  We investigate the optical response of two sub-wavelength grooves on a
metallic screen, separated by a sub-wavelength distance. We show that the
Fabry-Perot-like mode, already observed in one-dimensional periodic gratings
and known for a single slit, splits into two resonances in our system : a
symmetrical mode with a small Q-factor, and an antisymmetric one which leads to
a much stronger light enhancement. This behavior results from the near-field
coupling of the grooves. Moreover, the use of a second incident wave allows to
control the localization of the photons in the groove of our choice, depending
on the phase difference between the two incident waves. The system exactly acts
as a sub-wavelength optical switch operated from far-field.
