Mie Voids for Single-Molecule Fluorescence Enhancement in Wavelength-Scale Detection Volumes

Mie voids have been recently demonstrated as a promising nanophotonic platform for light manipulation and optical sensing. Moreover, the detection volumes of Mie void cavities exceed those of optical nanoantennas, making them appropriate for low-concentration single-molecule fluorescence biosensing. However, the fluorescence enhancement quantification of diffusing molecules in such optical antenna systems has not been addressed. Here, we explore the Mie void ability to enhance single-molecule fluorescence of diffusing fluorophores AF647 with the help of fluorescence correlation spectroscopy. The optimized structure confines 635 nm laser light within a well-defined excitation volume in the Mie void and numerically promotes the excitation gain. We monitor the reduction of the number of molecules, signifying the detection volume reduction in the Mie void and an increase in single-molecule brightness up to 2.8 times. However, we reveal that the observed fluorescence enhancement appears limited owing to the azimuthally symmetric emission direction away from the optical axis when the molecules diffuse in the vicinity of the Mie-void entrance. Altogether, this study demonstrates exploration of Mie-void-based nanoantenna potential for single-molecule fluorescence spectroscopy applications.