The interaction of light with matter in usually described in terms of
photons absorbed, emitted, or scattered by massive objects. The
conceptual simplicity of such a framework rests on the small strength
of the electromagnetic force, which allows us to describe
electromagnetic interactions in terms of first order (absorption,
emission) and second order (scattering) processes.

In carefully engineered nanostructures it is nevertheless possible, by
confining photons in sub-wavelength volumes and increasing the density
of dipoles, to engineer very large interactions between light and
matter.
A novel interaction regime can then be achieved, usually referred to
as ultrastrong coupling, in which electromagnetism becomes
non-perturbative. This leads to a completely novel phenomenology in
which light and matter loose their individual identity [1].

In this talk I will introduce the basic theory of non-perturbative
cavity quantum electrodynamics, present some of its most important
experimental realisations, and discuss a few aspects of its rich
phenomenology.

Slides can be found here.

[1] Ultrastrong coupling between light and matter
A. F. Kockum, A. Miranowicz, S. De Liberato, S. Savasta, and F. Nori
Nature Reviews Physics 1, 19 (2019)