  We report on an extensive experimental study of intersubband excitations in
the THz range arising from the coupling between a quantum well and a
zero-dimensional metal-metal microcavities. Because of the conceptual
simplicity of the resonators we obtain an extremely predictable and
controllable system to investigate light-matter interaction. The experimental
data is modelled by combining a quantum mechanical approach with an effective
medium electromagnetic simulation that allows us to take into account the
losses of the system. By comparing our modelling with the data we are able to
retrieve microscopic information, such as the electronic populations on
different subbands as a function of the temperature. Our modelling approach
sets the base of a designer tool for intersubband light-matter coupled systems.
