Scientists develop optically switchable chiral terahertz metamolecules

Los Alamos, NM--Scientists with Los Alamos National Laboratory and a multi-institutional team of researchers have created the first artificial molecules—metamolecules—whose chirality can be rapidly switched from a right-handed to a left-handed orientation when illuminated with light. The researchers say that these switchable molecules hold potentially huge possibilities for the application of terahertz technologies across a wide range of fields, including biomedical research, homeland security, and ultrahigh-speed communications.

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. "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. Antoinette Taylor of Los Alamos and her co-authors say that the general design principle of their optically switchable chiral terahertz metamolecules is not limited to handedness switching but could also be applied to the dynamic reversing of other electromagnetic properties.

Working with terahertz metamaterials engineered from nanometer-sized gold strips with air as the dielectric, the team fashioned a delicate artificial chiral molecule that 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 terahertz light. Furthermore, the photoexcitation enabled this chirality flipping and the circular polarization of the terahertz light to be dynamically controlled.

The Nature Communications paper is entitled "Photoinduced handedness switching in terahertz chiral metamolecules." 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. Silicon pads were introduced to each chiral meta-atom in the metamolecule but at different locations. In one meta-atom, the silicon pad bridged two gold strips, and in the other meta-atom, the silicon pad replaced part of a gold strip. The silicon pads broke the mirror symmetry and induced chirality for the combined metamolecule. The pads also functioned as the optoelectronic switches that flipped the chirality of the metamolecule under photoexcitation.

Terahertz radiation is an ideal none-invasive tool for analyzing the chemical constituents of organic and non-organic materials. Being able to flip the handedness of chiral metamolecules and control the circular polarization of terahertz light could be used to detect toxic and explosive chemicals, or for wireless communication and high-speed data processing systems.

Most biological molecules are chiral, including DNA, RNA and proteins, so terahertz-based polarimetric devices should also benefit medical researchers and developers of pharmaceutical drugs among others. "The switchable chirality we can engineer into our metamaterials provides a viable approach towards creating high performance polarimetric devices that are largely not available at terahertz frequencies,” says 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. It may enable control of electronic states in novel material systems, such as cyclotron resonances in graphene and topological insulators."

SOURCE: Los Alamos National Laboratory; www.lanl.gov/news/releases/artificial-molecules-that-switch-handedness-at-light-speed.html