The car light reveals a new optical phenomenon

A scientist may want to do cartwheels when making a discovery, but this time the discovery itself is based on cartwheels.

Researchers at Rice University have uncovered details about a novel type of interaction of polarized light matter with light that literally rotates end-to-end when propagated from a source. Their finding could help study molecules like those of light-collecting antennas that are expected to have a unique sensitivity to the phenomenon.

The researchers observed the effect they call trochoidal dichroism on the light scattered by two coupled dipole scatterers, in this case a pair of closely spaced plasmonic metal nanorods, when they were excited by the light from the car.

The light polarization the researchers used is fundamentally different from the linear polarization that makes sunglasses work and the circularly polarized corkscrew light used in circular dichroism to study the conformation of proteins and other small molecules.

Instead of taking a helical shape, the light field is flat as it moves, rotating clockwise or counterclockwise, away from the source like a hula hoop. This type of light polarization, called trochoidal polarization, has been observed in the past, said Rice graduate student and lead author Lauren McCarthy, but no one knew that plasmonic nanoparticles could be used to see how it coiled.

“We now know how trochoidal polarizations relate to existing light matter interactions,” he said. “There is a difference between understanding light and its physical properties and understanding the influence of light on matter. Differential interaction with matter, based on the geometry of the material, is the new piece here.”

The discovery by the rice laboratory of the chemist Stephan Link is detailed in the procedures of the National Academy of Sciences.

The researchers were not specifically looking for trochoidal dichroism. They generated an evanescent field in a technique they developed to study chiral gold nanoparticles to see how left and right circularly polarized light interacts with matter. Freely propagating circularly polarized light interactions are key to several technologies, including 3D glasses made of materials that discriminate between opposite light polarizations, but are not so well understood when light is limited to small spaces at interfaces .

Instead of the circularly polarized light previously used, the authors changed the polarization of the incident light used to generate an evanescent field with tumbling waves. The researchers found that the trochoidal polarizations clockwise and counterclockwise interacted differently with pairs of plasmonic nanorods oriented 90 degrees from each other. Specifically, the wavelengths of light scattered by the nanorod pairs changed when the trochoidal polarization changed from clockwise to counterclockwise, which is a characteristic of dichroism.

“Trochoidal waves have been discussed and different groups have investigated their properties and applications,” said McCarthy. “However, to the best of our knowledge, no one has observed that the geometry of a material can allow differential interactions with trochoidal waves counterclockwise versus clockwise.”

Molecules interact with light through their electric and magnetic dipoles. The researchers observed that molecules with electric and magnetic dipoles that are perpendicular to each other, as with 90-degree nanoparticles, have a charge motion that rotates in the plane when excited. Trochoidal dichroism could be used to determine the direction of this rotation, which would reveal molecular orientation.

The exciting self-assembling gold nanorod dimers also revealed subtle trochoidal dichroism effects, showing that the phenomenon is not limited to strictly manufactured nanoparticles arranged at 90 degrees.

“Having worked with polarized light interacting with plasmonic nanostructures for a long time, the current discovery is certainly special in several ways,” Link said. “Finding a new form of polarized light matter interaction is exciting in itself. However, the discovery process was equally rewarding as Lauren and my former student, Kyle Smith, pushed me to follow their results. In the end it was a true team effort from all the co-authors, of which I am very proud. ”