Optical binding is an optomechanical effect exhibited by systems of micro-and nanoparticles, suitably irradiated with off-resonance laser light. Physically distinct from standing-wave and other forms of holographic optical traps, the phenomenon arises as a result of an interparticle coupling with individual radiation modes, leading to optically induced modifications to Casmir-Polder interactions. To better understand how this mechanism leads to the observed assemblies and formation of patterns in nanoparticles, we develop a theory in terms of optically induced energy landscapes exhibiting the three-dimensional form of the potential energy field. It is shown in detail that the positioning and magnitude of local energy maxima and minima depend on the configuration of each particle pair, with regards to the polarization and wave vector of the laser light. The analysis reveals how the positioning of local minima determines the energetically most favorable locations for the addition of a third particle to each equilibrium pair. It is also demonstrated how the result of such an addition subtly modifies the energy landscape that will, in turn, determine the optimum location for further particle additions. As such, this development represents a rigorous and general formulation of the theory, paving the way toward full comprehension of nanoparticle assembly based on optical binding.
The efficiency and directedness of resonance energy transfer, by means of which electronic excitation passes between molecular units or subunits, fundamentally depend on the spectral features of donor and acceptor components. In particular, the flow of energy between chromophores in complex energy harvesting materials is crucially dependent on a spectral overlap integral reflecting the relative positioning and shapes of the absorption and fluorescence bands. In this paper, analytical results for this integral are derived for bands of Gaussian and log normal line shape; the methods also prove applicable to double Gaussian curves under suitable conditions. Underlying principles have been ascertained through further development of theory, with physically reasonable assumptions. Consideration of the Gaussian case, widely applicable to spectra of symmetric form, reveals that the directional efficiency of energy transfer depends equally on a frequency shift characterizing the spectroscopic gradient and the Stokes shift. On application to tryptophan residues, calculations based on a minimal parameter set give excellent agreement with experiment. Finally, an illustrative application highlights the critical role that the spectroscopic gradient and Stokes shift can exercise in extended, multichromophore energy harvesting systems.
Modeling the multistep flow of energy in light-harvesting dendrimers presents a considerable challenge. Recent studies have introduced an operator approach based on a matrix representation of the connectivity between constituent chromophores. Following a review of the theory, detailed applications are now shown to exhibit the time development of the core excitation following pulsed laser irradiation and the steady-state behavior that can be expected under conditions of constant illumination. It is also shown how energy capture by whole dendrimers can be analytically related to chromophore pair-transfer properties and, in particular, the spectroscopic gradient toward the core. Indicative calculations also illustrate the consequences of tertiary folding. In each respect, the model affords opportunities to derive new, physically meaningful information on the photophysical and structural features of dendrimeric systems.
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