A multi-institutional team of researchers that included scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has created the first artificial molecules whose chirality can be rapidly switched from a right-handed to a left-handed orientation with a †beam of light. This holds potentially important possibilities for the application of terahertz technologies across a wide range of fields, including reduced energy use for data-processing, homeland security and ultrahigh-speed.
Chirality is the distinct left/right orientation or "handedness" of some types of molecules, meaning the molecule can take one of two mirror image forms. The right-handed and left-handed forms of such molecules, called "enantiomers," can exhibit strikingly different properties. For example, one enantiomer of the chiral molecule limonene smells of lemon, the other smells of orange. The ability to observe or even switch the chirality of molecules using terahertz (trillion-cycles-per-second) electromagnetic radiation is a much coveted asset in the world of high technology.
“Natural materials can be induced to change their chirality but the process, which involves structural changes to the material, is weak and slow. With our artificial molecules, we’ve demonstrated strong dynamic chirality switching at light-speed," says Xiang Zhang, one of the leaders of this research and a principal investigator with Berkeley Lab’s Materials Sciences Division.
Working with terahertz (THz) metamaterials engineered from nanometer-sized gold strips with air as the dielectric – Zhang and his colleagues fashioned a delicate artificial chiral molecule which they then incorporated with a photoactive silicon medium. Through photoexcitation of their metamolecules with an external beam of light, the researchers observed handedness flipping in the form of circularly polarized emitted THz light. Furthermore, the photoexcitation enabled this chirality flipping and the circular polarization of THz light to be dynamically controlled.
"In contrast to previous demonstrations where chirality was merely switched on or off in metamaterials using photoelectric stimulation, we used an optical switch to actually reverse the chirality of our THz metamolecules," Zhang says.
Zhang, who holds the Ernest S. Kuh Endowed Chair Professor of Mechanical Engineering at the University of California (UC)† Berkeley, where he also directs the Nano-scale Science and Engineering Center, is one of three corresponding authors of a paper describing this work in Nature. The paper is titled "Photoinduced handedness switching in terahertz chiral metamolecules." The other corresponding authors are Shuang Zhang of the University of Birmingham in the United Kingdom, and Antoinette Taylor of DOE’s Los Alamos National Laboratory.
The optically switchable chiral THz metamolecules consisted of †a pair of 3D meta-atoms of opposite chirality made from precisely structured gold strips. Each meta-atom serves as a resonator with a coupling between electric and magnetic responses that produces strong chirality and large circular dichroism at the resonance frequency.