Using a tunable midinfrared light source we study optical transmission changes of an electrically driven GaAs/AlGaAs quantum-cascade structure without resonator. For forward bias, we observe a transmission increase in the spectral range around the electroluminescence maximum which is due to resonant optical amplification. The observed transmission increase is enhanced up to 20-fold with respect to a bare active region by the internal field enhancement in the quantum-cascade structure and by interference effects in the midinfrared beam. Model calculations account quantitatively for this behavior.

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