Optical properties of two‐dimensional dye aggregate

Molecular Monolayer Strong Coupling in Dielectric Soft Microcavities

We report strong coupling of a monolayer of J-aggregated dye molecules to the whispering gallery modes of a dielectric microsphere at room temperature. We systematically studied the evolution of strong coupling as the number of layers of dye molecules was increased and found the Rabi splitting to rise from 56 meV for a single layer to 94 meV for four layers of dye molecules. We compare our experimental results with two-dimensional (2D) numerical simulations and a simple coupled oscillator model, finding good agreement. We anticipate that these results will act as a stepping stone for integrating molecule-cavity strong coupling in a microfluidic environment since microspheres can be easily trapped and manipulated in such an environment and provide open access cavities.

Electromagnetic interactions of dye molecules surrounding a nanosphere

Enhanced interaction between light and molecules adsorbed on metallic nanoparticles is a cornerstone of plasmonics and surface-enhanced spectroscopies. Recent experimental access to the electronic absorption spectrum of dye molecules on silver colloids at low molecular coverage has revealed subtle changes in the spectral shape that may be attributed to a combination of factors, from a chemical modification of the molecule in contact with a metal surface to electromagnetic dye-dye and dye-metal interactions. Here we develop an original model to rigorously address the electromagnetic effects. The dye molecules are described as coupled anisotropic polarisable dipoles and their interaction with the core metal particle is described using a generalised Mie theory. The theory is readily amenable to numerical implementation and yields far-field optical cross-sections that can be compared to experimental results. We apply this model to specific adsorption geometries of practical interest to highlight the effect of molecular orientation on predicted spectral shifts and enhancement factors, as a function of surface coverage. These are compared to experimental results and reproduce the measured spectral changes as a function of concentration. These results have direct implications for the interpretation of surface selection rules and enhancement factors in surface-enhanced spectroscopies, and of orientation and coverage effects in molecular/plasmonic resonance coupling experiments.

Anderson transition in low-dimensional disordered systems driven by long-range nonrandom hopping

The single-parameter scaling hypothesis predicts the absence of delocalized states for noninteracting quasiparticles in low-dimensional disordered systems. We show analytically, using a supersymmetric method combined with a renormalization group analysis, as well as numerically that extended states may occur in the one- and two-dimensional Anderson model with a nonrandom hopping falling off as some power of the distance between sites. The different size scaling of the bare level spacing and the renormalized magnitude of the disorder seen by the quasiparticles finally results in the delocalization of states at one of the band edges of the quasiparticle energy spectrum.

Comparison between J-aggregates in a self-assembled multilayer and polymer-bound J-aggregates in solution: a steady-state and time-resolved spectroscopic study

The self-assembly of a thiacarbocyanine dye on a charged substrate via the layer-by-layer alternate adsorption technique was studied in detail. The influence of the chemical structure of the oppositely charged polyelectrolyte (PEL) in the multilayer on J-aggregate formation was investigated by means of absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. J-Aggregates were formed yielding a bathochromic narrow absorption band. Fine tuning of the position of the absorption band is possible by incorporating other polyelectrolytes in the multilayer. The results obtained for the dye-polyelectrolyte multilayers were compared with measurements made on the aqueous dye-polyelectrolyte mixtures. The parameters obtained by fitting the fluorescence decays to several different models allowed us to suggest a mechanism for the non-radiative decay channel. The low fluorescence quantum yields and fast non-exponential fluorescence decays obtained for both the dye-polyelectrolyte multilayers and the aqueous dye polyelectrolyte mixtures were attributed to the trapping of a fast migrating exciton by a non-luminescent trap. The results related to J-aggregate formation, decay time and dimensionality of the aggregates were compared in solution and in a self-assembled multilayer.