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1. Chiral and degenerate perfect absorption on exceptional surfaces

   https://doi.org/10.1038/s41467-022-27990-w  

   Soleymani, S.; Zhong, Q.; Mokim, M.; Rotter, S.; El-Ganainy, R.; Özdemir, Ş. K. (February 2022, Nature Communications)

   Abstract Engineering light-matter interactions using non-Hermiticity, particularly through spectral degeneracies known as exceptional points (EPs), is an emerging field with potential applications in areas such as cavity quantum electrodynamics, spectral filtering, sensing, and thermal imaging. However, tuning and stabilizing a system to a discrete EP in parameter space is a challenging task. Here, we circumvent this challenge by operating a waveguide-coupled resonator on a surface of EPs, known as an exceptional surface (ES). We achieve this by terminating only one end of the waveguide with a tuneable symmetric reflector to induce a nonreciprocal coupling between the frequency-degenerate clockwise and counterclockwise resonator modes. By operating the system at critical coupling on the ES, we demonstrate chiral and degenerate perfect absorption with squared-Lorentzian lineshape. We expect our approach to be useful for studying quantum processes at EPs and to serve as a bridge between non-Hermitian physics and other fields that rely on radiation engineering. 

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2. Coherent perfect absorption at an exceptional point

   https://doi.org/10.1126/science.abj1028  

   Wang, Changqing; Sweeney, William R.; Stone, A. Douglas; Yang, Lan (September 2021, Science)

   Recently, exceptional points, a degeneracy of open wave systems, have been observed in photonics, acoustics, and electronics. They have mainly been realized as a degeneracy of resonances; however, a degeneracy associated with the absorption of waves can exhibit distinct and interesting physical features. Here, we demonstrate such an absorbing exceptional point by engineering degeneracies in the absorption spectrum of dissipative optical microcavities. We experimentally distinguished the conditions to realize an absorbing exceptional point versus a resonant exceptional point. Furthermore, when the optical loss was tuned to achieve perfect absorption at an absorbing exceptional point, we observed its signature, an anomalously broadened line shape in the absorption spectrum. The distinct scattering properties of the absorbing exceptional point create opportunities for both fundamental study and applications of non-Hermitian degeneracies. 

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3. Ultra-sparse near-perfect sound absorbers

   https://doi.org/10.1121/10.0017520  

   Ji, Jun; Li, Junfei; Cummer, Steven_A; Jing, Yun (March 2023, JASA Express Letters)

   There is a trade-off between the sparseness of an absorber array and its sound absorption imposed by wave physics. Here, near-perfect absorption (99% absorption) is demonstrated when the spatial period of monopole-dipole resonators is close to one working wavelength (95% of the wavelength). The condition for perfect absorption is to render degenerate monopole-dipole resonators critically coupled. Frequency domain simulations, eigenfrequency simulations, and the coupled mode theory are utilized to demonstrate the acoustic performances and the underlying physics. The sparse-resonator-based sound absorber could greatly benefit noise control with air flow and this study could also have implications for electromagnetic wave absorbers. 

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4. In-plane linear stress waves in layered media: I. Non-Hermitian degeneracies and modal chirality

   https://doi.org/10.1016/j.wavemoti.2025.103610  

   Alizadeh, Vahidreza; Amirkhizi, Alireza V (November 2025, Wave Motion)

   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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5. Control of the Scattering Properties of Complex Systems by Means of Tunable Metasurfaces

   https://doi.org/10.12693/APhysPolA.144.421  

   Erb, J.; Shrekenhamer, D.; Sleasman, T.; Antonsen, T.M.; Anlage, S.M. (December 2023, Acta Physica Polonica A)

   Sirko, Leszek (Ed.)

   We demonstrate the ability to control the scattering properties of a two-dimensional wave-chaotic microwave billiard through the use of tunable metasurfaces located on the interior walls of the billiard. The complex reflection coefficient of the metasurfaces can be varied by applying a DC voltage bias to varactor diodes on the mushroom-shaped resonant patches, and this proves to be very effective at perturbing the eigenmodes of the cavity. Placing multiple metasurfaces inside the cavity allows us to engineer desired scattering conditions, such as coherent perfect absorption, by actively manipulating the poles and zeros of the scattering matrix through the application of multiple voltage biases. We demonstrate the ability to create on-demand coherent perfect absorption conditions at a specific frequency, and document the near-null of output power as a function of four independent parameters tuned through the coherent perfect absorption point. A remarkably low output-to-input power ratio P_{out}/P_{in} = 3:71 x 10^{-8} is achieved near the coherent perfect absorption point at 8.54 GHz. 

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