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Polarization switches crystal growth

17 Oct 2002

Switching the polarization of laser light shone onto a crystal in solution determines its precise structure when it forms.

The precise structure of crystals can be controlled simply by illuminating a solution with polarized light, say scientists in the US. The technique could eventually be used to promote the formation of a preferred crystal structure, for instance when one structure is biologically active but another is not.

Bruce Garetz and colleagues at Polytechnic University in New York and the Illinois Institute of Technology in Chicago illuminated an aqueous solution of glycine (an amino acid) with either linearly-polarized or circularly-polarized light from a Q-switched Nd:YAG laser.

Not only does the laser dramatically speed up the nucleation of glycine crystals, but the polarization determines which of three polymorphs (crystal structures) of glycine, called alpha, beta and gamma, forms. The alpha structure is made up of stacked planes, while the gamma structure consists of many parallel, twisting strands.

"Using circularly polarized light the alpha-polymorph is always produced," claim the researchers. On the other hand, linearly-polarized light from the laser promoted exclusive formation of the gamma structure.

Previous work by the group had suggested that polarized light accelerated crystal formation by helping to align the dissolved molecules in clusters. However, skeptics pointed out that contaminants in the solution could have seeded the crystal growth, or that it may have been triggered by photochemical or photothermal effects.

The latest work appears to confirm that it is the light's polarization that triggers the growth. "Any holes [in the argument] that might have existed in the past have been closed by this very pretty experiment," said University of Chicago researcher David Oxtoby. He adds that the effect could become a useful tool for material scientists or drugs manufacturers.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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