  Concentrating light within subwavelength spatial regions is a central topic
in nanophotonics. In this letter, we introduce a general principle for the
subwavelength localization and enhancement of electric fields in high-index
resonators, based on the flux conservation of the displacement current density.
We apply this design rule to a ring resonator by locally squeezing its section:
since the flux is conserved, the electric field is necessarily enhanced to
compensate the reduction of the section area. The introduced principle may
constitute an important step toward the control of the displacement current
density at the nanoscale, guiding the design of the topology and the geometry
of complex dielectric structures.
