A key for advancing the emerging artificial photosynthetic systems is to control the system of excitation energy transfer between photosynthetic light-harvesting complexes, which is vital for highly efficient primary photosynthesis. The results suggest that optical microcavities can be a strategic tool for modifying excitation energy transfer between molecular complexes, offering a promising approach towards efficient artificial light harvesting.
“These results show that we can influence how energy moves between the light-harvesting units used by nature in photosynthesis. In plants and photosynthetic bacteria, absorbed sunlight has to be transported very efficiently to the place where it can be converted into chemical energy. We show that placing light-harvesting complexes inside a very small optical cavity – essentially between tiny mirrors – can make this energy transfer more efficient. This is interesting because it gives us a new physical tool for steering energy flow in artificial light-harvesting systems”, says Tönu Pullerits.
What is the most important thing you have learned from this study?
“The most important lesson is that the optical cavity does more than just modify the energy levels of the molecules. It can also create an additional connection between molecules, so that excitation energy can move between them more effectively. We expected this in the strong light–matter coupling regime, but the key finding is that the effect is still visible even in the weak coupling regime, where one might have expected the cavity influence to be too small.”
The results, he thinks, can be useful because they point to a possible design principle for future artificial photosynthetic and solar-energy materials.
If we can control how absorbed light energy moves through a material, we may be able to guide it more efficiently to where it is needed.
“If we can control how absorbed light energy moves through a material, we may be able to guide it more efficiently to where it is needed, for example, to a reaction centre or an interface where charge separation occurs. Optical microcavities could therefore become a way to tune and improve energy transport in molecular devices. At the same time, our study also shows that one has to manage unwanted energy losses, such as exciton-exciton annihilation, when designing such systems.”
How may the study be important to the public?
“The study is part of the broader effort to learn from nature’s way of using sunlight. Photosynthesis is one of the most efficient natural processes for collecting solar energy, and understanding how to control similar processes in artificial materials may help the development of future sustainable energy technologies. Our work is fundamental research, so it is not a ready-made solar cell, but it gives new knowledge about how light and matter can be combined to improve the movement of energy at the nanoscale.
Was there something in the results that took you by surprise?
Yes. The surprising result was that the cavity-enhanced energy transfer was observed not only under strong coupling, but also in the weak coupling regime. In the weak regime, the usual clear signatures of strong light–matter hybridization are absent or very small. Nevertheless, the cavity still helped the light-harvesting complexes communicate with each other. This suggests that even a subtle interaction with the cavity mode can influence energy transport.