Metamolecules Change Chirality Under Light

For the first time, the chirality of artificial molecules has been switched from a right-handed to a left-handed orientation using a light beam. The discovery holds potential for a range of terahertz technology applications, including biomedical research, ultrahigh-speed communications and homeland security.

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.

The ability to observe or switch a molecule’s chirality using terahertz electromagnetic radiation is a coveted asset in 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, 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 (Berkeley Lab’s) Materials Sciences Div. “With our artificial molecules, we’ve demonstrated strong dynamic chirality switching at light-speed.”

The top shows a scanning electron microscopy image of optically switchable chiral THz metamolecules. The bottom shows the purple, blue and tan colors representing the gold meta-atom structures at different layers; two silicon pads are shown in green. (Images: Xiang Zhang et al)

Using THz metamaterials engineered from nanometer-size gold strips with air as the dielectric, Zhang and a multi-institutional team of colleagues 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 THz light.

“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 terahertz metamolecules,” Zhang said.

The optically switchable chiral THz 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 assembled to form a metamolecule, the mirror symmetry is preserved, resulting in the vanishing of optical activity,” he said. The optical activity that arises from the opposite meta-atoms cancels one another out.

The researchers introduced silicon pads to each chiral meta-atom in the metamolecule, but at different locations. 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, which also functioned as the optoelectronic switches that flipped the chirality of the metamolecule under photoexcitation, broke the mirror symmetry and induced chirality for the combined metamolecule.

THZ electromagnetic radiation, also known as T-rays, 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 THz light, scientists could use the technology to detect toxic or explosive chemicals, or use it in high-speed data processing systems and wireless communication.

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.

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

“The observed giant switchable chirality we can engineer into our metamaterials provides a viable approach toward creating high-performance polarimetric devices that are largely not available at terahertz frequencies,” said corresponding author Antoinette Taylor. “This frequency range is particularly interesting because it uniquely reveals information about physical phenomena, such as the interactions between or within biologically relevant molecules, and may enable control of electronic states in novel material systems, such as cyclotron resonances in graphene and topological insulators.”

The design principle for optically switchable chiral THz metamolecules is not limited to just handedness switching, Taylor said. It also could be applied to the dynamic reversing of other electromagnetic properties.

The findings were published in Nature Communications.

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