Physicists in the UK have developed a new and highly efficient way of detecting the chirality or “handedness” of molecules using tailored laser fields. The technique could improve the process of drug development in medicine and might also become the basis for compact devices that rapidly separate left- and right-handed molecules.

“Chirality is a universal type of asymmetry that arises naturally in many areas of science,” explains David Ayuso, a physicist at Imperial College London who led the research. “In general, an object is chiral when it is different from its mirror image – with our hands being the typical example.”

Just as a chiral glove would either fit a left hand or a right hand, but not both, the two superimposable, mirror-reflected versions of a chiral molecule (called enantiomers) behave very differently when they interact with another chiral entity such as circularly polarized light or a different chiral molecule. Since most biomolecules are chiral, and some of these different behaviours have detrimental or even toxic effects, reliable methods of detecting, quantifying and manipulating molecular chirality are highly desirable in the context of biochemistry and pharmaceuticals.

A fundamental limitation to overcome

Conventional optical methods for measuring chirality in matter rely on the helical – and thus chiral – pattern that the polarization of circularly polarized light “draws” as the light propagates through space. The problem is that the pitch of this helix is determined by the light’s wavelength, which is orders of magnitude longer than the molecules that interact with it. Consequently, the molecules “see” the helix as a simple planar circle – a non-chiral structure. As a result, the light-molecule interaction is only very weakly sensitive to the molecule’s handedness, with sensitivities usually below 0.1%.

Ayuso and colleagues have been working on several approaches over the last few years to overcome this fundamental limitation. “The key is to stop relying on the chiral helix that circularly polarized light draws in space, but instead try and create new optical fields in which the tip of the electric field vector (or the field’s ‘arrow’) of the laser creates a chiral structure in time,” says Ayuso. “In this way we create light that encodes chirality in time rather than space. The molecules therefore no longer perceive a circle but rather a chiral temporal structure.”

Light that encodes chirality in time is efficient at driving chiral electronic currents inside the molecules, he tells Physics World. These currents then interact with the molecules’ chiral structures (their natural “corkscrew”) in a way that is highly enantio-sensitive – that is, strongly influenced by the molecule’s handedness. “In this way, we can ‘force’ one of the two versions of a chiral molecule, for example, the left-handed one, to emit bright light at new optical frequencies, while the right-handed one remains dark,” he explains. “This allows us to detect the chirality of a molecule with 100% efficiency.”

Devices for efficient chiral recognition

According to the researchers, being able to drive strongly enantio-sensitive signals with lasers could aid the development of compact devices that recognize and manipulate chiral molecules in a highly efficient way. One example might be a pair of chiral optical tweezers that distinguishes between left-and right-handed molecules in space.

Helical light tells chiral molecules apart

The Imperial College team is now collaborating with experimentalist colleagues Mary Matthews and Jon Marangos to develop the technique further. As part of this effort, a PhD student in the team, Rose Piccuito, is applying “the unique laser capabilities available at Imperial” to the task, Ayuso says. “In her experiments, she uses femtosecond laser pulses (that is, ultrashort pulses that last just around 10^-15 seconds). The goal here is to image the chiral molecules not only with extremely high chiral sensitivity, but also at the natural timescales of electronic motion,” he explains.

Another interesting direction, he adds, would be to bring these ideas to the nanoworld. “We want to take advantage of the unique opportunities enabled by nanophotonic technology to shape light for highly enantio-sensitive imaging and manipulation of chiral molecules.”

The method is described in Science Advances.

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• Source: https://physicsworld.com/a/chiral-detection-of-molecules-gets-a-laser-driven-boost/