Mode conversion and resonant absorption in inhomogeneous materials with flat bands

TL;DR

This paper demonstrates that phenomena like mode conversion and resonant absorption, well-known in plasma physics, also occur in electronic and photonic systems with flat and dispersive bands, revealing new insights into wave interactions in these materials.

Contribution

The study shows that mode conversion and resonant absorption occur in systems with flat bands, extending plasma physics concepts to electronic and photonic materials with complex band structures.

Findings

Mode conversion occurs in pseudospin-1 Dirac systems with flat bands.

Similar phenomena are observed in pseudospin-2 Dirac systems and lattice models.

Explicit calculation of mode conversion coefficients using invariant imbedding method.

Abstract

Mode conversion of transverse electromagnetic waves into longitudinal oscillations and the associated resonant absorption of wave energy in inhomogeneous plasmas is a phenomenon that has been studied extensively in plasma physics. We show that precisely analogous phenomena occur generically in electronic and photonic systems where dispersionless flat bands and dispersive bands coexist in the presence of an inhomogeneous potential or medium parameter. We demonstrate that the systems described by the pseudospin-1 Dirac equation with two dispersive bands and one flat band display mode conversion and resonant absorption in a very similar manner to $p$-polarized electromagnetic waves in an unmagnetized plasma by calculating the mode conversion coefficient explicitly using the invariant imbedding method. We also show that a similar mode conversion process takes place in many other systems with a flat band such as pseudospin-2 Dirac systems, continuum models obtained for one-dimensional stub and sawtooth lattices, and two-dimensional electron systems with a quadratic band and a nearly flat band. We discuss some experimental implications of mode conversion in flat-band materials and metamaterials.