The diversity of light-matter interactions accessible to a system is limited by the small size of an atom relative to the wavelength of the light it emits, as well as by the small value of the fine-structure constant. We developed a general theory of light-matter interactions with two-dimensional systems supporting plasmons. These plasmons effectively make the fine-structure constant larger and bridge the size gap between atom and light. This theory reveals that conventionally forbidden light-matter interactions--such as extremely high-order multipolar transitions, two-plasmon spontaneous emission, and singlet-triplet phosphorescence processes--can occur on very short time scales comparable to those of conventionally fast transitions. Our findings may lead to new platforms for spectroscopy, sensing, and broadband light generation, a potential testing ground for quantum electrodynamics (QED) in the ultrastrong coupling regime, and the ability to take advantage of the full electronic spectrum of an emitter.

In this article, we review strong light-matter coupling at the interface of materials science, quantum chemistry, and quantum photonics. The control of light and heat at thermodynamic limits enables exciting new opportunities for the rapidly converging fields of polaritonic chemistry and quantum optics at the atomic scale from a theoretical and computational perspective. Our review follows remarkable experimental demonstrations that now routinely achieve the strong coupling limit of light and matter. In polaritonic chemistry, many molecules couple collectively to a single-photon mode, whereas, in the field of nanoplasmonics, strong coupling can be achieved at the single-molecule limit. Theoretical approaches to address these experiments, however, are more recent and come from a spectrum of fields merging new developments in quantum chemistry and quantum electrodynamics alike. We review these latest developments and highlight the common features between these two different limits, maintaining a focus on the theoretical tools used to analyze these two classes of systems. Finally, we present a new perspective on the need for and steps toward merging, formally and computationally, two of the most prominent and Nobel Prize-winning theories in physics and chemistry: quantum electrodynamics and electronic structure (density functional) theory. We present a case for how a fully quantum description of light and matter that treats electrons, photons, and phonons on the same quantized footing will unravel new quantum effects in cavity-controlled chemical dynamics, optomechanics, nanophotonics, and the many other fields that use electrons, photons, and phonons.

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A fundamental building block for nanophotonics is the ability to achieve negative refraction of polaritons, because this could enable the demonstration of many unique nanoscale applications such as deep-subwavelength imaging, superlens, and novel guiding. However, to achieve negative refraction of highly squeezed polaritons, such as plasmon polaritons in graphene and phonon polaritons in boron nitride (BN) with their wavelengths squeezed by a factor over 100, requires the ability to flip the sign of their group velocity at will, which is challenging. Here we reveal that the strong coupling between plasmon and phonon polaritons in graphene-BN heterostructures can be used to flip the sign of the group velocity of the resulting hybrid (plasmon-phonon-polariton) modes. We predict allangle negative refraction between plasmon and phonon polaritons and, even more surprisingly, between hybrid graphene plasmons and between hybrid phonon polaritons. Graphene-BN heterostructures thus provide a versatile platform for the design of nanometasurfaces and nanoimaging elements.negative refraction | plasmon polariton | phonon polariton | graphene-boron nitride heterostructure P olaritons with high spatial confinement, such as plasmon polaritons in graphene (1-5) and phonon polaritons in a thin hexagonal boron nitride (BN) slab (6-15), enable control over the propagation of light at the extreme nanoscale, due to their in-plane polaritonic wavelength that can be squeezed by a factor over 100. Henceforth we use the term squeezing factor (or confinement factor) to define the ratio between the wavelength in free space and the in-plane polaritonic wavelength. The combination of tunability, low losses, and ultraconfinement (1,2,8,10,11,15) makes them superior alternatives to conventional metal plasmons and highly appealing for nanophotonic applications (3-5, 10-13, 15). Their extreme spatial confinement, however, also limits our ability to tailor their dispersion relations.Unlike the case of 2D plasmons, the coupling between metal plasmons in a metal-dielectric-metal structure dramatically changes their dispersion relation and can even flip the sign of their group velocities (16,17). This has led to exciting applications by tailoring the in-plane plasmonic refraction, giving flexibility in controlling the energy flow of light. Specifically, by flipping the sign of the group velocity of metal plasmons, plasmonic negative refraction has been predicted (16) and demonstrated (17). The negative refraction has also been extensively explored in metamaterials, metasurfaces, and photonic crystals (18-26), but they become experimentally very challenging to realize when dealing with polaritons with high squeezing factors. In contrast to metal plasmons, the group velocity of graphene plasmons (2,11,27) and all other 2D plasmons (28-32) is always positive, including that in graphene-based multilayer structures (33). This has made the in-plane negative refraction for highly squeezed 2D plasmon polaritons seem impossible to achieve.Contrary to 2D plas...