Cooperative Recombination of a Quantized High-Density Electron-Hole   Plasma
  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. 
Y. D. Jho, X. Wang, J. Kono, D. H. Reitze, X. Wei, A. A. Belyanin, V.   V. Kocharovsky, Vl. V. Kocharovsky, and G. S. Solomon