Superradiance (SR) refers to spontaneous quantum phase transition, in which a self-organized build up of coherent radiation within an ensemble of quasi-degenerate emitters occurs. It was first noted by Dicke back in 1954 that, with increasing their density, the collection of N radiators starts to emit much faster and stronger comparing to spontaneous emission of individuals. More precisely, when packing up N identical emitters into the space comparable with a cube of the radiation wavelength, instead of observing isotropic and exponentially decaying emission, one produces a fierce, directional radiation burst, having a lagged peak intensity scaling like N2 and N-times reduced duration with respect to the spontaneous emission. Apart from the fundamental aspect, the research for SR is motivated by the prospects in producing ultra-short, intense, coherent light pulses - in alternative to lasers. The prerequisite for creating a SR emission is a spectral uniformity of participating transitions. For that reason SR was first successfully observed in atomic and molecular ensembles. As concerns semiconductor nanostructures, nowadays widely used in optoelectronics, the evidence for SR has never been provided, as the SR effect is obscured by the spectral inhomogeneous broadening in a semiconductor matrix. The goal of this project is to achieve the first proof for SR within an ensemble of individual emitters embedded in a nanostructured semiconductor. To this aim, we will focus our efforts on donor-bound-excitons (D0X), which are excitons (Coulomb correlated electron-hole pairs) localized on donor impurities in a semiconductor, for instance Si replacing Ga atoms in a GaAs lattice. D0Xs are characterized by small, sub-meV, inhomogeneous broadening, large oscillator strength and a few hundreds ps lifetime - all acting in favor for inducing SR. Our approach will employ methods of both linear (PL, streak camera) and nonlinear (up-conversion) time-resolved optical spectroscopy.
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