Superfluorescence from Ultrahigh-Density Magneto-excitons in Quantum Wells
The spontaneous appearance of macroscopic coherence is among the most dramatic cooperative phenomena in physics. In quantum electrodynamics, there exists a self-organization process of fundamental importance, called superfluorescence (SF), in which a collection of inverted atoms is incoherently prepared but macroscopic coherence builds up spontaneously. The resultant macroscopic dipole decays superradiantly, producing a burst of coherent radiation. SF has been observed in atomic gases and rarefied impurities in crystals but hitherto unobserved in semiconductors because of ultrafast decoherence. Here, we present experimental evidence for SF through cooperative recombination of ultrahigh-density excitons in semiconductor quantum wells in a strong perpendicular magnetic field. At a critical magnetic field and excitation fluence, we observe a sudden exciton depopulation accompanied by picosecond bursts of radiation. In addition, a clear transition from omnidirectional emission to a randomly directed but highly collimated beam was observed.