Quantum cascade laser device

A quantum cascade laser device comprising:

• a semiconductor substrate that includes a first surface and a second surface opposite to the first surface:
• an active layer provided on the first surface of the semiconductor substrate and in which a cascade structure in which a light emission layer and an injection layer are alternately laminated is formed by laminating a unit laminate including the light emission layer and the injection layer in multiple stages;
• a first clad layer provided on a side of the active layer opposite to the semiconductor substrate side and having a doping concentration of impurities of less than 1×1017 cm−3;
• a first electrode provided on a side in which the active layer is disposed with respect to the semiconductor substrate and electrically connected to the first clad layer;
• and
• a second electrode provided on the second surface of the semiconductor substrate and electrically connected to the semiconductor substrate, wherein
• the unit laminates included in the active layer each include, in their subband level structure:
• a first emission upper level;
• a second emission upper level having an energy level higher than that of the first emission upper level;
• and
• at least one emission lower level having an energy level lower than that of the first emission upper level,
• the active layer is configured to generate light having a center wavelength of 10 μm or more due to electron transition between at least two levels of the first emission upper level, the second emission upper level, and the at least one emission lower level in the light emission layer in each of the unit laminates,
• a doping concentration of impurities in the semiconductor substrate is 5×1015 cm−3 or more and less than 1×1017 cm−3, and
• where the second surface overlaps with the active layer in a thickness direction of the semiconductor substrate, the entire second surface is formed flat.