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The interaction of light and chiral matter is subject of recent research both in fundamental science and applications.
Among these are the helicity of electromagnetic fields described with the optical chirality density and emitters sensitive to
circular polarization employed in quantum communications.
In the weak coupling regime of chiral emitters, we analyze the conversion of chirality which can be regarded as an analogue
to absorption of energy describing the change of circular polarization of the incident field. This enables the tailoring of
chiral near-fields close to metamaterials, e.g. composed of gold helices, and gives insights into extinction measurements
such as circular dichroism.
We show relation of the weak and strong coupling regime. The latter can be modelled with cross electric-magnetic polarizabilities or
with effective chiral materials, i.e. bi-anisotropic media. Accordingly, we motivate the necessity for rigorous numerical
simulations to accurately describe chiral light-matter interaction.
The helicity of light is of great interest
in both fundamental research and in applications such as dichroism spectroscopy. Its
time-harmonic formulation is directly proportional to the density of optical chirality.
Recently, both an helicity optical theorem
(HOT) and a chirality conservation law
(CCL) have been formulated for arbitrary
scatterers taking into account an underlying
continuity equation of this quantity. We
summarize these two equivalent fundamental laws and analyze their potential applications.
The introduction of the near-field quantity of optical chirality has emerged in various numerical and few experimental studies of local chirality enhancement due to its relation to the excitation rate of chiral molecules. This time-even pseudoscalar has been dismissed as being a higher-order version of helicity. Nevertheless, we revisit the derivation of the underlying conservation law and define optical chirality in media similar to. We identify the mechanism of chirality conversion by either inhomogeneous or anisotropic space to complement the conservation of optical chirality.
The conservation law of optical chirality in arbitrary space enables the extension of the concept of polarization to the near-field where no distiniguished propagation direction of light is present. We show that the connection of electromagnetic energy and optical chirality provide the ability to define a circular polarization basis in time-harmonic near-field analysis.
In order to illustrate our theory, we present electromagnetic field simulations of simple as well as more complex nanostructures. Results using the well-known far-field polarization concept are readily reproduced and extended from the point of view of chirality conversion.
A chiral structure is not super-imposable with its mirror image. Most commonly found in organic molecules, chirality can also occur in other systems, such as electromagnetic fields, where circularly polarized light is the most widespread example. Chiral electromagnetic fields can be a useful tool for biosensing applications. In particular, it has been shown that chiral plasmonic nanostructures have the ability to produce strongly enhanced chiral near-fields. Recently, our group has developed chiral plasmonic nanopyramids, which have the ability to focus chiral near-fields at their tip. This could enable chiral sensing at the single-molecule level. Chiral near-fields can be characterized in terms of the “optical chirality density”. This time-even and parity-odd pseudoscalar was first derived by Lipkin and was found to follow a conservation law analogous to the energy conservation of electromagnetic fields. More recently, Tang and Cohen identified the physical meaning of the “optical chirality density” as the degree of asymmetry in the excitation rate of a chiral molecule. However, how this near-field interpretation of the optical chirality could translate into the far-field is not well understood. Here, we formulate a far-field interpretation by investigating the conservation law for optical chirality in matter, and performing time-averaging in analogy to Poynting’s Theorem. In parallel to extinction energy, we define the “global chirality” as the sum of chirality dissipation within a material and the chirality flux leaving the system. With finite-element simulations, we place a dipole source at locations of enhanced local chirality and investigate the global chirality and ellipticity of emitted light in the far-field. Interestingly, we find that lossy materials with a complex dielectric function have the ability to generate global chirality when excited by achiral light. In particular, chiral plasmonic nanostructures are found to act as effective global chirality generators. The global interpretation of optical chirality provides a useful tool for biosensing applications with chiral plasmonic nanostructures, where the detection is routinely performed in the far-field.