Light–matter interactions with photonic quasiparticles


Interactions between light and matter play an instrumental role in spectroscopy, sensing, quantum information processing and lasers. In most of these applications, light is considered in terms of electromagnetic plane waves propagating at the speed of light in vacuum. As a result, light–matter interactions can usually be treated as very weak and captured at the lowest order in quantum electrodynamics. However, progress in understanding the coupling of photons to material quasiparticles (plasmons, phonons and excitons) brings the need for a generalized view of the photon at the core of every light–matter interaction. In this new picture, the photon can have greatly different polarization and dispersion and be confined to the scale of a few nanometres. Such photonic quasiparticles enable a wealth of otherwise unobservable light–matter interaction phenomena, in interactions with both bound and free electrons. This Review focuses on the theoretical and experimental developments in realizing new light–matter interactions with photonic quasiparticles. Examples include room-temperature strong coupling, ultrafast ‘forbidden’ transitions in atoms and new applications of the Cherenkov effect, as well as breakthroughs in ultrafast electron microscopy and new concepts for compact X-ray sources.

Key points

  • Photonic quasiparticles are quantized time-harmonic solutions of Maxwell’s equations in an arbitrary inhomogeneous, dispersive and possibly non-local medium. Surface plasmon–polaritons, phonon–polaritons, exciton–polaritons and all other polaritons are examples of photonic quasiparticles. Moreover, photons in cavities, localized and bulk plasmons, and even acoustic phonons are also special cases of photonic quasiparticles.

  • Certain photonic quasiparticles can confine electromagnetic fields to dimensions much smaller than the wavelength of a photon. Specifically, polaritons in 2D materials, such as graphene and hexagonal boron nitride, allow simultaneously high confinement and low optical losses.

  • Macroscopic quantum electrodynamics prescribes the quantization of the photonic quasiparticles in an arbitrary medium, and can describe the interaction of any photonic quasiparticle with any type of quantum matter (for example, arbitrary emitters) in terms of elementary emission and absorption processes.

  • For bound-electron emitters, the confinement of photonic quasiparticles enables ultrafast spontaneous emission and few-molecule strong coupling, as well as possible new phenomena, such as forbidden transitions and multiphoton spontaneous emission.

  • For free-electron emitters, photonic quasiparticles enable new applications of the Cherenkov effect in particle detectors, as well as new concepts for compact X-ray sources and new applications in ultrafast electron microscopy.

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Fig. 1: Diagrammatic representation of physical processes contained within macroscopic quantum electrodynamics.
Fig. 2: Microscopic origin of photonic quasiparticles.
Fig. 3: Photonic manipulations with highly confined polaritons.
Fig. 4: Bound-electron interactions with photonic quasiparticles.
Fig. 5: Free-electron spontaneous radiation.
Fig. 6: The similarity of electron–photon, electron–plasmon and electron–phonon interactions.
Fig. 7: Effects enabled by strong fields of photonic quasiparticles.


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The authors acknowledge Y. Kurman, A. Gorlach, O. Eyal, J. Sloan, T. Christensen, D. Basov, S. Scheel and M. Segev for their helpful comments on the Review. The authors also acknowledge M. Soljacic and J. Joannopoulos for the fruitful collaborations that led to this Review. N.R. was supported by Department of Energy Fellowship DE-FG02-97ER25308 and a Dean’s Fellowship of the MIT School of Science. I.K. was supported by the Azrieli Faculty Fellowship, the ERC starting grant NanoEP 851780 from the European Research Council, the Israel Science Foundation grant number 831/19 and the GIF Young Scientists’ Program by the German-Israeli Foundation for Scientific Research and Development.

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Correspondence to Nicholas Rivera or Ido Kaminer.

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Nonlinear Compton/Thomson scattering

The scattering of multiple photons from a free electron, leading to many photons being converted into a single photon with a much higher frequency.

Hong–Ou–Mandel effect

A quantum effect in which two indistinguishable photons, incident on a 50/50 beamsplitter, never appear in different output ports of the beamsplitter, owing to quantum interference.

Lamb shifts

Shifts in the energy levels of a quantum emitter, due to virtual absorption and re-emission of photons, or more generally, photonic quasiparticles (Fig. 1).

Casimir–Polder forces

Forces on an emitter in the vicinity of an inhomogeneous optical structure, arising from the spatial inhomogeneity of the Lamb shift.

Purcell effect

The enhancement of spontaneous emission of an excited quantum emitter in the vicinity of an optical structure, relative to its spontaneous emission in free space.

Vacuum field

The fluctuating electromagnetic field that exists in the absence of photonic quasiparticles, owing to quantum mechanics.

Epsilon-near-zero materials

Materials for which the real part of the permittivity, at some frequency, is nearly zero (limited by losses).

Cherenkov cone

Photons emitted via the Cherenkov effect take the form of a cone, symmetrical around the motion of the emitting particle.

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Rivera, N., Kaminer, I. Light–matter interactions with photonic quasiparticles. Nat Rev Phys 2, 538–561 (2020).

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