Something from nothing: linking molecules with virtual light

Abstract

A new research field is emerging in which ensembles of molecules are collectively hybridised with light in a process known as strong coupling. This hybridisation leads to the formation of new states that are part light and part matter, states known as polaritons. Here we offer an entry point into the field of molecular strong coupling. We include an overview of the essential phenomena and an introduction to the conceptual framework – considerable use is made of simple classical physics models since they are helpful in developing an intuitive understanding. Open questions are identified and discussed, as well as some of the exciting experimental and theoretical challenges that lie ahead.

Keywords:

• Molecule
• vacuum
• dipole
• nanophotonics
• polariton
• cavity

Acknowledgements

It is our pleasure to acknowledge many of those with whom we have discussed this topic over several years. The authors would like in particular to thank: Jean-Michel Raimond, Simon Horsley, Pepijn Pinske, Vladan Vuletic, Steve Barnett, Henry Fernandez, Kishan Menghrajani, Philip Thomas (Figure 2), Adarsh Vasista and Wai Jue Tan. The authors report there are no competing interests to declare.

Data access statement

The research data supporting this publication are openly available from ORE at: https://doi.org/10.24378/exe.4084.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 The idea of thinking of strong coupling as a kind of alchemy of the vacuum is a lovely play on words and comes from the pioneer in molecular strong coupling, Thomas Ebbesen.

2 Note that in the literature what we refer to here as ‘the cavity mode’ is also often referred to generically as ‘the confined light field’.

3 See Section 4 for a discussion on the difference between these types of state.

4 For the mechanical oscillator model discussed in this section, losses are usually in the form of friction.

5 The lineshape is given by $[(\omega -{\omega }_{+}{)}^2+4(\alpha \gamma +\beta \kappa {)}^2{]}^{-1}+[(\omega -{\omega }_{-}{)}^2+4(\alpha \gamma +\beta \kappa {)}^2{]}^{-1}$, where α and β are the molecular and photonic weights of each mode respectively, and $\alpha +\beta =1$. At zero detuning, $\alpha =\beta =1/2$.

6 Note that Rydberg states continue to be an important testing ground for strong coupling physics, see Ref. [32].

7 Note that making a quantitative connection between the oscillations seen in panel (a) and the predicted Rabi frequency is quite subtle, see Ref. [31] for details.

8 We used the N + 1 model in Section 3 to describe N molecules coupled to a single photonic mode, but the maths has precisely the same form for multiple photonic modes coupled to a single molecular mode. ${\omega }_{\mathrm{A}}$ and ${\omega }_{\mathrm{B}}$ must simply be associated with the appropriate modes.

Additional information

Funding

The authors also acknowledge funding from the EPSRC proiject ‘Molecular Photonic Breadboards’ grant EP/T012455/1, and W. L. B. acknowledges funding from the ERC ‘photmat’ project (ERC-2016-AdG-742222, www.photmat.eu).

Notes on contributors

M. S. Rider

M. S. Rider is a postdoctoral research fellow at the University of Exeter working in the areas of theoretical nanophotonics and plasmonics. She uses analytical and computational approaches to study the rich behaviour of matter in complex photonic environments, increasing our understanding of fundamental light-matter interactions with subsequent applications in technology. She works closely with experimental colleagues, as combining the methods of theory and experiment is crucial for understanding physical phenomena.

W. L. Barnes

W. L. Barnes is Professor of Photonics at the University of Exeter. He has a long-standing interest in the interaction between light and molecules, especially in trying to understand how nano-structured environments modify such interactions. Areas of current interest are intermolecular energy transfer and ensemble strong coupling. Trained as an experimentalist he sees the interaction between experiment and theory as critical in developing conceptual understanding.