Otherwise ordinary semiconductor nanocrystals can exhibit strong optical signatures of chirality, as Rukhlenko et al. demonstrate in this interesting theoretical study in Optics Letters. A chiral object is an object which does not coincide with its mirror image; therefore it exists in distinct left-handed and right-handed flavors, called enantiomers. Chiral objects are a topic of considerable curiosity, not only due to the homochiral nature of life’s building blocks, but also because of their unique interaction with light that puts into evidence the nature and consequences of parity symmetry. Optical signatures of chiral objects include their ability to rotate the plane of polarization of light and their differential absorption of left- and right-handed forms of light. These signatures can serve as diagnostic probes of biomacromolecular structure, of the enantiomeric purity of drug molecules, and potentially of living matter in extraterrestrial space. Beyond the carbon-based chirality of organic molecules, researchers have learned to fabricate synthetic inorganic nanostructures that exhibit chirality. Special nano and microscale architectures, such as plasmonic helices and gammadions, are striking examples where strong optical activity has been demonstrated. The findings of Rukhlenko and co-authors are interesting in this context because they show that a special architecture is not a necessity. Rather, ordinary semiconductor nanocrystals can exhibit chirality and optical activity as an outcome of defects naturally present on their typically rough, irregular surfaces. The resulting optical activity can be considerably strong, particularly in nanostructures possessing three-dimensional anisotropy, where calculated g-factors (a parameter that quantifies the chirality) greatly exceed those of chiral molecules (10−4
) and approach the giant g-factors of chiral plasmonic structures up to approximately 0.3). My hope is that experimentalists will uncover these predicted signatures of chirality in spectroscopic studies of semiconductor quantum dots, provided that the inherent challenges associated with nanocrystal-to-nanocrystal surface and structural heterogeneity can be circumvented.
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