Extended quasimodes within nominally localized random waveguides

TL;DR

This study investigates the spatial and spectral properties of microwave fields in a disordered waveguide, revealing that overlapping quasimodes lead to extended fields with multiple peaks, challenging the notion of strict localization.

Contribution

It introduces a decomposition of the electromagnetic field into Lorentzian quasimodes in a localized waveguide, demonstrating their spectral overlap causes extended field patterns.

Findings

Overlapping quasimodes produce multiple spatial peaks.

Substantial energy extends beyond the waveguide's midpoint.

Lorentzian decomposition reveals position-independent frequencies and widths.

Abstract

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.