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This paper reports the observation of ultra-superluminal pulse propagation inGaAs/AlGaAs multiple contact heterostuctures in a superradiant emission regime,and shows definitively that it is a different class of emission fromconventional spontaneous or stimulated emission. It is shown that coherentpopulation gratings induced in the semiconductor medium under strong electricalpumping have great impact in causing a decrease of the group refractive indexin the range of 5-40%. This decrease is much greater than that which would beobserved due to conventional carrier depletion or chirp mechanisms. Thedecrease in refractive index in turn causes faster-than-c propagation offemtosecond pulses. The measurement also shows unequivocally the exist ofcoherent amplification of electromagnetic pulses in semiconductors at roomtemperature, the coherence being strongly enhanced by interactions of the lightwith coherent transient gratings locked to carrier gratings. This pulsegeneration technique can be anticipated to have great potential in applicationswhere highly coherent femtosecond optical pulses must be generated on demand

Pulse generation with ultra-superluminal pulse propagation in  semiconductor heterostructures by superradiant phase transition enhanced by  transient coherent population gratings