  We investigate photoluminescence from a high-density electron-hole plasma in
semiconductor quantum wells created via intense femtosecond excitation in a
strong perpendicular magnetic field, a fully-quantized and tunable system. At a
critical magnetic field strength and excitation fluence, we observe a clear
transition in the band-edge photoluminescence from omnidirectional output to a
randomly directed but highly collimated beam. In addition, changes in the
linewidth, carrier density, and magnetic field scaling of the PL spectral
features correlate precisely with the onset of random directionality,
indicative of cooperative recombination from a high density population of free
carriers in a semiconductor environment.
