  We propose a dielectric structure which focuses the laser light well beyond
the diffraction limit and thus considerably enhances the exerted optical
trapping force upon dielectric nanoparticles. Although the structure supports a
Fabry-Perot resonance, it actually acts as a nanoantenna in that the role of
the resonance is to funnel the laser light into the structure. In comparison
with the lens illuminating the structure, the proposed structure offers roughly
a 10000-fold enhancement in the trapping force - part of this enhancement comes
from an 80-fold enhancement in the field intensity, and the remaining comes
from a 130-fold enhancement in the normalized gradient of the field intensity
(viz. the gradient of the field intensity divided by the field intensity).
Also, the proposed structure offers roughly a 100-fold enhancement in the depth
of the trapping potential. It is noteworthy that 'self-induced back-action
trapping' (SIBA), which has recently been the focus of interest in the context
of optical resonators, does not take place in the proposed resonator. In this
paper, we also point out some misconceptions about SIBA together with some
hitherto unappreciated subtleties of the dipole approximation.
