Physicists Use Twisted Laser Light to Tell Left-Handed Molecules From Right-Handed Ones

Physicists in India have found a way to distinguish mirror-image molecules by firing laser light that both spins and twists as it travels, producing a signal far larger than the delicate optical measurements chemists currently depend on.

The problem the work addresses is old and expensive. Many molecules come in two forms that are identical in every respect except that each is the mirror image of the other, the way a left hand mirrors a right. Chemists call the property chirality, and the two versions enantiomers. They have the same mass, the same boiling point and the same spectrum, but biology treats them as entirely different substances, because the receptors they bind to are themselves handed. "A right-handed screw fits a right-handed thread, while a left-handed screw does not," as the researchers put it. In pharmaceuticals the distinction can be the difference between a drug and a toxin, which is why regulators require manufacturers to prove which hand they are selling.

The team, from the Tata Institute of Fundamental Research, the Indian Institute of Technology Bombay and the Indian Institute of Technology Hyderabad, engineered laser beams carrying both spin and orbital angular momentum — light that not only rotates but corkscrews forward along its own axis. That gives the beam a handedness of its own, a thread that can match or clash with the molecule it hits.

Working at TIFR Hyderabad's laser facility, the researchers directed ultrashort pulses of this twisted light into gaseous camphor, a naturally chiral molecule, and blew the molecules apart. A time-of-flight mass spectrometer counted the charged fragments. The number of fragments depended on whether the twist of the light matched the handedness of the molecule — a straightforward count, not a subtle rotation of a polarization plane measured to fractions of a degree.

That is the practical advance. Conventional techniques such as circular dichroism rely on tiny differences in how the two forms absorb polarized light, differences so small that they demand long integration times, high sample purity and carefully controlled geometry. The new approach produces a larger signal and, the authors report, reduces the need for complicated angular measurements and specialized equipment.

The work appears in Science Advances, with Haritha Venugopal, Parishkrith Aravind and Angitha Sajeevan among the lead authors. Camphor is a well-characterized test case rather than a commercially interesting target, and the method still has to be demonstrated on the larger, floppier molecules that dominate drug development, where fragmentation patterns get considerably messier. But structured light has moved in a few years from a curiosity of optics to a working laboratory tool, and this is one of the clearer demonstrations of what it buys.