Chiral Nanostructures Could Be Used to Tailor Applications

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BATH, England, May 3, 2018 — 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.

An impression of a chiral molecule moving through various configurations as it transitions from one handedness to another. Courtesy of Ventsislav Valev and Joel Collins, University of Bath.
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).

Published: May 2018
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
Chirality is a property of certain molecules and objects in which they are non-superimposable on their mirror images. In other words, a chiral object or molecule cannot be exactly superimposed onto its mirror image, much like a left and right hand. The term "chirality" comes from the Greek word cheir, meaning hand, emphasizing the handedness or asymmetry of the object or molecule. A molecule or an object with this property is said to be chiral, while its non-superimposable mirror image is...
Plasmonics is a field of science and technology that focuses on the interaction between electromagnetic radiation and free electrons in a metal or semiconductor at the nanoscale. Specifically, plasmonics deals with the collective oscillations of these free electrons, known as surface plasmons, which can confine and manipulate light on the nanometer scale. Surface plasmons are formed when incident photons couple with the conduction electrons at the interface between a metal or semiconductor...
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