  Plasmonic nano-optical tweezers enable the non-invasive manipulation of
nano-objects under low illumination intensities, and have become a powerful
tool for nanotechnology and biophysics. However, measuring the trap stiffness
of nanotweezers remains a complicated task, which hinders the development of
plasmonic trapping. Here, we describe an experimental method to measure the
trap stiffness based on the temporal correlation of the fluorescence from the
trapped object. The method is applied to characterize the trap stiffness in
different double nanohole apertures and explore the influence of their design
parameters in relationship with numerical simulations. Optimizing the double
nanohole design achieves a trap stiffness 10x larger than the previous
state-of-the-art. The experimental method and the design guidelines discussed
here offer a simple and efficient way to improve the performance of
nano-optical tweezers.
