Casimir-Polder potential on an excited atom near an atomic array

Two-dimensional (2D) arrays of atoms arranged with subwavelength periodicity can behave as mirrors due to emergent cooperative resonances from collective interactions, manifesting novel boundaries at the limits of miniaturization. Given that the modification of the electromagnetic (EM) field mode structure by boundaries lies at the heart of various quantum electrodynamics (QED) phenomena, such atomically thin mirrors open a new avenue for QED. In this work, we analyze two paradigmatic QED effects - atomic spontaneous emission and Casimir-Polder shifts - on an excited two-level test atom ‘emitter’  near a 2D atomic mirror. While such atomic QED phenomena have been extensively studied with atoms near macroscopic boundaries, we investigate the emergence of these effects from a microscopic perspective, analyzing their dependence on the geometry of the atomic array and quantum state of the mirror atoms. We investigate the Casimir-Polder energy shift and spontaneous emission of an emitter placed near an array, demonstrating a resonant contribution and an off-resonant contribution. Furthermore, we propose a scenario wherein the atoms constituting 2D arrays can exist in superposition of quantum states which couple to light differently, thus engendering boundaries that can exist in macroscopic quantum superpositions of different optical behaviors.