A synthetic biological quantum optical system

Abstract

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 Qy 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 Qx and Qy transitions of the chlorin. This observation is attributed to strong coupling of the LSPR to an H-dimer state not observed under weak coupling.

Fig. 1. (a) Schematic diagrams showing the structure of BT6 maquettes, and the location of the two chlorins. (b) Schematic diagram showing the method used to site-specifically bind maquettes to gold nanostructures.

Fig. 2. Ellipsometric thickness of the maquette film as a function of the time of immersion of NTA/Ni²⁺ functionalized gold films in a 500 nM solution of BT6-SE369₂ in buffer.

Fig. 3. (a) Normalized extinction spectra of BT6-SE369₂ maquettes in buffer solution (purple), clean gold nanostructures (blue) and gold nanostructures after attachment of BT6 maquettes with fractional coverages of 0.27 (green) and 1.00 (red). (b) Measured extinction spectrum (red symbols) and calculated spectrum obtained using the coupled harmonic oscillator model (black line) for a monolayer of BT6-SE369₂. (c) Variation in the exciton energy (triangles) and scaled coupling energy (circles) as a function of the LSPR energy for a monolayer of BT6-SE369₂ attached to gold nanostructures. (d) Variation in the scaled coupling energy as a function of the fractional surface coverage θ of BT6-SE369₂.

Fig. 4. Dispersion curves for the plexcitonic states determined from experimental data (circles and squares) together with curves fitted using eqn (1).

Fig. 5. (a) Possible alignment of pairs of SE369 chlorins in maquettes. The blue arrow represents the Q_y transition dipole moment, and the red arrow the direction of the field associated with the surface plasmon mode. (b) Possible coupling schemes for chlorin dimers.

Fig. 6. (a) Extinction spectra of BT6-SE369₁ maquettes in buffer solution (purple) and of an array of gold nanostructures before (black) and after attachment of monolayers of BT6-SE369₁ (red) and BT6-SE369₂ (blue). (b) Measured extinction spectrum (red symbols) and calculated spectrum obtained using the coupled harmonic oscillator model (black line) for a monolayer of BT6-SE369₁. (c) Variation in the exciton energy (triangles) and scaled coupling energy (circles) as a function of the LSPR energy for a monolayer of BT6-SE369₁ attached to gold nanostructures.

Fig. 7. (a) Extinction spectra for clean array of gold nanostructures before (black) and after attachment of BT6-SE369₁ (red) and chlorophyll a (blue). (b) Plot of mean coupling energy E_C as a function of the exciton energy E_mol for a variety of light-harvesting complexes and for chlorophyll a.