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Laser Controls Photosynthesis

Photonics Spectra
Aug 2002
Daniel C. McCarthy

Photosynthesis converts sunlight into stored carbon bond energy. Metabolic processes convert this energy into direct benefits for plants and bacteria and into indirect benefits for the rest of us. Although scientists understand the basic mechanism of photosynthesis, they have failed to synthesize its energy. The problem is a familiar one to chemists: How do you control a chemical process without essentially altering it?

Harvest.jpg
Ultrafast laser pulses can control photosynthesis without essentially altering its mechanism. The gratings and lenses of a pulse shaper (lower right) split the laser beam into different spectral components, and a liquid crystal mask arranges them into a specific profile. The shaped pulses are directed at an intrabacterial complex that harvests light for photosynthesis, and a computer evaluates their influence on the complex's conversion efficiency. Evolutionary algorithms calculate how to shape subsequent pulses to find the desired influence on the complex. Courtesy of Max Planck Institut für Quantenoptik.

A group of pan-European collaborators may have found the answer in the form of an ultrafast laser. The scientists learned that they could exert control over the efficiency of the photosynthesis by shaping ultrashort pulses to pump and probe light-harvesting structures in a purple bacterium. The research, performed at Max Planck Institut für Quantenoptik in Garching, Germany, relied on contributions from Glasgow University in the UK, Lund University in Sweden, and Vrije Universiteit in Amsterdam, the Netherlands.

During photosynthesis, the sun's radiant energy creates a chain of excited molecules in the plant tissue. Some of the molecules release their energy as heat, but most transfer it to molecular reaction centers that split hydrogen atoms from water molecules to form adenosine triphosphate (ATP), a compound that living systems commonly use to store energy. The process -- from photon to ATP formation -- lasts well under a second.

The researchers were primarily interested in the light-harvesting stage, wherein specialized pigment molecules capture and transfer solar energy. The amount of energy transferred vs. the amount lost as heat provided a quantifiable measure of their control over its photosynthetic process.

Working from the knowledge of how these molecular antenna complexes work, the team hit them with 30-fs pulses from a noncollinear optical parametric amplifier source. The excitation light had a central wavelength of 525 nm.

The laser emitted excitation pulses with a specific profile based on wavelength, pulse width, phase and amplitude and then probed the antenna to measure the impact on photosynthetic conversion. Typically, the antenna complexes in the bacterial sample converted about 50 percent of incident photons into photosynthetic energy.

But unlike sunlight, which is akin to white noise, the laser could be tuned to the antenna's reaction. Evolutionary algorithms tracked each pulse and adjusted subsequent emissions in search of a profile that the light-harvesting complex would dissipate instead of transfer.

The researchers found that they could demonstrate coherent control of the antenna's conversion efficiency, dropping it from 50 percent to about one-third. More importantly, the essential components and nature of the photosynthetic process were unchanged.


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