The role of virtual photons in nanoscale photonics

Abstract

The fundamental theory of processes and properties associated with nanoscale photonics should properly account for the quantum nature of both the matter and the radiation field. A familiar example is the Casimir force, whose significant role in nanoelectromechanical systems is widely recognised; the correct representation invokes the creation of short-lived virtual photons from the vacuum. In fact, there is an extensive range of nanophotonic interactions in which virtual photon exchange plays a vital role, mediating the coupling between particles. This review surveys recent theory and applications, also exhibiting novel insights into key electrodynamic mechanisms. Examples are numerous and include: laser-induced inter-particle forces known as optical binding; non-parametric frequency-conversion processes especially in rare-earth doped materials; light-harvesting polymer materials that involve electronic energy transfer between their constituent chromophores. An assessment of these and the latest prospective applications concludes with a view on future directions of research.

Quantum delocalization in photon-pair generation

“…First, however, we note that few previous works have attempted to tackle, at quantum electrodynamical level, the multicenter sums required for this kind of analysis. By far the largest body of such calculations is limited to two-center interactions, as featured extensively in works by Power [86], Craig and Thirunamachandran [44], Andrews and Bradshaw [42], and Salam [45] amongst others. These are generally calculations that concern the energies, forces, and spectroscopic transitions associated with specifically pairwise interactions.…”

“…Moreover, allowing for each photon of the correlated pair to emerge from spatially distinct (and separated) points in space introduces a positional uncertainty of a fundamentally quantum origin. In the following analysis, utilizing a quantum electrodynamical (QED) approach cast in terms of virtual photon coupling [42], we fully account for both the localized and nonlocalized generation of correlated photon pairs. The results, cast in the form of quantum amplitudes, are further developed for computational implementation, which we then use to quantify the net effect of SPDC nonlocalization on the total rate of pair production, within a model lattice structure.…”

Raman scattering mediated by neighboring molecules

“…For example, a transition between states of formally opposite parity, gerade-ungerade, may become allowed in a centrosymmetric molecule due to the symmetry-lowering presence of a nearby molecule. The precise form of electronic coupling between centers that has the capacity to accomplish such an effect can be accommodated by considering one or more virtual photon exchanges, [20][21][22] which thereby link the evolution in quantum state for the two particles. Here, we consider both single and double exchanges, and all associated quantum interferences that can lead to modifications of the rules governing fundamental single-center Raman transitions.…”

Symmetry analysis of Raman scattering mediated by neighboring molecules

“…The origin of this effect lies in twocenter near-field coupling that involves the scattering of light by the molecular pair with a single or double virtual photon exchange between them. This form of interaction is responsible for a wide range of other effects in the field of chemical physics, 17 most notably resonant energy transfer (where the input photon annihilation and output creation events are necessarily on different sites). A similar type of coupling can be exploited to enhance second order hyperpolarizabilities, in which connection such an interaction is referred to as cascading.…”