  Quantum emitters in confined arrays exhibit geometry dependent collective
dynamics. In particular, nanoscopic regular polygon-shaped arrays can possess
sub-radiant states with an exciton lifetime growing exponentially with emitter
number. We show that by placing an extra resonant absorptive dipole at the ring
center, such a structure becomes a highly efficient single-photon absorber with
tailorable frequency. Interestingly, for exactly nine emitters in a nonagon, as
it appears in a common biological light-harvesting complex (LHC2), we find a
distinct minimum for its most dark state decay rate and a maximum of the
effective absorption cross-section, surpassing that for a single absorptive
emitter. The origin of this optimum for nine emitters can be geometrically
traced to the fact that the sum of coupling strengths of a single ring emitter
to all others including the center ring closely matches the coupling of the
center to all ring emitters. The emerging dark collective eigenstate has
dominant center occupation facilitating efficient energy absorption and fast
transport. The resonance frequency can be tuned via ring size and dipole
polarization. In analogy to parabolic antennas, the ring concentrates the
incoming radiation at the center without being significantly excited, which
minimizes transport loss and time.

Quantum Physics;Mesoscale and Nanoscale Physics;Optics