  Photosynthetic organisms rely on a series of self-assembled nanostructures
with tuned electronic energy levels in order to transport energy from where it
is collected by photon absorption, to reaction centers where the energy is used
to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum
tepidum (Cba. tepidum), a member of the green sulphur bacteria (GSB) family,
light is absorbed by large antenna complexes called chlorosomes. The exciton
generated is transferred to a protein baseplate attached to the chlorosome,
before traveling through the Fenna-Matthews-Olson (FMO) complex to the reaction
center. The energy levels of these systems are generally defined by their
chemical structure. Here we show that by placing bacteria within a photonic
microcavity, we can access the strong exciton-photon coupling regime between a
confined cavity mode and exciton states of the chlorosome, whereby a coherent
exchange of energy between the bacteria and cavity mode results in the
formation of polariton states. The polaritons have an energy distinct from that
of the exciton and photon, and can be tuned in situ via the microcavity length.
This results in real-time, non-invasive control over the relative energy levels
within the bacteria. This demonstrates the ability to strongly influence living
biological systems with photonic structures such as microcavities. We believe
that by creating polariton states, that are in this case a superposition of a
photon and excitons within a living bacteria, we can modify energy transfer
pathways and therefore study the importance of energy level alignment on the
efficiency of photosynthetic systems.
