Researchers Generate New Form of Light

A team composed of researchers from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), the Israel Institute of Technology (Technion), and Technische Universitaet Berlin (TU Berlin) has generated and characterized a new form of light that can clearly identify molecules’ handedness.

The handedness, or chirality, of molecules plays an essential role in chemistry, biology, and drug development: While one type of a chiral molecule might cure a disease, its mirror twin, or enantiomer, could be toxic or even lethal. Opposite chiral molecules look identical, behave identically, and are hard to tell apart unless they interact with another chiral object.

Light can be used to detect chirality. Oscillations in the electromagnetic field draw a chiral helix in space along the direction of light propagation. Depending on whether the helix twirls clockwise or counterclockwise, the light wave is either right- or left-handed and chiral molecules will interact differently with it. However, the helix pitch, set by the light wavelength, is about 1000 times bigger than the size of a molecule. The tiny molecules perceive the light helix as a gigantic circle and hardly feel its chirality.

Synthetic chiral light selectively interacts with one of the two versions of a chiral molecule (left or right). The selected version responds by emitting very bright light, while its “mirror twin” remains dark. Courtesy of Steven Roberts.

To surmount this obstacle and gain control of chiral light-matter interaction, the scientists synthesized a new form of light that draws a chiral structure in time, at every point in space. “The handedness of this new light can be tuned in such a way that one enantiomer will actively interact with it and emit bright light in response, while the opposite enantiomer will not interact with it at all," MBI researcher David Ayuso said.

The scientists described the new chiral light mathematically and tested their model by simulating how it will interact with chiral molecules. They also demonstrated how to create the chiral light in a lab. They fused two converging laser beams that carry lightwaves of two different frequencies. When they tuned the phase shift between the frequencies, the scientists were able to control the handedness of the synthetic chiral light and thus were able to select which type of molecules the light would strongly interact with.

The synthetic chiral light enables full control over the intensity, polarization, and propagation direction of the nonlinear optical response of randomly oriented chiral molecules, the team said. This response can be enhanced or quenched in an enantiomer, opening up efficient ways to control chiral matter and perform ultrafast imaging of chiral dynamics in gases, liquids, and solids. “Synthetic chiral light is described by completely new intrinsic symmetry properties for electromagnetic fields, which is very exciting,” Technion researcher Ofer Neufeld said.

The researchers envision several potential applications for their new method of generating chiral light. For example, synthetic chiral light could allow scientists to monitor chiral chemical reactions in real time or detect a change in the molecules’ handedness. “We also hope to utilize this new approach to spatially separate molecules with the opposite handedness using ultrafast lasers,” said Olga Smirnova, professor at the TU Berlin and head of an MBI theory group.

The research was published in Nature Photonics (https://doi.org/10.1038/s41566-019-0531-2).