Experimental evidence of condensation of electron-hole pairs at room temperature during femtosecond cooperative emission

Experimental results of the investigation of spectral characteristics of cooperative emission of high-density e-h plasmas in bulk GaAs have been presented. It has been shown that the properties of electrons and holes during the cooperative recombination are essentially different from these under lasing or normal spontaneous emission. The center of line of the cooperative emission (1.405-1.406 eV) is shifted inside the band gap. It lies by about 19 meV and by nearly 40 meV lower than the peak wavelength of lasing and amplified spontaneous emission, respectively. This corresponds to the e-h condensation at the bottoms of the bands. The features of the cooperative emission can be explained in terms of e-h pairing and build-up of a transient coherent e-h BCS-like state. The estimated value of the order parameter Delta is around 2-3 meV.