  Polarization-controlled synchrotron radiation was used to map the electronic
structure of buried conducting interfaces of LaAlO$_3$/SrTiO$_3$ in a resonant
angle-resolved photoemission experiment. A strong dependence on the light
polarization of the Fermi surface and band dispersions is demonstrated,
highlighting the distinct Ti 3d orbitals involved in 2D conduction. Samples
with different 2D doping levels were prepared and measured by photoemission,
revealing different band occupancies and Fermi surface shapes. A direct
comparison between the photoemission measurements and advanced first-principle
calculations carried out for different 3d-band fillings is presented in
conjunction with the 2D carrier concentration obtained from transport
measurements.
