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We identify a new kind of physically realizable exceptional point (EP) corresponding to degenerate coherent perfect absorption, in which two purely incoming solutions of the wave operator for electromagnetic or acoustic waves coalesce to a single state. Such non-Hermitian degeneracies can occur at a real-valued frequency without any associated noise or nonlinearity, in contrast to EPs in lasers. The absorption line shape for the eigenchannel near the EP is quartic in frequency around its maximum in any dimension. In general, for the parameters at which an operator EP occurs, the associated scattering matrix does not have an EP. However, in one dimension, when the S matrix does have a perfectly absorbing EP, it takes on a universal one-parameter form with degenerate values for all scattering coefficients. For absorbing disk resonators, these EPs give rise to chiral absorption: perfect absorption for only one sense of rotation of the input wave.
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This content will become publicly available on November 1, 2026
In-plane linear stress waves in layered media: I. Non-Hermitian degeneracies and modal chirality
We study the band structure and scattering of in-plane coupled longitudinal and shear stress waves in linear layered media and observe that exceptional points (EP) appear for elastic (lossless) media, when parameterized with real-valued frequency and tangential wave vector component. The occurrence of these EP pairs is not limited to the original stop bands. They could also appear in all mode pass bands, leading to the formation of new stop bands. The scattered energy near these locations is studied along with the associated polarization patterns. The broken phase symmetry is observed inside the frequency bands book-ended by these EP pairs. This is especially manifested by the chirality of the trajectory of the particle velocity, which gets selected by a ‘‘direction’’ of the wave, e.g. the imaginary part of normal component of the wave vector, or the energy flux direction just outside the band. Additionally, EP pairs also appear in the spectrum of the (modified) scattering matrix when mechanical gain is theoretically included to balance the loss in a parity-time symmetric finite structure. These EP pairs lead to amplification of transmission to above 1 and single-sided reflectivity, both phenomena associated with broken phase symmetry, with intriguing potential applications.
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- Award ID(s):
- 2219087
- PAR ID:
- 10631653
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Wave Motion
- Volume:
- 139
- Issue:
- C
- ISSN:
- 0165-2125
- Page Range / eLocation ID:
- 103610
- Subject(s) / Keyword(s):
- Mixed mode stress waves Transfer matrix method Elastic and viscoelastic layered media Exceptional points Chiral polarization
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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