Universal Quasi-Static Limit for Plasmon-Enhanced Optical Chirality

Chirality is a fundamental feature in life sciences and biochemistry, since all biosystems are composed by molecules such as sugars and amino-acids displaying only one type of chirality. A broad and multidisciplinary scientific community is therefore dedicating increasing efforts to developing new strategies for the characterization of the chiral properties of organic molecules. Among the most widespread techniques, chiroptical methods exploit small differences in the response of a chiral material interacting with either left- or right-circularly-polarized light.

It has been recently demonstrated [1,2] that it is possible to create ‘superchiral’ optical fields displaying a much larger chiral asymmetry than circularly polarized plane waves. Such superchiral fields can be employed to considerably enhance the chiroptical sensitivity. Starting from this seminal work, the scientific community is trying to realize superchiral fields with nanostructures, in order to exploit the large local fields that can be achieved in such environments [3,4].

We analytically demonstrate that the maximum average optical chirality attainable in proximity of plasmonic nanostructures encounters a fundamental limitation, which originates from the quasi static-like nature of the electromagnetic fields around a metal nanoparticle and from the fact that the magnetic response of matter at optical frequencies is much weaker than the electric one. In practice, the spatial average of the optical chirality over a domain (surface or volume) wrapping the nanoparticle cannot be larger than that of circularly polarized plane waves. Systematic numerical simulations of different nanoparticle geometries displaying either electric or magnetic localized plasmon resonances fully confirm our analytical finding, providing important guidelines to design new assets for the characterization of the chiroptical properties of organic materials in very small detection volumes.

[1] Y. Tang and A. E. Cohen, Phys. Rev. Lett. 104, 163901(2010).

[2] Y. Tang and A. E. Cohen, Science 332, 333 (2011).

[3] M. Schäferling, D. Dregely, M. Hentschel, and H. Giessen, Phys. Rev. X “, 031010 (2012).

[4] A. García-Etxarri and J. A. Dionne, Phys. Rev. B 87, 235409 (2013).

paolo.biagioni@polimi.it