Flat minibands with spatially symmetric wavefunctions in intersubband superlattice light emitters

A quantum cascade (QC) superlattice (SL) laser comprising:

• a core region including a multiplicity of repeat units, each repeat unit including a unipolar radiative transition (RT) SL region, each of said RT SL regions comprising a plurality of quantum well (QW) regions interleaved with a plurality of first barrier regions, said QW regions having energy states characterized by upper and lower minibands,
• a pair of cladding regions bounding said core region, and
• electrodes for applying an electric field to said laser effective to cause said RT regions to generate laser action at an energy determined by upper and lower energy states of said QW regions, said upper and lower levels being located within said upper and lower minibands, respectively, and in the presence of said field the upper miniband of one RT region being aligned with the lower miniband of an adjacent RT region, in at least a first subset of said QW regions within each of said RT regions the thicknesses of said QW regions increasing from QW region to QW region in the direction of said applied field, and in at least a second subset of said first barrier regions within each of said RT regions the thicknesses of said first barrier regions varying from barrier region to barrier region in the direction of said applied field, so that, in the absence of said applied field, said upper and lower energy states are located each at different energies from region to region within said first subset, and so that, in the presence of said applied field, an essentially flatband condition of both said upper and lower minibands exists across adjacent RT regions, characterized in that
• each of said QW regions includes at least one second barrier region that splits said QW region into a multiplicity of coupled sub-wells, the thicknesses of said second barrier layers being such that the wavefunctions of at least said upper and lower states are essentially spatially symmetric with respect to the midplane of each RT region and said minibands can be independently positioned within said QW regions, and
• said second barrier regions are sufficiently thin that said states (1) are split beyond their natural broadening and (2) contribute to different minibands in each RT region.