Abstract
Electromagnetic fields with strong optical chirality can be formed in the near field of chiral plasmonic nanostructures. We calculate and visualize the degree of chirality to identify regions with relatively high values. This analysis leads to design principles for a simple utilization of chiral fields. We investigate planar geometries, which offer a convenient way to access the designated fields, as well as three-dimensional nanostructures, which show a very high local optical chirality.
4 More- Received 24 August 2011
DOI:https://doi.org/10.1103/PhysRevX.2.031010
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Published by the American Physical Society
Popular Summary
Many molecules encountered in chemistry and biology are “chiral,” in that they come in two structural enantiomers: the left- and the right-handed. Each is the mirror image of the other but cannot be superimposed onto its own mirror image through any rotation. The two enantiomers may taste different, as in the case of sugars. And they may behave differently, as in the case of chiral drugs interacting with the human body. Detecting the handedness of a chiral molecule is, therefore, crucial to understanding and controlling many biological or chemical processes.
The method that is currently used most commonly explores the fortunate fact that the two versions of a chiral molecule interact differently with circularly polarized light. But there is still a fundamental snag: The differences are usually small, and they need to be amplified for detection. A recent advent in chiral-molecule detection put forward a new concept of amplifying the differences through enhanced optical chirality: Nanoscaled metallic structures, onto which molecules are adsorbed or attached, are used to turn spatially uniform incident probing light into “superchiral” hot spots, where the light is either right- or left-circularly polarized and with enhanced intensity. How should one design structures for such tasks? In this paper, we answer this important open question.
There are a number of goals for a good design. Not only are high values of optical chirality needed, but large, continuous “hot“ regions with the enhanced optical chirality that are easily accessible for the chiral species are also most desirable. Easier fabrication is yet another goal. We have investigated different designs and compared their usefulness for practical chiral applications. We’ve discovered a number of basic design rules. For example, nanoscale elements with strong twist but without sharp corners lead to the best results, and three-dimensional chiral elements generate notably stronger optical chirality than two-dimensional ones. A superstructure that is composed of both handed species of a chiral metallic element should be used for simultaneous sensing of both species of a chiral molecule.
We believe that our investigation provides timely and vital insights needed for the further development of the new chirality-sensing approach.