Abstract
When light-harvesting complex II (LHCII), isolated from spinach, is adsorbed onto arrays of gold nanostructures formed by interferometric lithography, a pronounced splitting of the plasmon band is observed that is attributable to strong coupling of the localised surface plasmon resonance to excitons in the pigment-protein complex. The system is modelled as coupled harmonic oscillators, yielding an exciton energy of 2.24 ± 0.02 eV. Analysis of dispersion curves yields a Rabi energy of 0.25 eV. Extinction spectra of the strongly coupled system yield a resonance at 1.43 eV that varies as a function of the density of nanostructures in the array. The enhanced intensity of this feature is attributed to strong plasmon-exciton coupling. Comparison of data for a large number of light-harvesting complexes indicates that by control of the protein structure and/or pigment compliment it is possible to manipulate the strength of plasmon-exciton coupling. In strongly coupled systems, ultra-fast exchange of energy occurs between pigment molecules: coherent coupling between non-local excitons can be manipulated via selection of the protein structure enabling the observation of transitions that are not seen in the weak coupling regime. Synthetic biology thus provides a means to control quantum-optical interactions in the strong coupling regime.
Highlights
Organic semiconductors, produced from earth-abundant elements via low-energy pathways, are attractive for the sustainable production of devices and materials for many new and emerging technologies, including consumer electronics, solar energy capture, quantum computing, quantum communications and photocatalysis
When light-harvesting complex II (LHCII), isolated from spinach, is adsorbed onto arrays of gold nanostructures formed by interferometric lithography, a pronounced splitting of the plasmon band is observed that is attributable to strong coupling of the localised surface plasmon resonance to excitons in the pigment–protein complex
Ultra-fast exchange of energy occurs between pigment molecules: coherent coupling between non-local excitons can be manipulated via selection of the protein structure enabling the observation of transitions that are not seen in the weak coupling regime
Summary
Organic semiconductors, produced from earth-abundant elements via low-energy pathways, are attractive for the sustainable production of devices and materials for many new and emerging technologies, including consumer electronics, solar energy capture, quantum computing, quantum communications and photocatalysis. The importance of coherence as a design concept for molecular photonic materials has been recognised.[3,4,5] Theory[6,7,8,9] and measurements by ultra-fast spectroscopy[10,11,12,13,14] have suggested that quantum coherent excitations of multiple pigment molecules in photosynthetic light-harvesting complexes (LHCs) facilitate efficient energy transfer, leading to the delocalisation of excitation around individual complexes and to more efficient transfer of energy between complexes via Forster resonance energy transfer (FRET).[15] There has been a resurgence of interest in quantum biology, and it has been suggested that photosynthetic mechanisms provide a model for the design of molecular photonic structures to achieve efficient transport of excitons.[5,16]
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