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Quantum Electronics

Properties of electrons and holes during femtosecond cooperative emission

Petr P Vasil'ev, Hirofumi Kan, H Ohta, Teruo Hiruma

Published Oct 31, 2001• 5 citations
Publisher

Abstract

Spectral and energetic characteristics of the cooperative recombination of high density electrons and holes in bulk GaAs are experimentally studied at room temperature. It is shown that the properties and parameters of femtosecond superradiant pulses are conditioned by the collective properties of electrons and holes. Electron — hole pairing and the formation of a short-living coherent e — h BCS state distinguish strongly the regime of cooperative emission from all radiative e — h recombination regimes, which have been observed earlier. The dependences of the energy gap (the order parameter), the Fermi energy, and the band gap of the coherent e — h BCS state on the concentration of electron-hole pairs are obtained.

DOI
10.1070/qe2001v031n10abeh002067

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Spectral and energetic characteristics of the cooperative recombination of high density electrons and holes in bulk GaAs are experimentally studied at room temperature. It is shown that the properties and parameters of femtosecond superradiant pulses are conditioned by the collective properties of electrons and holes. Electron — hole pairing and the formation of a short-living coherent e — h BCS state distinguish strongly the regime of cooperative emission from all radiative e — h recombination regimes, which have been observed earlier. The dependences of the energy gap (the order parameter), the Fermi energy, and the band gap of the coherent e — h BCS state on the concentration of electron-hole pairs are obtained.
The effect of the nonstationary condensation of electron-hole pairs in GaAs/AlGaAs p-i-n heterostructures at room temperature under the conditions of generation of superradiance pulses was demonstrated in an explicit form. It was found that a dramatic decrease in spontaneous emission from the entire conduction band and a rapid electron transition to the very bottom of the band occurred at the earliest stages of the development of a superradiance pulse. The condensation of electrons at the band bottom resulted in the formation of a nonequilibrium coherent cooperative state, the recombination of which was observed previously as high-power femtosecond superradiance pulses.
A review of an experimental study of superradiance in semiconductor inorganic structures is presented. It is demonstrated that unique properties of superradiant emission are determined by unusual properties of electrons and holes, namely, the formation of BCS-like state in a system of collectively paired electrons and holes. This can adequately explain all features of superradiance, including its femtosecond pulse duration, record peak power, optical spectrum, spatial and temporal coherency and macroscopically large fluctuations. The effect of non-equilibrium condensation of electrons and holes in the phase domain at room temperature is experimentally demonstrated. The critical temperature of condensation in a strongly degenerate system of electrons and holes is finally theoretically estimated.
The spectra of spontaneous emission accompanying the generation of femtosecond superradiant pulses in GaAs/AlGaAs heterostructures are studied. It is clearly demonstrated that spontaneous emission spectra of electron–hole pairs, which have been left in the semiconductor after the formation of a coherent collective e–h state, correspond to the strong overheating of carriers. This phenomenon can be explained by the effect of dynamic cooling and nonequilibrium condensation of collectively paired carriers at the bottoms of the bands during the superradiant emission, which was observed earlier.
We present results of detailed investigations of light emission from semiconductor multiple quantum wells at low temperatures and high magnetic fields excited by intense femtosecond laser pulses. The intensity and linewidth as well as the directional and statistical properties of photoemission strongly depended on the magnetic field strength and pump laser fluence. We also investigated the effects of spot size, temperature, excitation geometry, and excitation pulse width on the emission properties. The results suggest that the initially incoherent photoexcited electron-hole pairs spontaneously form a macroscopic coherent state upon relaxation into the low-lying magnetoexcitonic states, followed by the emission of a superfluorescent burst of radiation. We have developed a theoretical model for superfluorescent emission from semiconductor quantum wells, which successfully explained the observed characteristics.
The paper examines the conditions for and mechanism of the onset of a nonequilibrium superradiant phase transition in a bulk semiconductor at room temperature in the strong coupling regime.Experimental data are analyzed, and the characteristic properties of this phase transition are compared with previously studied quantum phase transitions in ultracold gases in traps, ensembles of quantum dots, superconducting qubits, and some others.All the basic properties of the previously discovered collective state formed in the process of induced condensation of electron-hole pairs at room temperature are shown to correspond to those of the superradiant phase transition observed in other media.
The quantum properties of superradiant pulses generated at the radiative recombination of an electron–hole condensate in semiconductor heterostructures at room temperature have been studied using optical homodyne tomography. Signatures of the quantum entanglement of superradiant states, which are superpositions of two coherent states, have been revealed. It has been demonstrated that reconstructed Wigner functions under certain conditions are very similar to the Wigner functions of the Schrödinger cat states.