Chiral separation and twin-beam photonics

Abstract

It is well-known that, in a homogeneous fluid medium, most optical means that afford discrimination between molecules of opposite handedness are intrinsically weak effects. The reason is simple: the wide variety of origins for differential response commonly feature real or virtual electronic transitions that break a parity condition. Despite being electric dipole allowed, they manifest the chirality of the material in which they occur by breaking a selection rule that would otherwise preclude the simultaneous involvement of magnetic dipole or electric quadrupole forms of coupling. Although the latter are typically weaker than electric dipole effects by several orders of magnitude, it is the involvement of these weak forms of interaction that are responsible for chiral sensitivity. There have been a number of attempts to cleverly exploit novel optical configurations to enhance the relative magnitude -and hence potentially the efficiency -of chiral discrimination. The prospect of success in any such venture is enticing, because of the huge impact that such an advance might be expected to have in the health, food and medical sectors. Some of these proposals have utilized mirror reflection, and others surface plasmon coupling, or optical binding methods. Several recent works in the literature have drawn attention to a further possibility: the deployment of optical beam interference as a means to achieve chiral separations of sizeable extent. In this paper the underlying theory is fully developed to identify the true scope and limitations of such an approach.

Quantum formulation for nanoscale optical and material chirality: symmetry issues, space and time parity, and observables

“…The same logic applies if we change the handedness of the incident light, in a system comprising only one enantiomer; the radiation tensor changes in odd radiation-parity terms but not the others, and all of the molecular tensors remain unchanged (again, see figure 10). A variety of means by which the associated differences in local force might be used to engineer the separation of molecular enantiomers, and of chiral nanoparticles, have been proposed in recent years [43,[98][99][100][101][102][103][104][105][106][107][108][109][110], and the principles will be further discussed in section 11. Now let us observe how the matrix element feeds into an observable when the initial and final states differ-even if only in the state of the radiation field (as for example occurs in Rayleigh scattering or optical rotation).…”