  Cooperative spontaneous recombination (superfluorescence) of electron-hole
plasmas in semiconductors has been a challenge to observe due to ultrafast
decoherence. We argue that superfluorescence can be achieved in
quantum-confined semiconductor systems and present experimental evidence for
superfluorescence from high-density photoexcited electron-hole plasmas in
quantum wells under high magnetic fields (> 20 T). 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. Changes in the linewidth, carrier density, and magnetic field
scaling of the emission spectra correlate precisely with the onset of random
directionality and are consistent with cooperative recombination. We further
investigate the effects of spot size, temperature, and excitation geometry on
the emission properties.
