Chiral Nanostructures Could Be Used to Tailor Applications

Scientists have modeled the interaction between light and twisted molecules as the molecules transition from left- to right-handed versions, or vice versa. Understanding the behaviors of these transitional forms could lead to improved design of telecommunications components.

Previously it had only been possible to study either the left- or right-handed chiral form but nothing in between. The ability to morph a molecule from one handedness to the other would allow researchers to observe how the effects of this change translate into changes in the molecule’s physical properties.

A chiral molecule moving through various configurations as it transitions from one handedness to another. Courtesy of Ventsislav Valev and Joel Collins.

Researchers from the University of Bath, in conjunction with University College London, built “artificial molecules” representing 35 stages along the way to a geometric transformation from one handedness to the other. The shape of the artificial molecule affected its optical properties at the nanoscale.

Researchers used twisted laser light to study the optical properties at each stage, as the artificial molecules morphed from left- to right-handedness.

“We were able to follow the properties of a chiral artificial molecule . . . through two different routes. No one has done this before. Surprisingly, we found that each route leads to a different behavior,” said researcher Joel Collins.

“We measured the difference in absorption of left and right circularly polarized light, known as circular-dichroism (CD). Along one route, the artificial molecules behave as might be expected, with progressively decreasing CD, and eventually a reversal of the CD, for the mirrored structure. However, along the second route, the CD reversed several times, even before the structure changed handedness,” Collins said.

Nonlinear multiphoton microscopy was used to demonstrate the new kind of double-bisignate CD caused by the enantiomorphing

Because of the lack of mirror symmetry, chiral nanostructures allow twisted electric field “hotspots” to form at the material surface. These hotspots depend strongly on the optical wavelength and nanostructure geometry. Researchers say that understanding the properties of these chiral hotspots is crucial for their applications, such as in enhancing the optical interactions with chiral molecules.

“In chemistry, you can’t tune the twist of a chiral molecule, so every scientist who studies such molecules needs to tune the wavelength of light," researcher Ventsislav Valev said. "We have demonstrated a new, complementary physical effect, where we fix the wavelength and tune the twist of the chiral artificial molecule. In many cases, our approach is more practical; for instance, when we’re designing telecoms components, where the optical wavelength is predetermined.”

Plasmonic nanostructures have demonstrated the ability to control light in ways never observed in nature because the optical response is closely linked to their flexible geometric design. The analysis of a chiral molecule as it transitions from one handedness to another could further the optimization of plasmonic chiroptical materials.

The research was published in Advanced Optical Materials (doi:10.1002/adom.201800153).