Lasing properties of GaAs/(Al,Ga)As quantum-cascade lasers as a function of injector doping density

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Abstract
The lasing properties of GaAs/Al 0.33 Ga 0.67 As quantum-cascade lasers are investigated as a function of injector doping concentration n s between 2×10 11 and 1×10 12 cm −2 per period. Lasing is observed for n s ⩾3.5×10 11 cm −2 , with optimal lasing properties (minimum of the threshold current and maximum of the modified characteristic temperature) for n opt ≈6×10 11 cm −2 . With increasing n s up to n opt , the lasing energy of 115 meV exhibits first a blueshift to 135 meV, followed by a redshift to 120 meV for higher doping levels. This shift of the lasing energy as a function of n s is discussed in terms of changes in the field distribution, occupation of additional levels above the upper laser level, and electron–electron interactions.