  It is well-known that classical optical cavities can exhibit localized
phenomena associated to scattering resonances, leading to numerical
instabilities in approximating the solution. This result can be established via
the ``quasimodes to resonances'' argument from the black-box scattering
framework. Those localized phenomena concentrate at the inner boundary of the
cavity and are called whispering gallery modes. In this paper we investigate
scattering resonances for unbounded transmission problems with sign-changing
coefficient (corresponding to optical cavities with negative optical
properties, for example made of metamaterials). Due to the change of sign of
optical properties, previous results cannot be applied directly, and interface
phenomena at the metamaterial-dielectric interface (such as the so-called
surface plasmons) emerge. We establish the existence of scattering resonances
for arbitrary two-dimensional smooth metamaterial cavities. The proof relies on
an asymptotic characterization of the resonances, and showing that problems
with sign-changing coefficient naturally fit the black box scattering
framework. Our asymptotic analysis reveals that, depending on the
metamaterial's properties, scattering resonances situated closed to the real
axis are associated to surface plasmons. Examples for several metamaterial
cavities are provided.
