A Synthetic Biological Quantum Optical System

In strong plasmon-exciton coupling, a surface plasmon mode is coupled to an array of localized emitters to yield new hybrid light-matter states (plexcitons), whose properties may in principle be controlled via modification of the arrangement of emitters. We show that plasmon modes are strongly coupled to synthetic light-harvesting maquette proteins, and that the coupling can be controlled via alteration of the protein structure. For maquettes with a single chlorin binding site, the exciton energy (2.06 +/- 0.07 eV) is close to the expected energy of the Q(y) transition. However, for maquettes containing two chlorin binding sites that are collinear in the field direction, an exciton energy of 2.20 +/- 0.01 eV is obtained, intermediate between the energies of the Q(x) and Q(y) transitions of the chlorin. This observation is attributed to strong coupling of the LSPR to an H-dimer state not observed under weak coupling.