Light-matter interaction in intersubband microcavities

The research work presented in this thesis is focused on the study of the optoelectronic coupling between the intersubband excitation in a two-dimensional electron gas (2DEG) and the resonant photonic mode of a planar semiconductor microcavity, in which the 2DEGs are embedded. When a generic electronic excitation interacts resonantly with a discrete cavity mode, a strong-coupling regime arises if the interaction strength of the electron-photon system (vacuum-field Rabi energy) is larger than the damping rates. This condition has been demonstrated in diverse research fields: from atomic physics to organic/semiconductor excitons coupled to a planar microcavity, to superconductor qubits coupled to microwave transmission lines. In semiconductor physics, the strong coupling results in the formation of quasi-particles termed cavity polaritons, which are the linear superposition of light and matter excitations. In 2003, the strong coupling of intersubband transitions in doped quantum wells with confined photons, and the corresponding formation of `intersubband cavity polaritons', were experimentally observed up to room temperature. In contrast to other strongly coupled systems, intersubband microcavities are more appealing due to the unique possibility of externally controlling light-matter interaction. The manipulation of polariton coupling hinges on the principle that the intensity of intersubband absorption in the active region can be controlled either through the carrier density modulation or by altering the oscillator strength of the transition. Owing to the large oscillator strength and relatively low-energy of the transition, in intersubband microcavities the vacuum-field Rabi splitting can be a significant fraction of the intersubband transition energy. Such a regime of light-matter interaction was predicted theoretically and termed as the `ultrastrong coupling regime'. The investigation of the optoelectronic coupling is here conducted in two different directions: (i) exploring suitable means for the external manipulation of intersubband cavity polaritons, (ii) realizing the conditions for observing the ultrastrong coupling regime of light-matter interaction. The devices employed in the investigation are multiple quantum well active structures embedded in intersubband microcavities - based either on dielectric mirrors or on plasmon mode resonators. The results presented in this thesis contain various experimental realizations of the external control of polariton coupling in a solid-state device, with unprecedented modulation depth and speed. Moreover the first experimental observation of the ultrastrong coupling of light-matter interaction is also reported. These are fundamental steps towards the generation of the photon pairs from vacuum fluctuations in a quantum electrodynamical scheme analogous to the well known dynamic Casimir effect, which is yet to be realized experimentally.