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  • Harnessing of the sun’s energy unraveled

Apr 2009
Amanda D. Francoeur,

LONDON – Scientists have understood photosynthesis for centuries, but until now, they had been unable to actually see the sun’s energy moving through the most inner mechanisms of a plant.

“Plants have been doing this very efficiently, and we will be needing to harness the smarts of evolved biology,” said Dr. Ian P. Mercer, visiting researcher at Imperial College London and lead author of a report on the subject published in Physical Review Letters in February. Visualizing the process could lead to the development of more efficient solar cells that channel photovoltaic energy to take the place of fossil fuels.

Inexpensive solar cell devices now operate at only 10 percent efficiency. Therefore, Mercer said, “any improvement in solar conversion efficiency is good news. A goal is not only to improve our understanding of the design rules behind electronic devices in order to improve the efficiency of solar to electrical conversion but, ultimately, to enable solar-to-chemical conversion.”

Witnessing natural energy

In a photosynthetic protein, energy from the sun is transferred between electrons and moved through the molecule. Subsequently, light energy is converted into chemical energy by chemical reaction and then stored as food.

According to scientists, the process involves coupling between electrons, and in the Mercer study, the researchers’ method could differentiate random energy transfers from coherent pairing.

To understand the energy transfer more completely, the researchers examined protein from a purple photosynthetic bacterium called light-harvesting complex II (LH2) that mimics the photovoltaic process. To excite the electrons, they used a nonlinear high-power ultrashort pulse method called angle-resolved coherent four-wave mixing, splitting three beams from a single laser emitting 1000 pulses every second with a bandwidth ranging from 650 to 950 nm.

Two of the beams are projected at the same time, overlapping upon hitting the sample. The third is delayed 0 to 3 ps, and the fourth, or signal light, is provided by the sample.

“This provides an instantaneous 2-D map that is rich in detail,” Mercer said. “The relative delay of the light pulses adds further dimensionality, revealing how energy transfer progresses in time in the sample.”

Two sets of two-dimensional maps are shown, with pulse time delays indicated. A broad range of light from the laser-based technique allows for reading multiple wavelengths from bright spots that reflect energy driven by laser pulses. Visualizing how photosynthesis transfers energy so efficiently could help researchers develop a mechanism with the same resourcefulness to replace fossil fuels. Courtesy of Physical Review Letters.

A single ultrafast laser pulse can return data from the sample before the molecules move dramatically. A 2-D image, taken at a 10,000th of a billionth of a second, can capture information immediately before or at the exact moment of a chemical reaction.

The pulses reveal the transfer of energy by electrons, showing up as bright spots on a map. The position of the spots relates to the angle at which the light is emitted by the sample, which corresponds to the light energies interacting inside the sample. Chlorophyll and other pigments within the protein are colored differently, absorbing and emitting particular colors of light. The examiners investigated B800 and B850 pigments from the LH2 molecule at ambient temperature.

Wavelengths read from the bright spots indicated that energy was coherently coupled between electrons, a more prevalent occurrence than hopping between uncoupled electrons. The reading also was associated with the energy transfer inside the molecule before it was emitted.

As a result, the images allow for observing the electrons’ interaction, providing a better idea of how photosynthesis transfers energy so efficiently by quantum mechanical coupling.

Other laser-based techniques require long periods of laser exposure, which could damage the sample by photodegradation or toxicity, and more computer analysis is needed to achieve the same results.

The researchers at Imperial College currently are using the method to investigate protein, enzyme and photovoltaic mechanisms. “Another area of interest is not only to measure what is going on, but to see if we can control what is going on inside molecules,” Mercer said.

Manipulating energy transfer within a photovoltaic cell could take us another step closer to harnessing the sun’s energy.

solar cell
A device for converting sunlight into electrical energy, consisting of a sandwich of P-type and N-type semiconducting wafers. A photon with sufficient energy striking the cell can dislodge an electron from an atom near the interface of the two crystal types. Electrons released in this way, collected at an electrode, can constitute an electrical current.
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