Longitudinal localization of a fluorescent bead in a tunable microcavity with an accuracy of lambda/60

The exact localization of a quantum emitter in a transparent dielectric medium is an important task in applications of precision confocal microscopy. Therefore we use a planar metallic subwavelength microcavity that can be reversibly tuned across the entire visible range, with the transparent medium between the cavity mirrors. By analyzing the excitation patterns resulting from the illumination of a single fluorescent bead with a radially polarized doughnut mode laser beam we can determine the longitudinal position of this bead in the microcavity with an accuracy of a few nanometers.