  As was shown by N. Engheta, electromagnetic fields in two adjacent slabs
bounded by two metal walls can satisfy the boundary conditions even if the
distance between the two walls is much smaller than the wavelength. This is
possible if one of the slabs has a negative permeability. Here we show that
these subwavelength "resonators" resonate only if the permeability of at least
one of the slabs is frequency dependent. Thus, there is no advantage of using
these structures with "left-handed" slabs as frequency-selective devices.
However, we have found that these systems can be in principle used as memory
devices for evanescent fields, because the boundary conditions are identically
satisfied for all plane evanescent waves inside the cavity. The physical
meaning and practical limitations for possible realizations are discussed. The
analysis is supported by FDTD simulations.
