Coherent electron — hole state and femtosecond cooperative emission in bulk GaAs

The conditions for obtaining a collective coherent electron – hole state in semiconductors are discussed. The results of the experimental study of the regime of cooperative recombination of high-density electrons and holes (more than 3 × 10¹⁸ cm^-3) in bulk GaAs at room temperature are presented. It is shown that the collective pairing of electrons and holes and their condensation cause the formation of a short-living coherent electron – hole BCS-like state, which exhibits radiative recombination in the form of high-power femtosecond optical pulses. It is experimentally demonstrated that almost all of the electrons and holes available are condensed at the very bottoms of the bands and are at the cooperative state. The average lifetime of this state is measured to be of about 300 fs. The dependences of the order parameter (the energy gap of the spectrum of electrons and holes) and the Fermi energy of the coherent BCS state on the electron – hole concentration are obtained.