Sintered body, sputtering target, film, quantum cascade laser, and method of film formation

A quantum cascade laser comprising:

• a semiconductor substrate;
• and
• an active layer provided on the semiconductor substrate, having a cascade structure in which quantum well emission layers and injection layers are alternately laminated by multistage-laminating unit laminates each formed of the quantum well emission layer and the injection layer, and configured to generate light due to inter-subband transition in a quantum well structure, wherein
• an anti-reflection film is provided on an emission end surface from which light having a predetermined wavelength generated in the active layer is emitted,
• the anti-reflection film is formed of a single layer film comprising (i) cerium oxide and cerium fluoride or (ii) cerium oxide and yttrium fluoride, and
• wherein a content of the cerium fluoride is 10 to 30% by mass with respect to a total mass of the cerium oxide and the cerium fluoride, or a content of the yttrium fluoride is 10 to 30% by mass with respect to a total mass of the cerium oxide and the yttrium fluoride.