  The spectroscopic properties of a spin polarized two-dimensional hole gas are
studied in modulation doped (Cd,Mn)Te quantum wells. The giant Zeeman effect
induces a significant spin splitting even at very small values of the applied
field. Several methods of measuring the carrier density (Hall effect, filling
factors of the Landau levels at high field, various manifestations of
Moss-Burstein shifts) are described and calibrated. The value of the spin
splitting needed to fully polarize the hole gas, evidences a strong enhancement
of the spin susceptibility of the hole gas due to carrier-carrier interaction.
At small values of the spin splitting, whatever the carrier density (non zero)
is, photoluminescence lines are due to the formation of charged excitons in the
singlet state. Spectral shifts in photoluminescence and in transmission
(including an "excitonic Moss-Bustein shift") are observed and discussed in
terms of excitations of the partially or fully polarized hole gas. At large
spin splitting, and without changing the carrier density, the singlet state of
the charged exciton is destabilized in favour of a triplet state configuration
of holes. The binding energy of the singlet state is thus measured and found to
be independent of the carrier density (in contrast with the splitting between
the charged exciton and the neutral exciton lines). The state stable at large
spin splitting is close to the neutral exciton at low carrier density, and
close to an uncorrelated electron-hole pair at the largest values of the
carrier density achieved. The triplet state gives rise to a characteristic
double-line structure with an indirect transition to the ground state (with a
strong phonon replica) and a direct transition to an excited state of the hole
gas.
