Metamaterial Analogues of Molecular Aggregate Systems

Metamaterials offer a great degree of control over their physical properties through the tuning of the interactions between their elements within larger structures. Molecular Aggregates are a group of organic molecules that assemble into structures with new optical properties arising from the inter-molecule interactions. Molecular aggregates are a fascinating and important class of materials, particularly in the context of optical (pigmented) materials. The aim of this thesis is combine these two fields in the meta-analogue approach by replicating aggregate behaviour with metamaterials. In this way aggregate phenomena normally observed on the nm scale can instead be probed in great detail at the mm scale of a metamaterial. Simultaneously, new metamaterials can be created that are inspired by the study of aggregates. Passive resonant metamaterials are limited by the narrow-band nature of the resonances they support. In order to probe the interaction between two resonators more deeply, and guide experimentation with meta-analgoues, I create a tunable Split Ring Resonator (SRR). I show that by incorporating an active component into the structure of the SRR it is possible to tune the resonance frequency of this type of metamaterial atom. I make use of this tuneability to examine the interaction between two resonators, one passive and one active, as the resonance frequency of the active resonator is swept through that of the passive resonator. The resultant modes of this coupled system exhibit an anti-crossing and, by changing the separation between, and relative orientation of, the split-ring resonators, I investigate how the magnetic and electric coupling terms change. I find that the relative orientation of the resonators significantly effects the strength of the coupling. Through both structural and active tuning we are able to alter the relative sizes and signs of the coupling terms. In the world of researching molecular aggregates it is difficult to precisely examine small numbers of molecules, relying instead on the behaviours of large groups to determine their properties. In the second section I present two meta-analogues of molecular aggregates. A positively dispersive chain of resonators, that couples in a manner analogous to J-aggregates, and a negatively dispersive chain that couples analogously with H-aggregates. I also present two novel metamaterial chains inspired by the aggregate like structures. In the first a simultaneous negative and positive group velocity is achieved and in the second those counter propagating modes are coupled through the breaking of one of the meta-atom's symmetries, resulting in an anti-crossing and a region of zero group velocity. In the third results section, in an effort to achieve greater understanding of these systems of aggregate like chains of meta-molecules on the millimeter scale, I exploited the fact that they can be directly probed and rearranged. Using this system I directly observed the effects of various termination and defect types, in the process showing a clear relation between these and those predicted in the SSH model. I intend for this work to open new avenues of interrogation of aggregate structures using metamaterial analogues as well as of direct observation of localised modes. In the final section I present a meta-analogue of the strong coupling experiment. The formation of polariton modes from the strong coupling of light and matter is an exciting and important field with ramifications from Chemistry to laser physics. It's study is in many ways hampered, however, by the atomic and molecular scale at which interactions take place. In this section I investigate the effects of strong coupling using metamaterial analogues. This allows for the meta-atoms themselves to be directly probed during excitation, and for the nature of hybridised polaritons, and the optically `dark' states to be shown directly.Engineering and Physical Sciences Research Council (EPSRC

Direct observation of defect modes in molecular aggregate analogs (article)

This is the final version. Available from the American Physical Society via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.2843In this work we investigate defect modes localized at the ends and within the bulk of 1D metamaterial analogs of molecular aggregates. The study is undertaken in the microwave regime, where the cm scale of the metamaterial analog provides an opportunity to directly probe the modes deep within their near fields in ways not easily achieved in molecular systems. To demonstrate the power of this approach we compare our observations to predictions from a simple Su, Schrieffer, and Heeger (SSH) model and find good qualitative agreement.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC

Metamaterial analogues of strongly coupled molecular ensembles

This is the final version. Available on open access from the American Chemical Society via the DOI in this recordData in support of our findings are available at 10.6084/m9.figshare.16441584.The formation of polariton modes due to the strong coupling of light and matter has led to exciting developments in physics, chemistry and materials science. The potential to modify the properties of molecular materials by strongly coupling molecules to a confined light f ield is so far-reaching and so attractive that a new field known as ‘polaritonic chemistry’ is now emerging. However, the molecular scale of the materials involved makes probing strong coupling at the individual resonator level extremely challenging. Here we offer a complimentary approach based upon metamaterials, an approach that enables us to use cm-scale structures, thereby opening a new way to explore strong coupling phenomena. As proof-of-principle we show that meta-molecules placed inside a radio-frequency cavity may exhibit strong coupling, and show that near-field radio-frequency techniques allow us, for the first time, to probe the response of individual meta-molecules under strong coupling conditions.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC)QinetiQ Ltd

Metamaterial analogues of molecular aggregates

This is the author accepted manuscriptMolecular 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.Engineering and Physical Sciences Research Council (EPSRC)QinetiQ LtdEuropean Research Counci

Investigation of the coupling between tunable split-ring resonators (dataset)

All the plots from the paper can be replicated using the two python scripts included in the data. Uncoupled and coupled folders contain the raw VNA results for the coupled and uncoupled systems. Extracted contains the extracted mode positions for the uncoupled system while Xdata contains the extracted modes for the coupled system. Finally the updn folder contains the same data as Xdata but at a lower resolution for faster computation.The article associated with this dataset is located in ORE at: http://hdl.handle.net/10871/33901This is the dataset used for the Baraclough et al. (2018) article "Investigation of the coupling between tunable split-ring resonators" published in Physical Review B: condensed matter and materials physics.We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). W.L.B. acknowledges the support of the European Research Council through project Photmat (ERC-2016-AdG-742222). I.R.H. acknowledges support from the EPSRC via the TEAM-A prosperity partnership (Grant No. EP/L015331/1)

Direct observation of defect modes in molecular aggregate analogues (dataset)

asymmetric.csv - Measured data for the resonator chain transitioning from BA to AB. A column is frequency, E is S21 magnitude and F is S21 phase. Other columns are not used in the paper. Data is used in figure 7 panels b and c symmetric.csv - Measured data for the resonator chain transitioning from AB to BA. A column is frequency, E is S21 magnitude and F is S21 phase. Other columns are not used in the paper. Data is used in figure 7 panels a and c Aterm.csv - Measured data for the resonator chain that terminates with an A cell. A column is frequency, E is S21 magnitude and F is S21 phase. Other columns are not used in the paper.Data is used in figure 6 panel a Bterm.csv - Measured data for the resonator chain that terminates with an B cell. A column is frequency, E is S21 magnitude and F is S21 phase. Other columns are not used in the paper.Data is used in figure 6 panel b Molecular aggregates paper plots.ipynb - Jupyter notebook to plot figures 6 and 7. Coded in Julia 1.1 SSH_model.ipynb - Jupyter notebook to plot figure 2. Coded in Python 3.7The article associated with this dataset is available in ORE at: http://hdl.handle.net/10871/127508In this work we investigate defect modes localised at the ends and within the bulk of 1D metamaterial analogues of molecular aggregates. The study is undertaken in the microwave regime, where the cm scale of the metamaterial analogue provides an opportunity to directly probe the modes deep within their near-fields in ways not easily achieved in molecular systems. To demonstrate the power of this approach we compare our observations to predictions from a simple Su, Schrieffer and Heeger (SSH) model, and find good qualitative agreement.Engineering and Physical Sciences Research Council (EPSRC)Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC

Investigation of the coupling between tunable split-ring resonators (article)

This is the author accepted manuscript. The final version is available from American Physical Society via the DOI in this recordThe dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.743Passive resonant metamaterials are limited by the narrow-band nature of the resonances they support. Here we show that by incorporating an active component into the structure of the commonly used split-ring resonator it is possible to tune the resonance frequency of this type of metamaterial atom. We make use of this tuneability to examine the interaction between two resonators, one passive and one active, as the resonance frequency of the active resonator is swept through that of the passive resonator. The resultant modes of this coupled system exhibit an anti-crossing and, by changing the separation between, and relative orientation of, the split-ring resonators, we investigate how the magnetic and electric coupling terms change. We find that the relative orientation of the resonators significantly effects the strength of the coupling. Through both structural and active tuning we are able to alter the relative sizes and signs of the coupling terms. We hope that the nature of these changes will be of use to those designing large actively tunable metamaterial systems.We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). W.L.B. acknowledges the support of the European Research Council through project Photmat (ERC-2016-AdG-742222). I.R.H. acknowledges support from the EPSRC via the TEAM-A prosperity partnership (Grant No. EP/L015331/1)