Two mechanisms for achieving lower terahertz range frequencies in quantum cascade structures
with two quantum wells based on GaAs/AlGaAs compounds are proposed. The first mechanism is based on
the introduction of composite quantum wells consisting of a narrow (~2 nm) quantum well with a low potential barrier, being within the main wide quantum well. The second mechanism is based on barriers with
unequal heights, arranged in front of and behind the composite quantum well. Optimized quantum cascade laser structures emitting in the regions of ~2.15 and ~1.35 THz are calculated
