Photon-based and classical descriptions in nanophotonics: a review

The centrality of the photon concept in modern physics is strongly evident in wide spheres of photonics and nanophotonics. Despite the resilience and persistence of earlier classical representations, there are numerous optical features and phenomena that only truly photon-based descriptions of theory can properly address. It is crucial to cast theory in terms of observables, and in this respect the quantum theory of light engages most directly and pragmatically with experiment. No other theory adequately reconciles the discreteness in energy of optical quanta, with their characteristic quantum mechanical delocalization in space. Examples of the distinctiveness of a photonic representation are to be found in nonclassical optical correlations; intensity fluctuations and phase; polarization, spin, and information content; measures of optical chirality; near-field interactions; and plasmonics. Increasingly, links between these fundamental properties and features are proving significant in the context of nanoscale interactions. Yet, even as new technologies are being built on the framework of modern photonics, a number of difficult questions surrounding the nature of the photon still remain. Both in its flourishing applications and in matters of fundamental entity, the photon is still a subject very much at the heart of current research

Energy harvesting: a review of the interplay between structure and mechanism

The science of energy harvesting has recently undergone radical change, with the advent of new materials exploiting mechanisms fundamentally different from those of traditional solar cells. Utilizing principles that are in many cases acquired from breakthroughs in molecular photobiology, the introduction of a range of new synthetic polymers, multichromophore arrays and nanoparticle-based materials heralds a marked resurgence of interest, a shift of focus and heightened expectations in the science of light-harvesting. The interplay between structure and mechanism significantly impinges upon issues extending from fundamental theory to the principles of energy-harvesting materials design. Understanding and exploiting the principles allows materials to be engineered that can harness absorbed energy with heightened efficiency. Two of the key areas of application are dendrimers and rare-earth doped solids

Optically controlled resonance energy transfer:Mechanism and configuration for all-optical switching

In a molecular system of energy donors and acceptors, resonance energy transfer is the primary mechanism by means of which electronic energy is redistributed between molecules, following the excitation of a donor. Given a suitable geometric configuration it is possible to completely inhibit this energy transfer in such a way that it can only be activated by application of an off-resonant laser beam: this is the principle of optically controlled resonance energy transfer, the basis for an all-optical switch. This paper begins with an investigation of optically controlled energy transfer between a single donor and acceptor molecule, identifying the symmetry and structural constraints and analyzing in detail the dependence on molecular energy level positioning. Spatially correlated donor and acceptor arrays with linear, square, and hexagonally structured arrangements are then assessed as potential configurations for all-optical switching. Built on quantum electrodynamical principles the concept of transfer fidelity, a parameter quantifying the efficiency of energy transportation, is introduced and defined. Results are explored by employing numerical simulations and graphical analysis. Finally, a discussion focuses on the advantages of such energy transfer based processes over all-optical switching of other proposed forms. © 2008 American Institute of Physics

Nonlinear energy pooling in nanophotonic materials

Recently there has been considerable interest in the construction of photoactive organic materials designed to exhibit novel forms of optical nonlinearity. By exploiting the unique properties of these nanomaterials at high levels of photon flux, new possibilities emerge for applications in energy harvesting, low-threshold lasing, quantum logic devices, photodynamic therapy, etc. In particular, a detailed appraisal of the theory spotlights novel mechanisms for directed energy transfer and energy pooling in nanophotonic dendrimers. Characterized by a nonlinear dependence on the optical irradiance, these mechanisms fall into two classes: (a) those where two-photon absorption by individual donors is followed by transfer of the sum energy to the acceptor; (b) where the excitation of two electronically distinct but neighbouring donor groups is followed by a collective migration of their energy to a suitable acceptor. In each case these transfer processes are subject to minor dissipative losses, associated with intramolecular vibrational relaxation in the donor species. In this paper we describe in detail the balance of factors and the constraints that determines the favored mechanism, which include the excitation statistics, structure of the energy levels, selection rules, molecular architecture, the distribution of donors and acceptors, spectral overlap and coherence factors. Knowledge of these factors and the means for their optimization offers fresh insights into nanophotonic characteristics, and informs strategies for the design of new photoactive materials

Near-field manipulation of interparticle forces through resonant absorption, optical binding, and dispersion forces

The relative motions of two or more neutral particles, subject to optical trapping forces within a beam, are influenced by intrinsic inter-particle forces. The fundamental character of such forces is well-known and usually derives from dispersion interactions. However, the throughput of moderately intense (off-resonant) laser light can significantly modify the form and magnitude of these intrinsic forces. This optical binding effect is distinct from the optomechanical interactions involved in optical tweezers, and corresponds to a stimulated (pairwise) forward-scattering mechanism. In recent years, attention has begun to focus on optical binding effects at sub-micron and molecular dimensions. At this nanoscale, further manipulation of the interparticle forces is conceivable on the promotion of optically bound molecules to an electronic excited state. It is determined that such excitation may influence the intrinsic dispersion interaction without continued throughput of the laser beam, i.e. independent of any optical binding. Nevertheless, the forwardscattering mechanism is also affected by the initial excitation, so that both the optical binding and dispersion forces can be manipulated on input of the electromagnetic radiation. In addition, the rate of initial excitation of either molecule (or any energy transfer between them) may be influenced by an off-resonant input beam which, thus, acts as an additional actor in the modification of the interparticle force. A possible experimental set-up is proposed to enable the measurement of such changes in the interparticle coupling. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE)

All-optical switching based on controlled energy transfer between nanoparticles in film arrays

The potentiality to exert optical control, over the migration of electronic excitation energy between particles with suitably disposed electronic levels, affords a basis for all-optical switching. Implemented in a configuration with nanoparticles arrayed in thin films, the process can offer an ultrafast parallel-processing capability. The mechanism is a nearfield transfer of energy from donor nanoparticles in one layer (written into an electronically excited state by the absorption of light) to counterpart acceptors; the transfer effect proves amenable to activation by non-resonant laser radiation. The possibility of optical control arises under conditions where the donor-acceptor energy transfer is rigorously forbidden in the absence of laser light, either on the grounds of symmetry or energetics. Under such conditions, optical switching can be produced by the throughput of a single off-resonant beam or, with more control options, by two coincident beams. In model electrodynamical calculations the transfer fidelity, signifying the accuracy of mapping an input to its designated output, can be identified and cast in terms of key optical and geometric characteristics. The results show that, at reasonable levels of laser intensity, cross-talk drops to insignificant levels. Potential applications extend beyond simple switching into all-optical elements for logic gates and optical buffers. © 2009 Society of Photo-Optical Instrumentation Engineers

A Quantum Electrodynamical Foundation for Molecular Photonics

In this review the authors describe some of the advances in the quantum electrodynamical formulation of theory for molecular photonics. Earlier work has been extended and reformulated for application to real dispersive media—as reflected in the new treatment of refractive, dissipative, and resonance properties. Applications of the new theory have revealed new quantum optical features in two quite different aspects of the familiar process of second harmonic generation, one operating through local coherence within small particles and the other, a coherence between the quantum amplitudes for fundamental and harmonic excitation. Where the salient experiments have been performed, they exactly match the theoretical predictions