We have measured the spatial and spectral dependence of the microwave field
inside an open absorbing waveguide filled with randomly juxtaposed dielectric
slabs in the spectral region in which the average level spacing exceeds the
typical level width. Whenever lines overlap in the spectrum, the field exhibits
multiple peaks within the sample. Only then is substantial energy found beyond
the first half of the sample. When the spectrum throughout the sample is
decomposed into a sum of Lorentzian lines plus a broad background, their
central frequencies and widths are found to be essentially independent of
position. Thus, this decomposition provides the electromagnetic quasimodes
underlying the extended field in nominally localized samples. When the
quasimodes overlap spectrally, they exhibit multiple peaks in space.Comment: 4 pages, submitted to PRL (23 December 2005
