Absorption in the fractional quantum Hall regime: trion dichroism and spin polarization.

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0

Fermi-edge singularity of spin-polarized electrons.

We study the absorption spectrum of a two-dimensional electron gas (2DEG) in a magnetic field. We find that at low temperatures, when the 2DEG is spin polarized, the absorption spectra, which correspond to the creation of spin up or spin down electrons, differ in magnitude, linewidth, and filling factor dependence. We show that these differences can be explained as resulting from the creation of a Mahan exciton in one case, and of a power law Fermi-edge singularity in the other.