Dynamical response of double parabolically graded quantum wells

Ultrastrong light-matter coupling regime with polariton dots

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.

Strong light-matter coupling in subwavelength metal-dielectric microcavities at terahertz frequencies

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 variant Planck's over 2pi 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.

Interplay of collective excitations in quantum-well intersubband resonances

Intersubband resonances in a semiconductor quantum well (QW) display fascinating features involving various collective excitations such as Fermi-edge singularity (FES) and intersubband plasmon (ISP). Using a density matrix approach, we treated many-body effects such as depolarization, vertex correction, and self-energy consistently for a two-subband system. We found a systematic change in resonance spectra from FES- to ISP-dominated features, as QW width or electron density is varied. Such an interplay between FES and ISP significantly changes both line shape and peak position of the absorption spectrum. We found that a cancellation of FES and ISP undresses the resonant responses and recovers the single-particle features of absorption for semiconductors with a strong nonparabolicity such as InAs, leading to a dramatic broadening of the absorption spectrum.