Quantum cascade laser design with stepped well active region

1. A quantum cascade laser structure comprising a plurality of quantum wells and a plurality of barriers, at least a portion of which define an active region, wherein:

a photon is emitted in the active region when an electron transitions from an upper laser state in the active region to a lower laser state in the active region;

a final quantum well in the plurality of quantum wells that define the active region, the final quantum well extending below an adjacent quantum well in the active region when no bias is applied; and

the final quantum well comprises a thickness that is less than a thickness of the adjacent quantum well in the active region.

2. The quantum cascade laser structure of claim 1 , wherein the active region comprises 4 quantum wells.

3. The quantum cascade laser structure of claim 1 , wherein a final barrier in the plurality of barriers comprises an height that is greater than other barriers of the plurality of barriers.

4. The quantum cascade laser structure of claim 3 , wherein an energy difference between two energy states is created and comprises a difference between an upper laser state 4 and a parasitic state 5, and wherein the energy difference is at least about 80 meV.

5. The quantum cascade laser structure of claim 3 , the final barrier comprises Aluminum and Indium.

6. The quantum cascade laser structure of claim 5 , wherein an Aluminum to Indium proportion in the final barrier is higher than that of the other barriers in the active region.

7. The quantum cascade laser structure of claim 3 , wherein the final barrier in the active region comprises a thickness that is greater than thicknesses of the other barriers in the active region.

8. The quantum cascade laser structure of claim 1 , wherein a thickness of the final quantum well is more than about 5 Angstroms less than a thickness the adjacent quantum well in the active region.

9. The quantum cascade laser structure of claim 1 , wherein the final quantum well comprises a material that includes GaxIn1-xAs, where x varies from 0 to 1.

10. The quantum cascade laser structure of claim 1 , wherein the plurality of barriers comprise a material that includes AlyIn1-yAs, where y varies from 0 to 1.

11. The quantum cascade laser structure of claim 1 , wherein the active region resides within an active core and wherein the quantum cascade laser structure further comprises a cladding layer.

12. The quantum cascade laser structure of claim 11 , wherein the active core and the cladding layer are grown on an Indium Phosphide (InP) substrate.

13. The quantum cascade laser structure of claim 1 , wherein the final quantum well comprises Gallium and Indium.

14. The quantum cascade laser structure of claim 13 , wherein a Gallium to Indium proportion in the final quantum well is less than that of at least one of the plurality of quantum wells in the active region.

15. The quantum cascade laser structure of claim 1 , wherein the plurality of quantum wells and the plurality of barriers defines an injector region, such that electrons transport from the active region to a next adjacent active region through the injector region.

16. The quantum cascade laser structure of claim 15 , wherein the injector region comprises an entrance quantum well that comprises a depth that is greater than at least one of the plurality of quantum wells in the active region.

17. The quantum cascade laser structure of claim 15 , wherein the injector region comprises an additional barrier that comprises a height that is greater than at least one of the plurality of barriers in the active region.

18. The quantum cascade laser structure of claim 1 , wherein a lasing wavelength of the quantum cascade laser structure is from about 3 μm to about 12 μm.

19. A quantum cascade laser structure comprising at least one laser core and at least one cladding layer, forming a waveguide structure, the at least one laser core comprising a first type of semiconductor material that defines a plurality of quantum wells, the at least one laser core comprising a second type of semiconductor materials that define a plurality of barriers, wherein:

the plurality of quantum wells and the plurality of barriers define an active region, such that photons are emitted in the active region when electrons transition from an upper laser state in the active region to a lower laser state in the active region;

the plurality of quantum wells and the plurality of barriers define an injector region, such that electrons transport from one active region to a next adjacent active region through the injector region; and

at least one final quantum well of the plurality of quantum wells comprises a bottom portion that extends below a major quantum well in the active region when no bias is applied.

20. A quantum cascade laser structure comprising 4 quantum wells and a plurality of barriers in an active region, the active region being configured for emitting a photon, wherein:

a final quantum well in the active region comprises a bottom portion that extends below other quantum wells in the active region when no bias is applied;

a final barrier in the active region comprises top portion that extends beyond other barriers in the active region, thereby increasing an energy difference E54 between energy state 4 and energy state 5; and

the final barrier comprises a thickness that is greater than thicknesses of the other barriers in the active region.