Hybrid Metalens for Miniaturised Ultraviolet Fluorescence Detection

The advantages of metalenses to enable miniaturized systems have been well established, especially in the visible and infrared wavelength regimes. The ultraviolet (UV) presents a final frontier because feature size scales as the wavelength, so realizing a large size metalens with a high numerical aperture (NA) in the UV is a major challenge. Here, a single-layer, thin-film (450 nm) hybrid metalens with an NA of 0.9 and a diameter of 6.2 mm is presented. By combining a Fresnel lens and optimised binary gratings, the well-known shadowing effect of Fresnel lenses at high NA is avoided while being able to realize a large area lens. It is demonstrated that the combination of high NA and large area affords efficient detection of tryptophan-like fluorescence, which is a well-studied proxy for water contamination with faecal coliforms. The detection of tryptophan at levels better than 1 ppb, which corresponds to the low-risk category for drinking water according to the World Health Organization (WHO), is shown. It is also confirmed that the hybrid metalens fluorescence collection efficiency is 3.5 times higher than a high NA plano-convex lens used in state-of-the-art fluorometers, which demonstrates that the versatile metalens approach opens up new opportunities in the UV.