About chirality, tunnels and light fields

Will an electron escaping a molecule through a quantum tunnel behave differently depending on whether the molecule is left- or right-handed?

Chemists have borrowed the terms “left-handed” and “right-handed” from anatomy to describe molecules characterized by a certain type of asymmetry. To explore the concept of chirality, look at your hands, palms up. Obviously the two are mirror images of each other. But no matter how hard we try to layer them, they won’t completely overlap. Such objects, termed “chiral,” are found in nature at all scales, from galaxies to molecules.

Every day we experience chirality not only when we grasp an object or put on our shoes, but also when we eat or breathe: our taste and smell can distinguish two mirror images of a chiral molecule. In fact, our bodies are so sensitive to chirality that a molecule can be a drug and its reflection a poison. Chirality is therefore of crucial importance in pharmacology, where 90 percent of the drugs synthesized are chiral compounds.

Chiral molecules have special symmetry properties that make them great candidates for studying fundamental phenomena in physics. Recently, the research teams led by Prof. Yann Mairesse from CNRS / Bordeaux University and Prof. Nirit Dudovich from the Department of Physics of Complex Systems at the Weizmann Institute used chirality to shed new light on one of the most fascinating quantum phenomena: the tunneling process.

Tunneling is a phenomenon in which quantum particles cross seemingly impossible physical barriers. Since this movement is forbidden in classical mechanics, it is very difficult to get an intuitive picture of its dynamics. To create a tunnel in chiral molecules, the researchers exposed them to an intense laser field. “The electrons of the molecules are naturally bound by an energy barrier around the nuclei,” explains Mairesse. “You can think of the electrons as air trapped in an inflatable balloon. The powerful laser fields have the ability to reduce the balloon’s thickness enough to allow some air to tunnel through it, even though the balloon doesn’t have a hole.”

Mairesse, Dudovich and their teams set out to study a still unexplored aspect of tunneling: the moment a chiral molecule encounters a chiral light field and how their brief encounter affects electron tunneling. “We were very excited to explore the link between chirality and tunneling. We really wanted to learn more about what tunneling would look like under these special circumstances,” says Dudovich.

It takes only a few hundred attoseconds for an electron to escape from an atom or molecule. Such tiny time frames characterize many of the processes studied in Mairesse and Dudovich’s laboratories. The two teams asked themselves the following question: How does the chirality of a molecule affect the escape of an electron?

“We used a time-rotating laser field to rotate the barrier around the chiral molecules,” says Mairesse. “To continue with the balloon metaphor, when the laser field rotates horizontally, one expects the air to exit the balloon in the horizontal plane, following the direction of the laser field. What we found is that when the balloon is chiral, the air exits the balloon and flies towards the floor or ceiling depending on the direction of rotation of the laser. In other words, the electrons exit the chiral tunnel with a memory of the direction of rotation of the barrier. This is very similar to the effect of a corkscrew, but on the nanometer and attosecond scale.”

The two teams thus discovered that the probability of an electron tunneling, the phase in which the electron tunnels out and the timing of the tunneling event depend on the chirality of the molecule. These exciting results set the stage for further studies that will exploit the unique symmetry properties of chiral molecules to study the fastest processes involved in light-matter interaction.

The work will be published in the journal Physical Check X.