  Single-molecule fluorescence techniques are essential for investigating the
molecular mechanisms in biological processes. However, achieving
sub-millisecond temporal resolution to monitor fast molecular dynamics remains
a significant challenge. The fluorescence brightness is the key parameter that
generally defines the temporal resolution for these techniques. Conventional
microscopes and standard fluorescent emitters fall short in achieving the high
brightness required for sub-millisecond monitoring. Plasmonic nanoantennas have
been proposed as a solution, but despite huge fluorescence enhancement have
been obtained with these structures, the brightness generally remains below 1
million photons/s/molecule. Therefore, the improvement of temporal resolution
has been overlooked. In this article, we present a method for achieving high
temporal resolution in single-molecule fluorescence techniques using plasmonic
nanoantennas, specifically optical horn antennas. We demonstrate about 90%
collection efficiency of the total emitted light, reaching a high fluorescence
brightness of 2 million photons/s/molecule in the saturation regime. This
enables observations of single molecules with microsecond binning time and fast
fluorescence correlation spectroscopy (FCS) measurements. This work expands the
applications of plasmonic antennas and zero-mode waveguides in the fluorescence
saturation regime towards brighter single-molecule signal, faster temporal
resolutions and improved detection rates to advance fluorescence sensing, DNA
sequencing and dynamic studies of molecular interactions.
