An interband cascade gain medium, comprising: an active gain region comprising a plurality of cascading stages, each of the cascading stages including an active gain quantum well region (101a, 201) comprising one or more active electron quantum wells (201b, 201c) and one or more active hole quantum wells, a hole injector region (101b, 202) comprising two hole quantum wells, and an electron injector region (101c, 203) comprising one or more electron quantum wells (112, 113, 203a), a final electron quantum well of the electron injector region of a first stage being separated from an adjacent first active electron quantum well of the active gain quantum well region of a second stage by an electron barrier; the thicknesses of the two hole quantum wells in the hole injector region and a thickness of at least one active hole quantum well in the active gain quantum well region being configured such that the valence band maximum of the hole injector states lies at least 100 meV lower in energy than the valence band maximum of the at least one active hole quantum well in the active gain quantum well region; wherein the two hole quantum wells in the hole injector region are GaSb hole quantum wells and the active hole quantum well in the active gain quantum well region is a GaInSb hole quantum well; characterized in that the final electron quantum well of the electron injector region (101c, 203) of the first stage has a thickness that is between 85 and 110 % of a thickness of the adjacent first active electron quantum well of the second stage; and in that the two hole quantum wells in the hole injector region have a combined thickness of 40 to 85. an active gain region comprising a plurality of cascading stages, each of the cascading stages including an active gain quantum well region (101a, 201) comprising one or more active electron quantum wells (201b, 201c) and one or more active hole quantum wells, a hole injector region (101b, 202) comprising two hole quantum wells, and an electron injector region (101c, 203) comprising one or more electron quantum wells (112, 113, 203a), a final electron quantum well of the electron injector region of a first stage being separated from an adjacent first active electron quantum well of the active gain quantum well region of a second stage by an electron barrier; the thicknesses of the two hole quantum wells in the hole injector region and a thickness of at least one active hole quantum well in the active gain quantum well region being configured such that the valence band maximum of the hole injector states lies at least 100 meV lower in energy than the valence band maximum of the at least one active hole quantum well in the active gain quantum well region; wherein the two hole quantum wells in the hole injector region are GaSb hole quantum wells and the active hole quantum well in the active gain quantum well region is a GaInSb hole quantum well; characterized in that the final electron quantum well of the electron injector region (101c, 203) of the first stage has a thickness that is between 85 and 110 % of a thickness of the adjacent first active electron quantum well of the second stage; and in that the two hole quantum wells in the hole injector region have a combined thickness of 40 to 85.