Quantum cascade laser element

A quantum cascade laser (QCL) element having a semiconductor superlattice sandwiched by a pair of electrodes,

• wherein the semiconductor superlattice has an active region that emits electromagnetic waves of a first frequency in a THz range during application of an external voltage for operation through the pair of electrodes,
• wherein the active region has a plurality of unit structures each of which is stacked with one another to form a multi-layered structure,
• wherein each unit structure is made of first to fourth well layers that are stacked in this order and are separated from one another by at least one barrier layer,
• wherein an energy level structure for an electron in one unit structure of the active region during application of a first bias electric field by the external voltage comprises, at least:
• a reception level that receives an electron transported from an upstream unit structure disposed adjacent to one side of the one unit structure, wherein the reception level is corresponding to the first well layer;
• an upper lasing level that has a lower energy value than the reception level by a first energy difference associated with an LO phonon energy in a crystal lattice forming the semiconductor superlattice and receives an electron scattered by an LO phonon in the crystal lattice from the reception level, wherein the upper lasing level is corresponding to the second well layer;
• a lower lasing level that has a lower energy value than the upper lasing level by a photon energy value of an electromagnetic wave for the first frequency and receives an electron, which has lost energy due to stimulated emission, from the upper lasing level, wherein the lower lasing level is corresponding to the third well layer;
• and
• a depopulation level that has a lower energy value than the lower lasing level by a second energy difference associated with the LO phonon energy, receives an electron scattered by LO phonons in the crystal lattice from the lower lasing level, and transports an electron toward a downstream unit structure disposed adjacent to the other side of the one unit structure, wherein the depopulation level is corresponding to the fourth well layer, and
• wherein, during application of a second bias electric field that is weaker than the first bias electric field, an unnecessary upper lasing level and an unnecessary lower lasing level are separated by an energy difference that is equivalent to a photon energy value corresponding to one of electromagnetic waves of frequencies different from the first frequency and whose wavefunctions for an electron have an overlap integral value with each other of 0.15 or less, wherein the unnecessary lasing upper level is either of the reception level or the depopulation level in the upstream unit structure and is one that will have a high probability of existence of an electron in the first well layer during application of the second bias electric field, and wherein the unnecessary lower lasing level is either of the upper lasing level or the lower lasing level and is one that will have a high probability of existence of an electron in the second well layer during application of the second bias electric field,
• whereby stimulated emissions of photons are suppressed for electromagnetic waves at frequencies different from the first frequency.