Observation of coherent perfect absorption at an exceptional point

Abstract: The past few years have witnessed growing interests in exceptional points (EPs) in various domains, including photonics, acoustics and electronics. However, EPs have mainly been realized based on the degeneracy of resonances of physical systems; distinct degeneracies occur relating to the absorption properties of waves, with distinct physical manifestations. Here we demonstrate this physically different kind of exceptional point, by engineering degeneracies in the absorption spectrum of optical microcavities with dissipation. We experimentally distinguish the conditions to realize a resonant EP and an absorbing EP. Furthermore, when the optical loss is optimized to achieve perfect absorption at such an EP, we observe an anomalously broadened lineshape in the absorption spectra, as predicted by theory. The distinct scattering properties enabled by this type of EP creates new opportunities for both the fundamental study and applications of non-Hermitian singularities.

Observation of Coherent Perfect Absorption at an Exceptional Point

This paper explores the novel concept of exceptional points (EPs) within the context of coherent perfect absorption (CPA) in optical microcavity systems. The authors focus on a specific category called absorbing EPs, distinct from the more conventional resonant EPs typically defined by the degeneracy of resonance frequencies. The research investigates the interplay between incoming and outgoing wave solutions, leading to singular characteristics in non-Hermitian systems when tuned precisely.

The primary experimentation revolves around observing coherent perfect absorption at EPs within coupled optical microcavities. The experimental setup employs whispering gallery mode (WGM) microcavities to realize the coupled system. The microcavities exhibit intrinsic loss rates, coupled to single-mode fiber waveguides. The authors meticulously control system parameters such as resonant frequencies and coupling strengths to observe the transition to EPs with absorption as the primary feature. This enables insights into the distinct scattering behaviors associated with CPA EPs, characterized by quartic absorption lineshapes.

The study reveals that CPA EPs provide a mechanism for achieving perfect absorption at real frequencies without the instabilities often associated with lasing processes. This contrasts with resonant EPs, which involve complex frequencies and are typically associated with poles of the scattering matrix.

The implications of this research are manifold. Practically, CPA EPs could be harnessed in photonics for applications requiring phase-sensitive absorption and scattering control. Theoretically, the work enriches our understanding of non-Hermitian systems and extends the study of EPs beyond conventional resonance degeneracies. This opens avenues for exploring quantum photonic systems devoid of gain, revealing potential pathways for controlling quantum states and interactions with non-classical light.

Future research could explore applying this novel EP understanding across various domains, including metamaterials, acoustic systems, and electrical circuits, potentially revealing new phenomena utilizing EPs. Such developments could have profound effects on quantum state control, noise suppression, and non-linear interaction enhancement at the quantum level, making CPA EPs a promising area of exploration in non-Hermitian physics and engineering.

Overall, this paper provides a comprehensive analysis of a less-explored type of exceptional point, offering a fresh perspective on wave absorption dynamics and singularities in engineered optical systems.