David S. Bradshaw

The transport of electronic excitation energy (EET) between ions, atoms, molecules or chromophores is an important process that occurs in a wide range of physical systems. The tantalising prospect of effective experimental control over such transfer is, in principle, amenable to a variety of different kinds of approach. Several of the most promising, which are analysed and compared in this paper, involve the influence of externally applied static electric or electromagnetic fields, or the exploitation of local media effects. A quantum electrodynamical framework is used as a common basis to describe the corresponding mechanisms, illustrated by specially adapted Feynman diagrams. It becomes evident that energy transfer between polar species engages an additional pairwise interaction beyond the EET coupling. Such an effect may also play an important role in interatomic Coulombic decay (ICD), a process that has recently attracted fresh interest. The control of ICD, in which the photoionisation of two nearby atoms via energy transfer, is determined to have analogous characteristics to conventional forms of EET.

One of the key frameworks for developing the theory of light–matter interactions in modern optics and photonics is quantum electrodynamics (QED). Contrasting with semiclassical theory, which depicts electromagnetic radiation as a classical wave, QED representations of quantized light fully embrace the concept of the photon. This tutorial review is a broad guide to cutting-edge applications of QED, providing an outline of its underlying foundation and an examination of its role in photon science. Alongside the full quantum methods, it is shown how significant distinctions can be drawn when compared to semiclassical approaches. Clear advantages in outcome arise in the predictive capacity and physical insights afforded by QED methods, which favors its adoption over other formulations of radiation–matter interaction.

In reporting and explaining the interactions of light with atoms and molecules, a photon-based description is not only appropriate but clearly essential when electronic or other kinds of quantum transition ensue. However, textbook treatments frequently go no further than the Planck relation, in representing the quantum nature of the light itself—often resorting to classical principles when discussing mechanism. Complete consistency and rigour can be achieved by treating both the matter and the radiation in a fully quantised form, which requires the electromagnetic fields to be cast in a quantum field representation. It proves possible to develop a rigorous approach to this theory that is simple to convey and apply, and which lends itself to a significantly enhanced level of appreciation of mechanism. This paper lays a concise foundation and exemplifies the application in three specific cases: absorption, emission and scattering. It is also shown how this formulation affords a basis for applications in higher-order, multiphoton and nonlinear optical processes.

The theory of non-resonant optical processes with intrinsic optical nonlinearity, such as harmonic generation, has been widely understood since the advent of the laser. In general, such effects involve multiphoton interactions that change the population of each input optical mode or modes. However, nonlinear effects can also arise through the input of an off-resonant laser beam that itself emerges unchanged. Many such effects have been largely overlooked. Using a quantum electrodynamical framework, this review provides detail on such optically nonlinear mechanisms that allow for a controlled increase or decrease in the intensity of linear absorption and fluorescence and in the efficiency of resonance energy transfer. The rate modifications responsible for these effects were achieved by the simultaneous application of an off-resonant beam with a moderate intensity, acting in a sense as an optical catalyst, conferring a new dimension of optical nonlinearity upon photoactive materials. It is shown that, in certain configurations, these mechanisms provide the basis for all-optical switching, i.e., the control of light-by-light, including an optical transistor scheme. The conclusion outlines other recently proposed all-optical switching systems.

Electronically excited molecules frequently exhibit two distinctive decay mechanisms that rapidly generate optical emission: one is direct fluorescence and the other is energy transfer to a neighboring component. In the latter, the process leading to the ensuing “indirect” fluorescence is known as FRET, or fluorescence resonance energy transfer. For chiral molecules, both fluorescence and FRET exhibit discriminatory behavior with respect to optical and material handedness. While chiral effects such as circular dichroism are well known, as too is chiral discrimination for FRET in isolation, this article presents a study on a stepwise mechanism that involves both. Chirally sensitive processes follow excitation through the absorption of circularly polarized light and are manifest in either direct or indirect fluorescence. Following recent studies setting down the symmetry principles, this analysis provides a rigorous, quantum outlook that complements and expands on these works. Circumventing expressions that contain complicated tensorial components, our results are amenable for determining representative numerical values for the relative importance of the various coupling processes. We discover that circular dichroism exerts a major influence on both fluorescence and FRET, and resolving the engagement of chirality in each component reveals the distinct roles of absorption and emission by, and between, donor and acceptor pairs. It emerges that chiral discrimination in the FRET stage is not, as might have been expected, the main arbiter in the stepwise mechanism. In the concluding discussion on various concepts, attention is focused on the validity of helicity transfer in FRET.

Resonance energy transfer (RET), the transport of electronic energy from one atom or molecule to another, has significant importance to a number of diverse areas of science. Since the pioneering experiments on RET by Cario and Franck in 1922, the theoretical understanding of the process has been continually refined. This review presents a historical account of the post-Förster outlook on RET, based on quantum electrodynamics, up to the present-day viewpoint. It is through this quantum framework that the short-range, R⁻⁶ distance dependence of Förster theory was unified with the long-range, radiative transfer governed by the inverse-square law. Crucial to the theoretical knowledge of RET is the electric dipole-electric dipole coupling tensor; we outline its mathematical derivation with a view to explaining some key physical concepts of RET. The higher order interactions that involve magnetic dipoles and electric quadrupoles are also discussed. To conclude, a survey is provided on the latest research, which includes transfer between nanomaterials, enhancement due to surface plasmons, possibilities outside the usual ultraviolet or visible range and RET within a cavity.

This section includes the table of contents, preface, list of symbols, and list of figures. PDF download only.

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In this chapter, we examine some of the principal attributes of electromagnetic radiation that feature in photons. At the outset, it is worth asserting that the photon concept owes its legitimacy to the quantum theory of light. As we shall see in later chapters, the photon is formally regarded as a quantum of excitation of any specific optical mode. In the literature, one can find occasional mentions of photons qualified by an adjective, such as dipole photons, electric photons, ballistic photons, etc.,* These should be regarded as potentially misleading because each alludes to a property that is more correctly associated with a particular kind of phenomenon. Even if the distinction between "real" and "virtual" photons represents one of the most widely used descriptors, it does not signify a clear-cut difference. We thus begin by listing the following photon properties, all of which can be involved in specific forms of optical interaction, with the exception of the first. Online access to SPIE eBooks is limited to subscribing institutions.