The magnetic field dependence of the midinfrared quantum-cascade laser emission spectra is used to identify the particular Wannier-Stark states responsible for the laser action in two different laser designs. The active regions in both quantum-cascade lasers are based on a modified bound-to-continuum design, but have differing degrees of coupling between the injector miniband and the bound state. The effects of the magnetic field and the injection-barrier width on the emission wavelength indicate that the laser emission in the quantum-cascade laser with less coupling between the injector and the bound state originates from a transition between the injector and extractor minibands. The transition from injector miniband to extractor miniband has both a lower energy and a lower oscillator strength than the transition originating from the bound state, but dominates because of the low population of the upper bound state. This result has important implications for further miniband engineering of quantum-cascade-laser active regions for laser action at the shortest possible wavelengths.

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The calculated shifts are depicted in Fig. 4 only for the transitions between the zeroth Landau orbits; transitions between higher-index Landau orbits are predicted to result in a much larger value of redshift and are not at all compatible with the measured values.

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