Miniature Achromatic Metalens Integrated With Optical Fiber (AMOF) for Remote Single‐Nanoparticle Fluorescence Probing

Single‐nanoparticle and single‐molecule fluorescence detection delivers unparalleled capabilities for quantifying concentrations, hydrodynamic sizes, and molecular dynamics for biosensing assays or environmental monitoring. Metalenses offer a promising route to miniaturizing high‐NA objective lenses, but their use in fiber‐integrated probes for quantitative single‐particle analysis remains largely unexplored. In this work, we introduce an achromatic metalens integrated with a solid‐core optical fiber (AMOF) as a compact sensing head that enables remote epi‐fluorescence correlation spectroscopy (FCS) in aqueous solutions. We achieve remote epi‐fluorescence monitoring of polystyrene nanoparticles of sizes from 110 nm to 4 µm and streptavidin biomolecules in the far‐red spectral range within a defined focal volume. By systematically varying the optical fiber core diameters, we reveal and quantify a design trade‐off between single‐nanoparticle sensitivity and detection volume, enabling accurate determination of particle concentration and size down to low picomolar‐femtomolar regimes through remote FCS. The robust operation of the AMOF head over meter‐scale fiber lengths without complicated alignment demonstrates that high numerical aperture (NA) achromatic metalenses can be effectively combined with optical fibers for remote single‐particle fluorescence analysis. This approach opens promising opportunities for in situ diagnostics, environmental nanoparticle monitoring, and remote emitter characterization.