  The extraordinary transmission of light through a nano slit in a metal film
is enhanced by introducing a nano cavity antenna formed by a nearby metallic
nano-strip over the slit opening. For a fixed wavelength, the width of the
metallic nano-strip should be chosen to make the horizontal
metal-insulator-metal waveguide of finite length resonant as a Fabry-Perot
cavity. When such a cavity antenna is used to enhance the transmission through
a non-resonant nano slit, the slit should be opened at a position with maximal
magnetic field in the horizontal resonant cavity. It is shown that an optimized
cavity antenna can enhance greatly the transmission of light through a
non-resonant nano slit (by about 20 times) or a resonant nano slit (by 124%).
The transmission spectrum of the nano slit can also be tuned by adjusting the
width of the metallic nano-strip. Such a transmission enhancement with a nano
cavity antenna is studied for the first time and the physical mechanism is
explained.
