for developing nanophotonic sensors that operate with metrological precision.