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A New Twist on Light

Photonics.com
Apr 2011
CAMBRIDGE, Mass., April 26, 2011 — Corkscrew-like light whose rotating waves spiral more tightly than other forms of circularly polarized light can discriminate between right- and left-handed versions of chiral molecules, which can’t be superimposed on their mirror images (enantiomer). This “superchiral light” could have possible applications in plasmonic sensing or in selecting the enantiomer of a drug that interacts with proteins or DNA in an optimal way.


An electromagnetic field can have the property of chirality, or handedness, which makes it impossible to superimpose the field lines on their mirror image. The report by Tang and Cohen describes “superchiral light,” which has greater selectivity in exciting molecules of one chirality relative to their mirror image than does any other known form of light. (Images: Science/AAAS)

The basic concept is that circularly polarized light interacts differently with a single chiral molecule than with its enantiomer. For most small molecules, however, this difference is relatively weak, because the light polarization varies on a spatial scale much larger than the atomic dimensions.

Yiqiao Tang and Adam Cohen have engineered a sharply rotating polarization pattern that is more than 10 times better than ordinary circular polarization at discriminating enantiomers. The pattern was generated by superimposing a circularly polarized wave on an oppositely polarized wave of lower intensity traveling in the opposite direction. The improvement observed in these experiments is well accounted for by theory, the authors reported.

This research appears in the April 15, 2011, issue of Science.

For more information, visit: www.sciencemag.org


GLOSSARY
circularly polarized light
A light beam whose electric vectors can be broken into two perpendicular elements that have equal amplitudes and that differ in phase by l/4 wavelength.
enantiomer
A molecule that is the mirror image of another molecule. The two mirror-image molecules have the same chemical properties; their behavior differs only in reactions with other enantiomers or with polarized light. Also known as an enantiomorph, optical antipode or optical isomer.
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