  We study the physics of a new type of subwavelength nanocavities. They are
based on U-shaped metal-insulator-metal waveguides supporting the excitation of
surface plasmon polaritons. The waveguides are simultaneously excited from both
sides of the U by incident plane waves. Due to their finite length discrete
modes emerge within the nanocavity. We show that the excitation symmetry with
respect to the cavity ends permits the observation of even and odd modes. Our
investigations include near and far field simulations and predict a strong
spectral far field response of the comparable small nanoresonators. The strong
near field enhancement observed in the cavity at resonance might be suitable to
increase the efficiency of nonlinear optical effects, quantum analogies and
might facilitate the development of active optical elements, such as active
plasmonic elements.
