Magneto-optics of strongly correlated two-dimensional electrons in single heterojunctions

Investigations of two-dimensional (2D) electron systems in semiconductors subjected to a strong perpendicular magnetic field with the use of photoluminescence are reviewed. The foundation of the optical spectroscopy method using the radiative recombination of 2D electrons with photoexcited holes bound to accepters in a delta-doped monolayer in GaAs/AlxGa1-xAs single heterojunctions is presented. Optical spectroscopy studies of the energy spectra of 2D electrons imposed on transverse magnetic fields in the regimes of the integer and fractional quantum Hall effects are discussed. The relationship between the mean energy of the 2D electron gas and the first moment of their radiative recombination is analysed. It is shown that the magnetic field dependence of the first moment provides a method to measure the cyclotron, enhanced spin and quasiparticle energy gaps at the same time. Therefore it is shown how magneto-optics 'see' the ground state of interacting 2D electrons in the extreme quantum limit and how an optical 'tool' is efficient for the determination of Coulomb gaps of incompressible Fermi fluids in the fractional quantum Hall effect. Finally optical observations and studies of the Wigner crystallization of 2D electrons are presented. The corresponding liquid-solid phase diagram is discussed.