  The optical response of a heavily doped quantum well, with two occupied
subbands, has been investigated as a function of the electronic density. It is
shown that the two optically active transitions are mutually coupled by
dipole-dipole Coulomb interaction, which strongly renormalizes their absorption
amplitude. In order to demonstrate this effect, we have measured a set of
optical spectra on a device in which the electronic density can be tuned by the
application of a gate voltage. Our results show that the absorption spectra can
be correctly described only by taking into account the Coulomb coupling between
the two transitions. As a consequence, the optical dipoles originating from
intersubband transitions are not independent, but rather coupled oscillators
with an adjustable strength.
