'Scuola Normale Superiore - Edizioni della Normale'
Abstract
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