Metamolecules switch handedness under light

Ashley N. Paddock, ashley.paddock@photonics.com

A new technique that uses light to change the “handedness” of artificial molecules could benefit terahertz technology applications from biomedical research to ultrahigh-speed communications and homeland security.

Using a light beam, the chirality of artificial molecules has been switched from a right-handed orientation to a left-handed one for the first time. Chirality is the distinct left or right orientation, or handedness, of some types of molecules – meaning it can take one of two mirror-image forms. Called enantiomers, the right- and left-handed forms of such molecules can exhibit strikingly different properties; for example, one enantiomer of the chiral molecule limonene has a lemon scent, while the other smells of orange.

Controlling the chirality of artificial molecules could enable advances in communications and biomedical imaging. Top, a scanning electron microscope image of optically switchable chiral terahertz metamolecules. Bottom, the purple, blue and tan colors represent the gold meta-atom structures at different layers; two silicon pads are shown in green. Images courtesy of Xiang Zhang et al, Berkeley Lab.

The ability to observe or switch a molecule’s chirality using terahertz electromagnetic radiation is a coveted asset in high technology.

“In electromagnetism, chirality or optical activity arises from the coupling between the electric and magnetic responses of the materials,” said Xiang Zhang, one of the leaders of the research and a principal investigator with the US Department of Energy’s Lawrence Berkeley National Laboratory’s Materials Sciences Div. “However, in natural materials, the magnetic response is extremely weak at THz and optical frequencies, and as a result, the chirality is also very weak.”

Using terahertz metamaterials engineered from nanometer-size gold strips with air as the dielectric, Zhang and a multi-institutional team of colleagues from Los Alamos National Lab and the University of Birmingham in the UK fashioned a delicate artificial chiral molecule that they incorporated with a photoactive silicon medium. By performing photoexcitation of their metamolecules with an external light beam, they observed dynamically controlled handedness flipping in the form of circularly polarized emitted terahertz light.

In this schematic, the chirality-switching metamolecule consists of four chiral resonators with fourfold rotational symmetry. An external beam of light instantly reverses the metamolecule’s chirality from right-handed to left-handed.

“Under strong optical irradiation, the handedness of the metamolecule is switched to its opposite handedness,” Zhang told BioPhotonics. “This state is temporary; it relaxes back to its original handedness in a time scale of 1 millisecond.” The process is repeatable.

The optically switchable chiral terahertz metamolecules consisted of a pair of 3-D 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.

When two chiral meta-atoms of the same shape but opposite chirality are put together, they form a metamolecule, and their symmetry is preserved, resulting in vanishing optical activity. Essentially, the optical activity that arises from the opposite meta-atoms cancels each other out, he said.

To break the mirror symmetry and induce chirality for the combined metamolecule, the researchers introduced silicon pads to each chiral meta-atom in the metamolecule. In one meta-atom, the silicon pad bridged two gold strips, while the silicon pad replaced part of the gold strip in the other meta-atom. The silicon pads functioned as the optoelectronic switches that flipped the chirality of the metamolecule under photoexcitation.

Terahertz electromagnetic radiation falls within the frequency range of molecular vibrations, making it a suitable noninvasive tool for analyzing the chemical constituents of organic and nonorganic materials. By having the ability to flip the handedness of metamolecules and control the circular polarization of terahertz light, scientists could use the technology to detect toxic or explosive chemicals, or use it in high-speed data processing systems and wireless communications.

Terahertz-based polarimetric devices also could benefit medical researchers and developers of pharmaceutical drugs because most biological molecules, including DNA, RNA and proteins, are chiral.

“In THz, most of the biological molecules show circular dichroism,” Zhang said. “However, there is a lack of spectroscopy tools to accurately measure the circular dichroism of biomolecules at THz in comparison with the visible range. The sensitive detection of circular dichroism requires dynamic modulation of the electromagnetic waves between the two circular polarizations. The chirality-switching metamaterial we demonstrated may bridge this gap.”

Their design principle for optically switchable chiral terahertz metamolecules is not limited to just handedness switching; it also could be applied to dynamic reversing of other electromagnetic properties.

“Dynamically reversing other electromagnetic properties would enable us complete control of electromagnetic waves, not only in polarization, but also in phase, intensity and propagation directions,” Zhang said. “For example, we can use a similar design principle to make a meta-surface with dynamically switchable high and low impedance. For a THz wave reflected by the meta-surface, the phase can be dynamically switched between 0 and 180 degrees.

“The metamaterials we demonstrated so far still have some drawbacks: Chirality is not strong enough to completely convert the THz waves into purely circular polarizations. We will work on the perfection of the metamaterial design to achieve a stronger chirality switching effect.”

The work was published in Nature Communications (doi:10.1038/ncomms 1908).