The regime of ultrastrong light-matter interaction has been investigated theoretically and experimentally, using zero-dimensional electromagnetic resonators coupled with an electronic transition between two confined states of a semiconductor quantum well. We have measured a splitting between the coupled modes that amounts to 48% of the energy transition, the highest ratio ever observed in a light-matter coupled system. Our analysis, based on a microscopic quantum theory, shows that the nonlinear polariton splitting, a signature of this regime, is a dynamical effect arising from the self-interaction of the collective electronic polarization with its own emitted field.

We have demonstrated that a metal-dielectric-metal microcavity combined with quantum well intersubband transitions is an ideal system for the generation of cavity polariton states in the terahertz region. The metallic cavity has highly confined radiation modes that can be tuned in resonance with the intersubband transition. In this system we were able to measure a very strong light-matter splitting (the Rabi splitting $2\hslash {\Omega }_R$), corresponding to 22% of the transition energy. We believe this result to be the first demonstration of intersubband polaritons in the terahertz frequency range.

We present an experimental and theoretical study of the optical properties of metal-dielectric-metal structures with patterned top metallic surfaces, in the THz frequency range. When the thickness of the dielectric slab is very small with respect to the wavelength, these structures are able to support strongly localized electromagnetic modes, concentrated in the subwavelength metal-metal regions. We provide a detailed analysis of the physical mechanisms which give rise to these photonic modes. Furthermore, our model quantitatively predicts the resonance positions and their coupling to free space photons. We demonstrate that these structures provide an efficient and controllable way to convert the energy of far field propagating waves into near field energy.

We provide a theoretical description of the coupling between the electromagnetic field and the intersubband excitations of a bidimensional electron gas. Our theory, based on the electrical dipole gauge, applies generally to an arbitrary quantum heterostructure embedded in a multilayer waveguide or in a microcavity. We show that the dipole gauge Hamiltonian takes into account the Coulomb interactions in the system, without the need of adding extra terms to the Hamilitonian. Furthermore, the bright excitations of the system appear as many-body collective plasmon modes, interacting between each other and with the light field through dipole coupling. The electrical dipole gauge therefore provides a suitable framework for the study of solid-state quantum electrodynamics phenomena that occur at very high electronic densities, such as the ultrastrong light-matter interaction.

In this Letter we investigate a low dimensional semiconductor system, in which the light-matter interaction is enhanced by the cooperative behavior of a large number of dipolar oscillators, at different frequencies, mutually phase locked by Coulomb interaction. We experimentally and theoretically demonstrate that, owing to this phenomenon, the optical response of a semiconductor quantum well with several occupied subbands is a single sharp resonance, associated with the excitation of a bright multisubband plasmon. This effect illustrates how the whole oscillator strength of a two-dimensional system can be concentrated into a single resonance independently from the shape of the confining potential. When this cooperative excitation is tuned in resonance with a cavity mode, their coupling strength can be increased monotonically with the electronic density, allowing the achievement of the ultrastrong coupling regime up to room temperature.