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The spectral degeneracies emerging as a consequence of parity-time (PT) symmetry exhibit a profound divergence from their conventional counterparts. They possess non-Hermitian nature and are designated as exceptional points (EPs), marking instances where the real and corresponding imaginary portions of specific eigenvalues align perfectly, alongside a coalescence of their associated eigenvectors.

This unique phenomenon underscores the intricate, scientifically fascinating behavior exhibited by PT-symmetric systems. In fact, the progress in understanding and creating EPs have resulted in the demonstration of many nontrivial effects, such as power oscillations, non-reciprocal light propagation, unidirectional invisibility, chiral modes, and orbital angular momentum lasers.

Sensors operating in the vicinity of EPs are asserted to be superior to those operating away from EPs due to sensitivity enhancement.

Researchers led by Prof. Xiaobei Zhang at Shanghai University (SHU), China, are interested in deformed Reuleaux-triangle resonator (RTR) which breaks rotational symmetry to simplify exceptional point achievement. In general, EP in the circular microcavity requires two scatters to form.

Their work, titled "Exceptional point enhanced nanoparticle detection in deformed Reuleaux-triangle microcavity," was in Frontiers of Optoelectronics on Aug. 8, 2024.

The first scatter is used to break the rotational symmetry of the microcavity, making the degenerated whispering-gallery modes (WGMs) split into even and odd modes, and another scatter coalesces two eigenvalues and corresponding eigenvectors. The formed EP produces high-chirality mode, the far field pattern of which responds drastically to the disturbance of external nanoparticle.

Consequently, the deformed RTR can effectively detect a single nanoparticle at distance of up to 4,000 nm.