Quantum cascade laser

A quantum cascade laser comprising:

• a substrate;
• and
• an active layer provided on the substrate, wherein
• the active layer has a cascade structure in which a unit laminate including a light emission layer which generates light and an injection layer to which electrons are transported from the light emission layer is laminated in multiple stages,
• the light emission layer and the injection layer each have a quantum well structure in which quantum well layers and barrier layers are alternately laminated,
• a separation layer including a separation quantum well layer which is the quantum well layer having a layer thickness smaller than an average layer thickness of the quantum well layers included in the light emission layer and smaller than an average layer thickness of the quantum well layers included in the injection layer is provided between the light emission layer and the injection layer in the unit laminate, and
• the unit laminate includes an upper light emission level, a lower light emission level, and a non-linear level due to a ground level of the separation quantum well layer in a subband level structure due to the quantum well structure.