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Quantum metasurfaces, i.e., two-dimensional subwavelength arrays of quantum emitters, can be employed as mirrors towards the design of hybrid cavities, where the optical response is given by the interplay of a cavity-confined field and the surface modes supported by the arrays. We show that, under external magnetic field control, stacked layers of quantum metasurfaces can serve as helicity-preserving cavities. These structures exhibit ultranarrow resonances and can enhance the intensity of the incoming field by orders of magnitude, while simultaneously preserving the handedness of the field circulating inside the resonator, as opposed to conventional cavities. The rapid phase shift in the cavity transmission around the resonance can be exploited for the sensitive detection of chiral scatterers passing through the cavity. We discuss possible applications of these resonators as sensors for the discrimination of chiral molecules., 6+11 pages, 2+3 figures

We study the photophysics of molecular aggregates from a quantum optics perspective, with emphasis on deriving scaling laws for the fast non-radiative relaxation of collective electronic excitations, referred to as Kasha's rule. At deep subwavelength separations, quantum emitter arrays exhibit an energetically broad manifold of collective states with delocalized electronic excitations originating from near field dipole-dipole exchanges between the aggregate's monomers. Photoexcitation with visible light addresses almost exclusively symmetric collective states, which for an arrangement known as H-aggregate, have the highest energies (hypsochromic shift). The extremely fast subsequent non-radiative relaxation via intramolecular vibrational modes then populates lower energy, subradiant states which results in the effective inhibition of fluorescence. Our treatment allows for the derivation of an approximate linear scaling law of this relaxation process with the number of available low energy vibrational modes and reveals its direct proportionality to the dipole-dipole interaction strength between neighbouring monomers.