The photon: A virtual reality

It has been observed that every photon is, in a sense, virtual - being emitted and then sooner or later absorbed. As the motif of a quantum radiation state, the photon shares these characteristics of any virtual state: that it is not directly observable; and that it can signify only one of a number of indeterminable intermediates, between matter states that are directly measurable. Nonetheless, other traits of real and virtual behavior are usually quite clearly differentiable. How 'real', then, is the photon? To address this and related questions it is helpful to look in detail at the quantum description of light emission and absorption. A straightforward analysis of the dynamic electric field, based on quantum electrodynamics, reveals not only the entanglement of energy transfer mechanisms usually regarded as 'radiative' and 'radiation less'; it also gives significant physical insights into several other electromagnetic topics. These include: the propagating and non-propagating character in electromagnetic fields; near-zone and wave-zone effects; transverse and longitudinal character; the effects of retardation, manifestations of quantum uncertainty and issues of photon spin. As a result it is possible to gain a clearer perspective on when, or whether, the terms 'real' and 'virtual' are helpful descriptors of the photon

Ascent control analysis for S-II derivative launch vehicles, digital computer program

Model is used for analysis of the six degrees-of-freedom dynamics of general launch vehicle during atmospheric boost. Equations of motion are developed for rigid body and flat earth. Case may be started at any time beginning with ignition of stage and may be ended upon, or prior to, stage burnout. End of case may be at a specified time or based on propellant expended

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

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

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

All-optical control of molecular fluorescence

We present a quantum electrodynamical procedure to demonstrate the all-optical control of molecular fluorescence. The effect is achieved on passage of an off-resonant laser beam through an optically activated system; the presence of a surface is not required. Following the derivation and analysis of the all-optical control mechanism, calculations are given to quantify the significant modification of spontaneous fluorescent emission with input laser irradiance. Specific results are given for molecules whose electronic spectra are dominated by transitions between three electronic levels, and suitable laser experimental methods are proposed. It is also shown that the phenomenon is sensitive to the handedness of circularly polarized throughput, producing a conferred form of optical activity

Surface optical vortices

It is shown how the total internal reflection of orbital-angular-momentum-endowed light can lead to the generation of evanescent light possessing rotational properties in which the intensity distribution is firmly localized in the vicinity of the surface. The characteristics of these surface optical vortices depend on the form of the incident light and on the dielectric mismatch of the two media. The interference of surface optical vortices is shown to give rise to interesting phenomena, including pattern rotation akin to a surface optical Ferris wheel. Applications are envisaged to be in atom lithography, optical surface tweezers, and spanners