Direct optical observation of band mixing in GaAs-(Al(x)Ga (1-x)As heterostructures.

We report photoluminescence measurements across the energy gap of modulation doped GaAs-(AlGa)As quantum well heterostructures. The special design of our samples enabled us to measurer spectra with polarizations along the normal and in the plane of the interfaces with the unique possibility of investigating optical transitions in the Brillouin zone from K=O to the Fermi wave vector (K(F)~= 2 x 10(6)cm(-1).