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We demonstrate an exceptional surface in a waveguide-coupled resonator by establishing unidirectional coupling between its frequency-degenerate counterpropagating modes. When operated on the ES, the system exhibits chiral perfect absorption with quartic lineshape. 

We introduce a new class of photonic resonators with resonant modes that feature hybrid standing-travelling waves. 

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. 

We propose a new optical amplifier geometry based on exceptional points. Compared to its standard counterpart device, the proposed structure relaxes the limitation imposed by the gain-bandwidth product.