Quantum cascade element

A quantum cascade element comprising:

• a first substrate;
• a quantum cascade layer provided at a front surface side of the first substrate, the quantum cascade layer including a plurality of quantum well layers and a plurality of barrier layers alternately stacked in a direction perpendicular to the front surface of the first substrate;
• and
• a second substrate bonded to the first substrate with the quantum cascade layer interposed, the second substrate including a photonic crystal contacting the quantum cascade layer, the photonic crystal including a plurality of recesses at a side contacting the quantum cascade layer, the plurality of recesses facing an uppermost layer of the plurality of barrier layers;
• an oxide film provided between the quantum cascade layer and the photonic crystal, the oxide film directly contacting the quantum cascade layer and the photonic crystal,
• wherein each recess of the photonic crystal includes a side face extending along the direction perpendicular to the front surface of the first substrate, and
• no oxide film is provided on the side face.