Experimental investigation of the interaction of an intense relativistic electron beam with a plasma

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Optically nonlinear energy transfer in light-harvesting dendrimers

Dendrimeric polymers are the subject of intense research activity geared towards their implementation in nanodevice applications such as energy harvesting systems,organic light-emitting diodes, photosensitizers, low-threshold lasers, and quantum logic elements, etc. A recent development in this area has been the construction of dendrimers specifically designed to exhibit novel forms of optical nonlinearity, exploiting the unique properties of these materials at high levels of photon flux. Starting from a thorough treatment of the underlying theory based on the principles of molecular quantum electrodynamics, it is possible to identify and characterize several optically nonlinear mechanisms for directed energy transfer and energy pooling in multichromophore dendrimers. Such mechanisms fall into two classes: first, those where two-photon absorption by individual donors is followed by transfer of the net energy to an acceptor; second, those where the excitation of two electronically distinct but neighboring donor groups is followed by a collective migration of their energy to a suitable acceptor. Each transfer process is subject to minor dissipative losses. 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, laser coherence factors, chromophore selection rules and architecture, possibilities for the formation of delocalized excitons, spectral overlap, and the overall distribution of donors and acceptors. Furthermore, it transpires that quantum interference between different mechanisms can play an important role. Thus, as the relative importance of each mechanism determines the relevant nanophotonic characteristics, the results reported here afford the means for optimizing highly efficient light-harvesting dendrimer devices

Laser-controlled fluorescence in two-level systems

The ability to modify the character of fluorescent emission by a laser-controlled, optically nonlinear process has recently been shown theoretically feasible, and several possible applications have already been identified. In operation, a pulse of off-resonant probe laser beam, of sufficient intensity, is applied to a system exhibiting fluorescence, during the interval of excited- state decay following the initial excitation. The result is a rate of decay that can be controllably modified, the associated changes in fluorescence behavior affording new, chemically specific information. In this paper, a two-level emission model is employed in the further analysis of this all-optical process; the results should prove especially relevant to the analysis and imaging of physical systems employing fluorescent markers, these ranging from quantum dots to green fluorescence protein. Expressions are presented for the laser-controlled fluorescence anisotropy exhibited by samples in which the fluorophores are randomly oriented. It is also shown that, in systems with suitably configured electronic levels and symmetry properties, fluorescence emission can be produced from energy levels that would normally decay nonradiatively. © 2010 American Chemical Society

Raman scattering mediated by neighboring molecules

Raman scattering is most commonly associated with a change in vibrational state within individual molecules, the corresponding frequency shift in the scattered light affording a key way of identifying material structures. In theories where both matter and light are treated quantum mechanically, the fundamental scattering process is represented as the concurrent annihilation of a photon from one radiation mode and creation of another in a different mode. Developing this quantum electrodynamical formulation, the focus of the present work is on the spectroscopic consequences of electrodynamic coupling between neighboring molecules or other kinds of optical center. To encompass these nanoscale interactions, through which the molecular states evolve under the dual influence of the input light and local fields, this work identifies and determines two major mechanisms for each of which different selection rules apply. The constituent optical centers are considered to be chemically different and held in a fixed orientation with respect to each other, either as two components of a larger molecule or a molecular assembly that can undergo free rotation in a fluid medium or as parts of a larger, solid material. The two centers are considered to be separated beyond wavefunction overlap but close enough together to fall within an optical near-field limit, which leads to high inverse power dependences on their local separation. In this investigation, individual centers undergo a Stokes transition, whilst each neighbor of a different species remains in its original electronic and vibrational state. Analogous principles are applicable for the anti-Stokes case. The analysis concludes by considering the experimental consequences of applying this spectroscopic interpretation to fluid media; explicitly, the selection rules and the impact of pressure on the radiant intensity of this process

Chiral discrimination in optical binding

The laser-induced intermolecular force that exists between two or more particles in the presence of an electromagnetic field is commonly termed “optical binding.” Distinct from the single-particle forces that are at play in optical trapping at the molecular level, the phenomenon of optical binding is a manifestation of the coupling between optically induced dipole moments in neutral particles. In other, more widely known areas of optics, there are many examples of chiral discrimination—signifying the different response a chiral material has to the handedness of an optical input. In the present analysis, extending previous work on chiral discrimination in optical binding, a mechanism is identified using a quantum electrodynamical approach. It is shown that the optical binding force between a pair of chiral molecules can be significantly discriminatory in nature, depending upon both the handedness of the interacting particles and the polarization of the incident light, and it is typically several orders of magnitude larger than previously reported

Interparticle interactions:Energy potentials, energy transfer, and nanoscale mechanical motion in response to optical radiation

In the interactions between particles of material with slightly different electronic levels, unusually large shifts in the pair potential can result from photoexcitation, and on subsequent electronic excitation transfer. To elicit these phenomena, it is necessary to understand the fundamental differences between a variety of optical properties deriving from dispersion interactions, and processes such as resonance energy transfer that occur under laser irradiance. This helps dispel some confusion in the recent literature. By developing and interpreting the theory at a deeper level, one can anticipate that in suitable systems, light absorption and energy transfer will be accompanied by significant displacements in interparticle separation, leading to nanoscale mechanical motion

Optical vortex generation from molecular chromophore arrays

The generation of light endowed with orbital angular momentum, frequently termed optical vortex light, is commonly achieved by passing a conventional beam through suitably constructed optical elements. This Letter shows that the necessary phase structure for vortex propagation can be directly produced through the creation of twisted light from the vacuum. The mechanism is based on optical emission from a family of chromophore nanoarrays that satisfy specific geometric and symmetry constraints. Each such array can support pairs of electronically delocalized doubly degenerate excitons whose azimuthal phase progression is responsible for the helical wave front of the emitted radiation. The exciton symmetry dictates the maximum magnitude of topological charge; detailed analysis secures the conditions necessary to deliver optical vortices of arbitrary order

Women's experiences of participating in a randomised trial comparing alternative policies for timing of cord clamping at very preterm birth: a questionnaire study

BACKGROUND: The Cord Pilot Trial compared two alternative policies for cord-clamping at very preterm birth at eight UK tertiary maternity units: clamping after at least 2 min and immediate neonatal care with cord intact, or clamping within 20 s and neonatal care after clamping. This paper reports views and experiences of the women who participated in the trial (261 randomised), based on data from two self-completed questionnaires. METHODS: Women were given or posted the first questionnaire between 4 and 8 weeks after birth, and posted a second similar questionnaire at 1 year. Both questionnaires included three questions about experiences of participating in the trial: (1) If time suddenly went backwards and you had to do it all over again, would you agree to participate in the Cord Pilot Trial?; (2) Please tell us if there was anything about the Cord Pilot Trial that you think could have been done better; and (3) Please tell us if there was anything about the Cord Pilot Trial, or your experiences of joining the trial, that you think were particularly good. RESULTS: One hundred and eighty-six women completed the first questionnaire and 133 completed the second. At both time points, 90% responded 'probably' or 'definitely' to participating in the trial again. More women randomised to deferred clamping responded 'definitely yes' than those allocated immediate clamping (78% versus 67% first questionnaire). Women were positive about the level of information and explanations, the friendly and caring staff, and the benefits for their baby and others as a result of participating in the trial. Suggestions for how the trial could be done better included being approached earlier, better staff communication about the trial, more information overall, and better timing of follow-up. CONCLUSIONS: Women were largely positive about participating in the trial. Nevertheless, they had suggestions for how the study could have been improved. These suggestions have implications for the design of future trials. TRIAL REGISTRATION: ISRCTN21456601 . Registered on 28 February 2013