  Plasmonic nanoapertures generate strong field gradients enabling efficient
optical trapping of nano-objects. However, because the infrared laser used for
trapping is also partly absorbed into the metal leading to Joule heating,
plasmonic nano-optical tweezers face the issue of local temperature increase.
Here, we develop three independent methods based on molecular fluorescence to
quantify the temperature increase induced by a 1064 nm trapping beam focused on
single and double nanoholes milled in gold films. We show that the temperature
in the nanohole can be increased by 10{\deg}C even at the moderate intensities
of 2 mW/$\mu$m$^2$ used for nano-optical trapping. The temperature gain is
found to be largely governed by the Ohmic losses into the metal layer,
independently of the aperture size, double-nanohole gap or laser polarization.
The techniques developed therein can be readily extended to other structures to
improve our understanding of nano-optical tweezers and explore heat-controlled
chemical reactions in nanoapertures.
