Abstract
Molecular aggregates are a fascinating and important class of materials, particularly in the context of optical (pigmented) materials. In nature, molecular aggregates are employed in photosynthetic light harvesting structures, while synthetic aggregates are employed in new generation molecular sensors and magnets. The roles of disorder and symmetry are vital in determining the photophysical properties of molecular aggregates, but have been hard to investigate experimentally, owing to a lack of sufficient structural control at the molecular level and the challenge of probing their optical response with molecular spatial resolution. We present a new approach using microwave analogues of molecular aggregates to study the properties of both individual meta-molecules and 1D molecular chains. We successfully replicate J- and H-aggregate behavior and demonstrate the power of our approach through the controlled introduction of structural symmetry breaking. Our results open a new area of study, combining concepts from molecular science and metamaterials.
Highlights
Molecular aggregates are a fascinating and important class of materials, in the context of optical materials
Aggregates of pigments are employed in nature by many photosynthetic structures used for light harvesting,[4,5] while from a synthetic standpoint, molecular aggregates are the building blocks of supramolecular science[6] and have applications in areas such as new-generation sensors,[7] molecular magnets,[8] single photon sources,[9] and nanophotonic materials.[10,11]
Electromagnetic metamaterials have been well-explored in the microwave regime, where the length scales allow a range of powerful fabrication approaches to be used, including photolithography,[24] and 3D printing.[25]
Summary
Article magnetic-dipole coupling can be employed to explore aggregation effects, and (v) take a first step in exploring some of the potential benefits of microwave metamaterial analogues. The dispersion of the modes on a chain of the combined, doublering+frame meta-molecules is shown in Figure 5a; for these data we used a magnetic (loop) probe antenna because such an antenna is able to pick up fields associated with both electric and magnetic dipole modes In this figure both positive (J-like) and negative (H-like) gradient modes are seen. A suitable antenna was placed in a region of high field (as determined from the finite element modeling) approximately 0.5 mm from the surface of each meta-molecule design and the return loss recorded (the power reflected back along the cable of the exciting antenna) These results were normalized by measuring the return loss when the exciting antenna was placed in the same position against a blank piece of substrate (all copper removed), so as to remove the frequency dependence of the antennas. Plotting the resulting Fourier spectrum for each frequency one can readily identify the dispersion relation of the collective modes of each chain, as shown in Figures 4 and 5
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