Direct observation of laser level occupation and injector coupling in undoped GaAs/(Al,Ga)As quantum-cascade structures

Abstract

The occupation of the laser levels in an undoped GaAs/(Al,Ga)As quantum-cascade structure is directly detected by interband photoluminescence spectroscopy. The upper laser level and the injector state exhibit an anticrossing behavior with a splitting of about 10 meV, which is attributed to a strong coupling between them. The energy splitting matches well the oscillation period observed in recent, ultrafast, coherent electron-transport experiments by F. Eickemeyer et al. [Phys. Rev. Lett. 89, 047402 (2002)]. A comparison with the calculated transition energies qualitatively confirms the anticrossing behavior in the vicinity of the threshold field strength. Furthermore, the measured energy difference between the upper and lower laser level agrees with the lasing energies of a set of complete quantum-cascade lasers. At the same time, the variation of the lasing energies corresponds to the energy splitting between the upper laser level and the injector state.