  Strong-coupling between light and matter produces hybridized states
(polaritons) whose delocalization and electromagnetic character allow for novel
modifications in spectroscopy and chemical reactivity of molecular systems.
Recent experiments have demonstrated remarkable distance-independent long-range
energy transfer between molecules strongly coupled to optical microcavity
modes. To shed light on the mechanism of this phenomenon, we present the first
comprehensive theory of polariton-assisted remote energy transfer (PARET) based
on strong-coupling of donor and/or acceptor chromophores to surface plasmons.
Application of our theory demonstrates that PARET up to a micron is indeed
possible via strong-coupling. In particular, we report two regimes for PARET:
in one case, strong-coupling to a single type of chromophore leads to transfer
mediated largely by surface plasmons while in the other case, strong-coupling
to both types of chromophores creates energy transfer pathways mediated by
vibrational relaxation. Importantly, we highlight conditions under which
coherence enhances or deteriorates these processes. For instance, while
exclusive strong-coupling to donors can enhance transfer to acceptors, the
reverse turns out not to be true. However, strong-coupling to acceptors can
shift energy levels in a way that transfer from acceptors to donors can occur,
thus yielding a chromophore role-reversal or "carnival effect." This
theoretical study demonstrates the potential for confined electromagnetic
fields to control and mediate PARET, thus opening doors to the design of remote
mesoscale interactions between molecular systems.
