Engaging new dimensions in nonlinear optical spectroscopy using auxiliary beams of light

By applying a sufficiently intense beam of off-resonant light, simultaneously with a conventional excitation source beam, the efficiencies of one- and two-photon absorption processes may be significantly modified. The nonlinear mechanism that is responsible, known as laser modified absorption, is fully described by a quantum electrodynamical analysis. The origin of the process, which involves stimulated forward Rayleigh-scattering of the auxiliary beam, relates to higher order terms which are secured by a time-dependent perturbation treatment. These terms, usually inconsequential when a single beam of light is present, become prominent under the secondary optical stimulus – even with levels of intensity that are moderate by today’s standards. Distinctive kinds of behaviour may be observed for chromophores fixed in a static arrangement, or for solution- or gas-phase molecules whose response is tempered by a rotational average of orientations. In each case the results exhibit an interplay of factors involving the beam polarisations and the molecular electronic response. Special attention is given to interesting metastable states that are symmetry forbidden by one- or two-photon absorption. Such states may be accessible, and thus become populated, on input of the auxiliary beam. For example, in the one-photon absorption case, terms arise that are more usually associated with three-photon processes, corresponding to very different selection rules. Other kinds of metastable state also arise in the two-photon process, and measuring the effect of applying the stimulus beam to absorbances of such character adds a new dimension to the information content of the associated spectroscopy. Finally, based on these novel forms of optical nonlinearity, there may be new possibilities for quantum non-demolition measurements

On the detection of characteristic optical emission from electronically coupled nanoemitters

Optical emission from an electronically coupled pair of nanoemitters is investigated, in a new theoretical development prompted by experimental work on oriented semiconductor polymer nanostructures. Three physically distinct mechanisms for photon emission by such a pair, positioned in the near-field, are identified: emission from a pairdelocalized exciton state, emission that engages electrodynamic coupling through quantum interference, and correlated photon emission from the two components of the pair. Each possibility is investigated, in detail, by examination of the emission signal via explicit coupling of the nanoemitter pair with a photodetector, enabling calculations to give predictive results in a form directly tailored for experiment. The analysis incorporates both near- and far-field properties (determined from the detector-pair displacement), so that the framework is applicable not only to a conventional remote detector, but also a near-field microscope setup. The results prove strongly dependent on geometry and selection rules. This work paves the way for a broader investigation of pairwise coupling effects in the optical emission from structured nanoemitter arrays

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

Public opinion in Poland\u27s transition to market economy

Public opinion research has changed dramatically in the last ten years in Poland, in terms of its methodology, scope, and role in political change. During the first Solidarity era (1980–81), the genie of public opinion was let out of the bottle, and even martial law could not entirely put it back. Public opinion polling in the 1980s became more sophisticated and more common, and began to tackle increasingly sensitive political issues. Public opinion came to play a role in the political process, and to give the Polish population a sense of its own purpose and values. It also revealed the depth of antipathy to the communist regime and leadership and, in doing so, further eroded the already fragile legitimacy of the regime. When, in the late 1980s, the regime realized it could not succeed at winning back the allegiance, or at least acquiescence, of the Polish population, it agreed to negotiate with the opposition. The result was the emergence of the first noncommunist regime in Eastern Europe