Abstract
Abstract Parity-Time ( P T $\mathcal{P}\mathcal{T}$ ) symmetry has become an important concept in the design of synthetic optical materials, with exotic functionalities such as unidirectional transport and nonreciprocal reflection. At exceptional points, this symmetry is spontaneously broken, and solutions transition from those with conserved intensity to exponential growth or decay. Here, we analyze a quantum-photonic surface formed by a single layer of atoms in an array with light mediating strong cooperative many-body interactions. We show how delocalized collective excitation eigenmodes can exhibit an effective P T $\mathcal{P}\mathcal{T}$ symmetry and nonexponential decay. This effective symmetry is achieved in a passive system without gain by balancing the scattering of a bright mode with the loss from a subradiant dark mode. These modes coalesce at exceptional points, evidenced by the emergence of coherent perfect absorption where coherent incoming light is perfectly absorbed and scattered only incoherently. We also show how P T $\mathcal{P}\mathcal{T}$ symmetry can be generated in total reflection and by balancing scattering and loss between different polarizations of collective modes.
Highlights
The search for novel and powerful methods to control light using artificially engineered optical properties is a current driving force in photonics [1]
Applications of PT symmetric systems include, e.g., non-reciprocal light propagation [5, 6], unidirectional invisibility [7, 8], and coherent perfect absorption (CPA) [9,10,11]. They stem from a real eigenvalue spectrum [12] that undergoes spontaneous PT symmetry breaking at exceptional points (EPs) where eigenmodes coalesce and eigenvalues become complex [13]
While non-exponential decay demonstrates a physical effect of EPs even in the absence of PT symmetry, EPs are of particular interest when they coincide with the spontaneous breaking of such a symmetry, resulting in a transition of the eigenvalues from purely real to complex
Summary
The search for novel and powerful methods to control light using artificially engineered optical properties is a current driving force in photonics [1]. Applications of PT symmetric systems include, e.g., non-reciprocal light propagation [5, 6], unidirectional invisibility [7, 8], and coherent perfect absorption (CPA) [9,10,11] They stem from a real eigenvalue spectrum [12] that undergoes spontaneous PT symmetry breaking at exceptional points (EPs) where eigenmodes coalesce and eigenvalues become complex [13]. Transmission resonance narrowing due to giant Dicke subradiance below the fundamental quantum limit of a single-atom linewidth was recently experimentally observed [20] for a planar atom array, where all the atomic dipoles oscillated in phase, while incident fields can drive giant subradiance in resonator metasurfaces [21] These quantum-photonic [22] surfaces of atoms that have no dissipative losses due to absorption have several advantages over artificial resonator metasurfaces We demonstrate PT symmetry in other effective models of the system, related to total reflection as well as balanced scattering and loss between different polarizations of collective modes
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