Optical forces between dielectric nanoparticles in an optical vortex

We report a study on the optical forces between a pair of dielectric particles, based on quantum electrodynamics. At a fundamental level these forces result from a stimulated scattering process which entails a virtual photon relay between the two particles. Results for a variety of systems are secured from a completely general analysis that accommodates a system with arbitrary dielectric properties (with regard to shape, frequency response etc.) in an optical field of arbitrary complexity. Specific results are obtained and exhibited for: (a) optical forces between nanoparticles, and specifically between carbon nanotubes; (b) the effects of optical ordering, clustering and trapping associated with twisted (Laguerre-Gaussian) laser beams

Electronic coupling mechanisms and characteristics for optically nonlinear photoactive nanomaterials

In a range of nanophotonic energy harvesting materials, resonance energy transfer (RET) is the mechanism for the intermolecular and intramolecular transfer of electronic excitation following the absorption of ultraviolet/visible radiation. In the nonlinear intensity regime, suitably designed materials can exhibit two quite different types of mechanism for channeling the excitation energy to an acceptor that is optically transparent at the input frequency. Both mechanisms are associated with two-photon optical excitation - of either a single donor, or a pair of donor chromophores, located close to the acceptor. In the former case the mechanism is two-photon resonance energy transfer, initiated by two-photon absorption at a donor, and followed by RET directly to the acceptor. The probability for fulfilling the initial conditions for this mechanism (for the donors to exhibit two-photon absorption) is enhanced at high levels of optical input. In the latter twin-donor mechanism, following initial one-photon excitations of two electronically distinct donors, energy pooling results in a collective channeling of their energy to an acceptor chromophore. This mechanism also becomes effective under high intensity conditions due to the enhanced probability of exciting donor chromophores within close proximity of each other and the acceptor. In this paper we describe the detailed balance of factors that determines the favored mechanism for these forms of optical nonlinearity, especially electronic factors. Attention is focused on dendrimeric nanostar materials with a propensity for optical nonlinearity

Optical control and switching of excitation transfer in nano-arrays

The possibility of influencing resonance energy transfer through the input of off-resonant pulses of laser radiation is the subject of recent research. Attention is now focused on systems in which resonance energy transfer is designedly precluded by geometric configuration. Here, through an optically nonlinear mechanism - optically controlled resonance energy transfer - the throughput of non-resonant pulses can facilitate energy transfer that is, in their absence, completely forbidden. The system thus functions as an optical buffer, with excitation throughput switched on by the secondary beam. For applications, a system based on two parallel nano-arrays is envisaged. This paper will establish and discuss the principles - those that can be exploited to enhance switching characteristics and efficiency, and others (such as off-axis excitation transfer) that may represent cross-talk limitations. Principles to be explored in detail are the interplay between geometric features, including the array architecture and repeat distance (lattice constant), the array spacing and translational symmetry, the orientations of the transition dipoles, and the magnitude of the relevant components of the nonlinear response tensors. The aim is, through a determination of key parameters, to inform a program of optimization that can deliver specific criteria for realizing the most efficient systems for implementation

The optical control of electronic energy transfer through single and dual auxiliary beams

The electronic transfer of energy from a donor particle to an acceptor is a mechanism that plays a key role in a wide range of optical and photophysical phenomena. The ability to exert control on this transfer is of immense importance. It now emerges that there are all-optical routes which can be introduced to achieve this very purpose. We demonstrate the possibility of promoting energy transfer, in the optical near field, that is rigorously forbidden (on geometric or symmetric grounds) in the absence of laser light. The effect operates through coupled stimulated Raman scattering by the donor-acceptor pair. The absorption of a photon takes place at one particle and stimulated emission at either, coupled with energy transfer between the pair. At this fundamental level, transfer phenomena arise for both single and dual input auxiliary beams. In the latter case the emitted photon may differ from the absorbed photon. In either situation energy transfer will not occur in the absence of auxiliary radiation, if either the donor or acceptor transition is single-quantum forbidden. By engaging input laser light, energy transfer may proceed through two or three quantum allowed transitions. The results for this novel type of optical control suggest transfer efficiency levels comparable to Förster transfer. Many applications are envisaged, chief of which is the potential for all-optical switching

Theory of directed transportation of electronic excitation between single molecules through photonic coupling

The primary result of UV-Visible photon absorption by complex organic molecules is the population of short-lived electronic excited states. Transportation of their excitation energy between single molecules, formally mediated by near-field interactions, may occur between the initial absorption and eventual fluorescence emission events, commonly on an ultrafast timescale. The routing of energy flow is typically effected by a sequence of pairwise transfer steps over numerous molecules, rather than a single step over the same overall distance. Directionality emerges when there is structure in the molecular organisation. For a chemically heterogeneous system with local order, and with suitable molecular dispositions, automatically unidirectional transfer can be exhibited as the result of a 'spectroscopic gradient'. However it is also possible to exert control over the directionality of excitation flow by the operation of external influences. Examples are the application of an electrical or optical stimulus to the system - achieved by the incorporation of an ancillary polar species, the application of a static electric field or electromagnetic radiation. Most significantly, based on the latter option, an all-optical method has recently been determined that enables excitation transportation to be completely switched on or off, such that the energy flow is subject to controllable photoactivated gating. It is already apparent that this photonic process, termed Optically Controlled Resonance Energy Transfer, has potentially numerous applications. For example, it represents a new basis for optical transistor action

Nanoparticle manipulation through inter-particle optical forces and torques

Recently, emerging from studies based on quantum electrodynamics, it has been shown possible to significantly modify the form and magnitude of inter-particle forces by the throughput of intense laser light. This paper identifies these laser-induced forces as being the result of coherent stimulated scattering by particle pairs. Such forces have the capacity to generate novel patterns of nanoscale response, entirely determined and controlled by the frequency, intensity, polarisation and other features of the laser input. Results are given, based on general calculations of the optical forces and torques operating between a pair of dielectric particles. It is subsequently shown, by further development of the analysis, that it is possible to address the case of a twisted (Laguerre-Gaussian) laser beam as the input radiation. Here, the results reveal additional and highly distinctive torques operating between pairs of nanoparticles. Significantly the results demonstrate that these laser-induced forces and torques can be either positive or negative according to conditions. As a consequence, new possibilities emerge for the optical control of nanoparticle ordering, clustering and trapping

Off-resonant activation of optical emission

Recent reports have identified a three-wave optically parametric mechanism for the active enhancement of fluorescence using off-resonant radiation. In this Letter it is shown by numerical simulation that the output of a laser system optically pumped just below threshold, can be strongly enhanced by this mechanism, using an ancillary beam of moderate intensity. The electrodynamics and kinetics of the nonlinear optical mechanism are analyzed, model calculations performed, and the output is illustrated graphically. The response demonstrates a novel method for achieving all-optical transistor action

Optical ordering of nanoparticles trapped by Laguerre-Gaussian laser modes

In earlier work, it has been established that laser-induced coupling between a pair of nanoparticles can enable the generation of novel patterns, entirely determined and controlled by the frequency, intensity, and polarization of the optical input. Jn this paper, the detailed spatial disposition about the beam axis is determined for two-, three- and four-nanoparticle systems irradiated by a Laguerre-Gaussian (LG) laser mode. The range-dependent laser-induced energy shift is identified by the employment of a quantum electrodynamical description, calculations are performed to determine the distribution of absolute minima as a function of the topological charge, and the results are graphically displayed. This analysis illustrates a number of interesting features, including the fact that on increasing the LG beam's topological charge the particles increasingly cluster, i.e. the order of the structure is significantly raised - also the number of minima for which the particles can be trapped is enhanced. Finally, it is shown that similar principles apply to other kinds of radially structured optical modes

Energy migration in molecular assemblies:The characterisation and differentiation of two-photon mechanisms

Energy migration between chromophores plays a prominent role in a range of energy harvesting assemblies. Recent advances in the design and production of light-harvesting polymers have led to the synthesis of novel two-photon absorbing dendrimers. To construct increasingly efficient multifunctional macromolecules of this type, understanding the inherent optical processes and disentangling them has become imperative. This paper explores the fundamental processes by means of which energy transfers from a donor chromophore to an acceptor through two-photon absorption from an input laser beam. It is determined that three distinct classes of mechanism can operate: (i) two-photon absorption by individual chromophores is followed by transfer of the energy to an acceptor group; (ii) a singly excited chromophore is excited to a virtual state by the additional absorption of a photon from the pump radiation field, coupled with resonance energy transfer to the acceptor, or; (iii) two-photon excitation of the acceptor results from acquisition of one quantum of energy from a singly excited neighbour group and another from the throughput radiation. These mechanisms may compete and, in certain cases, lead to manifestations of quantum interference. Generally, the most favoured mechanism is determined by a balance of factors and constraints. Principal amongst the latter are the choice of wavelength (connected with the possibility of exploiting certain electronic resonances, whilst judiciously avoiding others) and the precise chromophore architecture (taking account of geometric factors concerned with the relative orientation of transition moments). As the relative importance of each mechanism determines the key nanophotonic characteristics of the assembly, the principles and results reported here afford the means for expediting highly efficient two-photon energy migration

Controlling the localization and migration of optical excitation

In the nanoscale structure of a wide variety of material systems, a close juxtaposition of optically responsive components can lead to the absorption of light by one species producing fluorescence that is clearly attributable to another. The effect is generally evident in systems comprising two or more light-absorbing components (molecules, chromophores or quantum dots) with well-characterised fluorescence bands at similar, differentiable wavelengths. This enables the fluorescence associated with transferred energy to be discriminated against fluorescence from an initially excited component. The fundamental mechanism at the heart of the phenomenon, molecular (resonance) energy transfer, also operates in systems where the product of optical absorption is optical frequency up-conversion. In contrast to random media, structurally organised materials offer the possibility of pre-configured control over the delocalization of energy, through molecular energy transfer following optical excitation. The Förster mechanism that conveys energy between molecular-scale components is strongly sensitive to specific forms of correlation between the involved components, in terms of position, spectroscopic character, and orientation; one key factor is a spectroscopic gradient. Suitably designed materials offer a broad scope for the widespread exploitation of such features, in applications ranging from chemical and biological sensing to the detection of nanoscale motion or molecular conformations. Recently, attention has turned to the prospect of actively controlling the process of energy migration, for example by changing the relative efficiencies of fluorescence and molecular energy transfer. On application of static electric fields or off-resonant laser light - just two of the possibilities - each represents a means for achieving active control with ultrafast response, in suitably configured systems. As the principles are established and the theory is developed, a range of new possibilities for technical application is emerging. For example, applications can be envisaged for new forms of all-optical switching and transistor action. There is also interest in engaging with the interplay of optical excitation and local nanoscale force, exploiting local responses to changes in dispersion forces, accompanying molecular energy transfer