Good vibrations could hold key to emulating energy capture and transfer in nature

Physicists at the University of Southampton are studying molecular vibrations that could shine new light on how plants convert sunlight into energy.

An experimental group, led by Dr Luca Sapienza, is investigating how photosynthetic organisms exploit mechanical vibrations in energy transfer processes.

The new research could help scientists reverse engineer the natural processes to realise new devices with enhanced energy capture and transfer capabilities.

Plants, algae and some bacteria rely on nano-scale molecular complexes to absorb sunlight and trigger the chemical energy conversion that sustains life processes on Earth.

These complexes are thought to benefit from molecular vibrations, but exactly how these affect the efficiency, directionality and quantum properties of energy dynamics is yet to be fully understood.

To gain this understanding, Southampton researchers are investigating how bio-molecules transfer energy within the controlled vibrational environment provided by opto-mechanical on-chip devices, where sunlight is replaced by excitation laser sources.

Dr Sapienza, of the Quantum, Light and Matter research group, says: "By investigating bio-molecules embedded within nano-fabricated devices that can control mechanical vibrations, this research will shine new light onto the microscopic processes that control energy dynamics at the molecular scale."

Within photosynthetic complexes, precisely arranged chromophores are bound to a protein scaffold and the absorption of light leads to the formation of collective electronic states, called excitons. The associated electronic energy is distributed and transferred to lower energy states at a pico-second rate.

There is mounting experimental and theoretical evidence that a sophisticated interplay between electronic and vibrational dynamics underpins the efficiency of the process.

"This line of thought has led to the counterintuitive idea that phonon-assisted processes, resulting from the coupling of biomolecules to their vibrational environment, rather than being detrimental, can actually improve the efficiency and directionality of the energy transfer and can sustain quantum coherent processes," Dr Sapienza says.

"The leading hypothesis to explain the mechanism at the basis of how these systems function is the presence of coherent vibronic interactions, whereby specific vibrational motions are driven out of thermal equilibrium to form exciton vibrational quantum states. The interpretation of how such process occurs, however, remains still controversial."

The latest research, funded by the Engineering and Physical Sciences Research Council (EPSRC), will use on-chip opto-mechanical devices, developed for semiconductor technology, as a new platform to control the phononic environment of photo-active biomolecules.

The devices are designed to control the amplitude of specific vibrational frequencies involved in photosynthetic processes. By characterising the emission dynamics and correlations of the photons emitted by the biomolecules, the research team will investigate the influence of the phononic environment.

Dr Sapienza says: "While opto-mechanics is a well-established research area, with great potential in metrology and force-sensing applications, the integration of biomolecules within phononic membranes is an exciting and completely novel field."

"The realisation of this platform opens the path to reverse-engineering biological architectures, that have been optimised by evolution over billions of years, to develop hybrid biomechanical units exploiting coherence to enhance the harvesting and transfer of energy."

Dr Sapienza is one of three academics from the University of Southampton awarded a total of £600,000 from the EPSRC New Horizons call; a programme aimed at high-risk discovery research focused on advancing knowledge and securing the pipeline of next-generation innovations.

Professor Jonathan Essex is investigating molecular simulations, which are an essential tool in the design of new drugs, while Dr Alain Zemkoho is exploring 'pessimistic bilevel optimisation' problems between various engineering, economic and human systems.