Light with Intrinsic 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. (Source: S. Roberts)

Light is the fastest way to distinguish right- and left-handed chiral molecules, which has important appli­cations in chemistry and biology. However, ordinary light only weakly senses molecular handed­ness. Researchers from the Max Born Institute for Nonlinear Optics and Short Pulse Spectro­scopy MBI,the Israel Institute of Technology (Technion) and Technische Univer­sität Berlin have now shown how to generate and charac­terize an entirely new type of light, synthetic chiral light, which identi­fies molecules’ handed­ness excep­tionally distinctly.

Like our left and right hands, some molecules in nature also have mirror twins. However, while these twin molecules may look similar, some of their properties can be very different. For instance, the handed­ness – or chirality – of molecules plays an essential role in chemistry, biology, and drug develop­ment: while one type of a molecule can cure a disease, its mirror twin – or enantiomer – may be toxic or even lethal.

It is extremely hard to tell opposite chiral molecules apart because they look identical and behave identically, unless they interact with another chiral object. Light has long been the perfect candidate: oscillations of the electro­magnetic field draw a chiral helix in space, along the light propa­gation direction. Depending on whether the helix twirls clockwise or counter­clockwise, the light wave is either right- or left-handed. Chiral molecules can interact dif­ferently with it. However, the helix pitch, set by the light wavelength, is about a thousand times bigger than the size of a molecule. So, the tiny molecules perceive the light helix rather as a gigantic circle, hardly feeling its chirality at all.

An inno­vative way around this problem, is to synthesize a wholly new type of chiral light – one that draws a chiral structure in time, at every single 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,” explains MBI researcher David Ayuso. The scientists described this new chiral light mathe­matically and tested their model by simulating how it interacts with chiral molecules. Further­more, they showed how to create such light in a lab: fusing two conver­ging laser beams that carry light waves of two different frequencies. By tuning the phase shift between the different frequencies, scientists can control the handed­ness of this synthetic chiral light and thus select with which type of molecules it will strongly interact.

“Synthetic chiral light is described by completely new intrinsic symmetry properties for electro­magnetic fields, which is very exci­ting”, says Ofer Neufeld, a PhD student in the Technion’s Physics Department. The researchers foresee a variety of potential appli­cations of the new method in chemistry and biology. For example, synthetic chiral light could allow one to monitor chiral chemical reactions in real-time or detect the switch in the molecules’ handed­ness. “We also hope to utilize this new approach to spatially separate molecules with the opposite handed­ness using ultra­fast lasers,” concludes Olga Smir­nova, professor at the TU Berlin and head of an MBI Theory group. (Source: FVB / MBI)

Reference: D. Ayuso et al.: Synthetic chiral light for efficient control of chiral light–matter interaction, Nat. Phot., online 18 October 2019; DOI: 10.1038/s41566-019-0531-2

Link: Fundamentals of Extreme Photonics (O. Smirnova), Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy MBI, Berlin, Germany

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