Semiconductor laser

1. A semiconductor laser comprising:

a substrate having a principal surface;

a mesa structure disposed on the principal surface of the substrate, the mesa structure extending in a direction of an axis parallel to the principal surface, the mesa structure including an active region that includes a quantum well structure, the active region having a top surface, a bottom surface, a first side surface, a second side surface, a third side surface, and a fourth side surface;

an emitter region including a first-conductivity-type semiconductor on at least one of the first side surface, the second side surface, the top surface, and the bottom surface of the active region; and

a collector region including a quantum filter structure on at least one side surface of the active region, the quantum filter structure having a plurality of layers, each of the layers extending along a plane perpendicular to the principal surface of the substrate, wherein

the emitter region is in contact with the active region so as to supply carriers to the active region

the quantum filter structure of the collector region is in contact with the active region so as to selectively receive the carriers from the active region,

the active region emits light by the carriers,

the collector region is separated from the emitter region by the active region being therebetween on the mesa structure,

the first side surface and the second side surface extend in the direction of the axis, and

the third side surface extends in a direction intersecting the axis.

2. The semiconductor laser according to claim 1 , wherein the quantum well structure has an energy band structure with a subband structure that provides a higher energy level and a lower energy level having an energy lower than that of the upper energy level, and

the quantum filter structure includes a superlattice structure that passes carriers at the lower energy level with a higher transmission probability than that of carriers at the higher energy level.

3. The semiconductor laser according to claim 2 ,

wherein the quantum well structure further provides a relaxation energy level having an energy lower than that of the lower energy level.

4. The semiconductor laser according to claim 1 , wherein

the quantum well structure includes a plurality of unit cells each having a first well layer, a second well layer, a first barrier layer, and a second barrier layer,

the first barrier layer separates the first well layer from the second well layer, and

the first well layer separates the first barrier layer from the second barrier layer.

5. The semiconductor laser according to claim 4 , wherein

the unit cells are arranged in a direction of an axis perpendicular to the principal surface,

and

the first barrier layer has a thickness smaller than that of the second barrier layer.

6. The semiconductor laser according to claim 4 ,

wherein the first barrier layer is partially or completely doped with a dopant of the first-conductivity-type.

7. The semiconductor laser according to claim 1 , wherein

the quantum filter structure includes a first III-V compound semiconductor layer containing three or more elements including aluminum as a group III constituent element and a second III-V compound semiconductor layer containing three or more elements including gallium as a group III constituent element,

the first III-V compound semiconductor layer serves as a barrier layer,

the second III-V compound semiconductor layer serves as a well layer, and

the first and second III-V compound semiconductor layers are alternatively arranged on at least one of the side surfaces of the active layer.

8. The semiconductor laser according to claim 1 , further comprising a first electrode disposed on the emitter region and a second electrode disposed on the collector region, wherein

the collector region includes a first-conductivity-type semiconductor,

the first electrode is electrically connected to the first-conductivity-type semiconductor of the emitter region, and

the second electrode is electrically connected to the first-conductivity-type semiconductor of the collector region.