Eigenmodes for the problem of extraordinary light transmission through subwavelength holes

We show that the widespread concept of eigenmodes in lossless waveguide structures, which assumes the presence of propagating and evanescent modes only, fails in the case of metal-dielectric structures, such as single holes/slits and periodic arrays of holes/slits in metals. In addition to these modes, there is a sequence of new eigenstates with complex values of the propagation constant and non-zero lateral energy flows. The whole eigenproblem ceases to be Hermitian because of changing sign of the optical dielectric constant. The new anomalous modes are shown to be essential for the description of the extraordinary light transmission through subwavelength holes. Copyright (C) EPLA, 2007.