  Nanoantennas capable of large fluorescence enhancement with minimal
absorption are crucial for future optical technologies from single-photon
sources to biosensing. Efficient dielectric nanoantennas have been designed,
however, evaluating their performance at the individual emitter level is
challenging due to the complexity of combining high-resolution nanofabrication,
spectroscopy and nanoscale positioning of the emitter. Here, we study the
fluorescence enhancement in infinity-shaped gallium phosphide (GaP)
nanoantennas based on a topologically optimized design. Using fluorescence
correlation spectroscopy (FCS), we probe the nanoantennas enhancement factor
and observed an average of 63-fold fluorescence brightness enhancement with a
maximum of 93-fold for dye molecules in nanogaps between 20 nm and 50 nm. The
experimentally determined fluorescence enhancement of the nanoantennas was
confirmed by numerical simulations of the local density of optical states
(LDOS). Furthermore, we show that beyond design optimisation of dielectric
nanoantennas, increased performances can be achieved via tailoring of
nanoantenna fabrication.
