Periodic Reporting for period 4 - PHOTMAT (Photonically fused molecular materials)
Reporting period: 2022-03-01 to 2023-08-31
Our world is made of molecules. Our goal was to explore a new way for molecules to interact with each other, and thus enable new opportunities in chemistry and materials science. Traditionally molecules interact through their electric charge. The new approach was to find ways to link molecules not with electric charge, but with light, something that produces two big changes. First, molecular energy levels can be modified, for example molecules that would naturally absorb blue light can be transformed into ones that absorb green light. Second, energy can be de-localised, for example the energy absorbed by one molecule could be made available not just on the nanometre scale – as in photosynthesis – but over centimetres, perhaps enabling alternative light-harvesting technologies.
Linking molecules with light in this way is known as strong coupling. At the start of the photmat project many ground-breaking results were still un-confirmed, and many of the mechanisms and underlying science poorly understood. The aim of photmat was to develop a significantly deeper scientific understanding, and to highlight more clearly where the potential transformative developments lie. Significant progress has been made, and a clearer picture of what still needs to be understood is now emerging.
Linking molecules with light is surprisingly simple, but rather subtle. It is achieved by confining light within a small volume of space, and then placing optically active molecules within this volume. In many experiments the light is confined by placing two metal mirrors close together to form a cavity, a sort of planar sandwich, the mirrors are the slices of bread, the molecules form the sandwich filling. If now we think of light inside the cavity in the form of a photon, it can be absorbed by the molecules, then released into the cavity. The light cannot escape because the cavity confines the light, instead it may again be absorbed by the molecules etc. etc. It is this exchange of the photon's energy between the molecules and the cavity that lies at the heart of strong coupling, it is how we link molecules with light. The subtlety is that we use virtual photons, thereby harnessing some of the weirdness of quantum mechanics.
The importance for society is that linking molecules with light potentially offers new ways to do chemistry, and opens new routes for materials science. Many desirable chemical reactions are difficult to achieve, often requiring high temperatures and/or catalysts that use scarce materials etc. Strong coupling offers the prospect of controlling some chemical reaction pathways ‘simply’ by placing the reactants in an optical cavity. Another example is organic electronics, where device efficiencies ore often limited by wasteful triplet states. Strong coupling offers a way to bypass these states, thus improving efficiency, e.g. for display device.
Linking molecules with light in this way is known as strong coupling. At the start of the photmat project many ground-breaking results were still un-confirmed, and many of the mechanisms and underlying science poorly understood. The aim of photmat was to develop a significantly deeper scientific understanding, and to highlight more clearly where the potential transformative developments lie. Significant progress has been made, and a clearer picture of what still needs to be understood is now emerging.
Linking molecules with light is surprisingly simple, but rather subtle. It is achieved by confining light within a small volume of space, and then placing optically active molecules within this volume. In many experiments the light is confined by placing two metal mirrors close together to form a cavity, a sort of planar sandwich, the mirrors are the slices of bread, the molecules form the sandwich filling. If now we think of light inside the cavity in the form of a photon, it can be absorbed by the molecules, then released into the cavity. The light cannot escape because the cavity confines the light, instead it may again be absorbed by the molecules etc. etc. It is this exchange of the photon's energy between the molecules and the cavity that lies at the heart of strong coupling, it is how we link molecules with light. The subtlety is that we use virtual photons, thereby harnessing some of the weirdness of quantum mechanics.
The importance for society is that linking molecules with light potentially offers new ways to do chemistry, and opens new routes for materials science. Many desirable chemical reactions are difficult to achieve, often requiring high temperatures and/or catalysts that use scarce materials etc. Strong coupling offers the prospect of controlling some chemical reaction pathways ‘simply’ by placing the reactants in an optical cavity. Another example is organic electronics, where device efficiencies ore often limited by wasteful triplet states. Strong coupling offers a way to bypass these states, thus improving efficiency, e.g. for display device.
We pioneered the use of ellipsometry to study the role of strong coupling in a photochemical reaction, the conversion of one dye molecule into another by the absorption of UV light. This reaction was the foundation of early claims of strong coupling being used to modify chemical reactions. Our exhaustive study led us to the conclusion that whilst the conversion rate is modified by placing the molecules in a cavity, strong coupling is not involved. This result has helped provide a corrective to a number of over-optimistic claims made recently, and was very well received at the major conference in this field in San Diego June 2023. Follow-up work continues, and results are currently under review at Advanced Materials.
One of the most puzzling early results on molecular strong coupling was the claim that strong coupling modifies the Raman scattering of molecules, a claim not supported by early theories. We used a combination of state-of-the-art experimental probes, differential Raman scattering, and micro-Raman scanning to investigate, and our results, showing that the Raman response is much more subtle than previously thought, have helped clarify the role of strong coupling, and are contributing to the emergence of a deeper understanding of vibrational strong coupling (ongoing, submission to Nature Nanotechnology).
We developed a new metamaterial approach using radio-frequency techniques to mimic molecular systems. We used split-ring resonators (copper rings, ~1 cm in diameter) as analogues of molecules, and showed that they can be used to simulate the behaviour of molecular aggregates. We showed that these meta-molecules can be used to demonstrate the equivalent of molecular strong coupling, highlighting the power of metamaterials in mimicking molecular photo-physics, they are being used by others to explore inter-molecular energy transfer, and the role of molecular disorder.
We also pioneered a custom set-up combining two organic field-effect transistors to look at how molecular-scale material properties can be exploited/manipulated. We showed that by electrically controlling the doping level in such a material inside a cavity, the extent of the coupling between with material and the cavity could be manipulated. The reverse process was not seen, helping to confirm an emerging understanding that strong coupling cannot be used to modify conductance, as had been hoped prior to the project starting. We further showed that by tuning the free carrier density in the molecular-scale material we could strongly modulate the transfer of energy between molecular materials.
An unexpected bonus was the development of ‘open access’ strong coupling structures. Using ellipsometry we probed a range of planar open cavities (producing some of the most highly cited work from the project), and in so doing discovered a new way of tracking strong coupling. In addition we developed a new cavity structure, plastic microspheres, and demonstrated proof-of-principle of this new class of structure, important because such microsphere can be manipulated by optical tweezers etc, and are compatible with microfluidics. We then incorporated a donor-acceptor energy transfer system into this new type of cavity. Our work on these soft microcavities has recently been highlighted as offering a new technology platform for strong coupling.
One of the most puzzling early results on molecular strong coupling was the claim that strong coupling modifies the Raman scattering of molecules, a claim not supported by early theories. We used a combination of state-of-the-art experimental probes, differential Raman scattering, and micro-Raman scanning to investigate, and our results, showing that the Raman response is much more subtle than previously thought, have helped clarify the role of strong coupling, and are contributing to the emergence of a deeper understanding of vibrational strong coupling (ongoing, submission to Nature Nanotechnology).
We developed a new metamaterial approach using radio-frequency techniques to mimic molecular systems. We used split-ring resonators (copper rings, ~1 cm in diameter) as analogues of molecules, and showed that they can be used to simulate the behaviour of molecular aggregates. We showed that these meta-molecules can be used to demonstrate the equivalent of molecular strong coupling, highlighting the power of metamaterials in mimicking molecular photo-physics, they are being used by others to explore inter-molecular energy transfer, and the role of molecular disorder.
We also pioneered a custom set-up combining two organic field-effect transistors to look at how molecular-scale material properties can be exploited/manipulated. We showed that by electrically controlling the doping level in such a material inside a cavity, the extent of the coupling between with material and the cavity could be manipulated. The reverse process was not seen, helping to confirm an emerging understanding that strong coupling cannot be used to modify conductance, as had been hoped prior to the project starting. We further showed that by tuning the free carrier density in the molecular-scale material we could strongly modulate the transfer of energy between molecular materials.
An unexpected bonus was the development of ‘open access’ strong coupling structures. Using ellipsometry we probed a range of planar open cavities (producing some of the most highly cited work from the project), and in so doing discovered a new way of tracking strong coupling. In addition we developed a new cavity structure, plastic microspheres, and demonstrated proof-of-principle of this new class of structure, important because such microsphere can be manipulated by optical tweezers etc, and are compatible with microfluidics. We then incorporated a donor-acceptor energy transfer system into this new type of cavity. Our work on these soft microcavities has recently been highlighted as offering a new technology platform for strong coupling.
We are the first to employ wavelength-modulated Raman spectroscopy and ellipsometry to investigate strong coupling. Our pioneering use of metamaterial systems employing meta-molecules is also a world first.
We helped to significantly clarify what role strong coupling may play in a number of molecular processes: Raman scattering (modification possible, but only in nanoscale environments); photochemical reactions (strong coupling based modification of excitonic reactions does not occur); electrical conductance and surface work-functions (strong coupling does not have an effect). We highlighted new ways to achieve strong coupling in open cavity structures, where easy access – e.g. for molecular chemistry – is enabled. We identified key questions for future research, including: a more meaningful strong coupling criterion, the important role of additional cavity modes (modes that are inevitably present but usually ignored in theories), and the role of molecular disorder (our metamaterial approach may be valuable here). In terms of harnessing the results in the context of applications, we are directly involved in two major follow-on projects with this goal.
We helped to significantly clarify what role strong coupling may play in a number of molecular processes: Raman scattering (modification possible, but only in nanoscale environments); photochemical reactions (strong coupling based modification of excitonic reactions does not occur); electrical conductance and surface work-functions (strong coupling does not have an effect). We highlighted new ways to achieve strong coupling in open cavity structures, where easy access – e.g. for molecular chemistry – is enabled. We identified key questions for future research, including: a more meaningful strong coupling criterion, the important role of additional cavity modes (modes that are inevitably present but usually ignored in theories), and the role of molecular disorder (our metamaterial approach may be valuable here). In terms of harnessing the results in the context of applications, we are directly involved in two major follow-on projects with this goal.