Ultrastrong and deep strong light-matter coupling with nanoparticle supercrystals

Most materials interact weakly with light, which is why we treat light as an external perturbation. There has been growing interest in systems where light-matter interaction is much stronger so that the it becomes comparable to other characteristic energies of the system. In the regime of ultrastrong and deep strong coupling the coupling energy approaches the bare excitation energy of a material. This regime leads to exotic effects like the population of the ground state with virtual photons, phase transitions, and a breakdown of the Purcell effect. Here I discuss how extreme light-matter coupling is systematically achieved in materials. I introduce densely packed supercrystals of gold nanoparticles that have coupling energies in the visible spectral range. I present measurements of their plasmon-polaritons and confirm the breakdown of the Purcell effect. Such supercrystals may also be used to place molecules in an extreme electromagnetic environment. If time permits, I discuss a universal model to treat light-matter coupling in systems of different dimension and the strongest light-matter coupling we can reasonably expect to achieve in materials.

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