A quantum cascade semiconductor laser includes: a semiconductor mesa having a core layer extending in a direction of a first axis, and an end face extending in a direction of a second axis intersecting the direction of the first axis, and the semiconductor mesa being disposed on a principal surface of a substrate; and a reflective layer disposed on the end face of the semiconductor mesa, the reflective layer including a first semiconductor film in contact with the core layer, the core layer having a superlattice structure, the superlattice structure including a quantum well layer and a barrier layer, and the first semiconductor film of the reflective layer having a bandgap equal to or smaller than that of the quantum well layer.

A quantum cascade laser includes a laser structure including first and second end faces, the laser structure including a semiconductor laminate region and a first embedding semiconductor region. The laser structure includes first and second regions arranged in a direction of a first axis extending from the first to second end faces. Each of the first and second regions includes the semiconductor laminate region. The semiconductor laminate region of the first region has a first recess. The semiconductor laminate region of the second region has a semiconductor mesa. The first recess and the semiconductor mesa extend in the direction of the first axis, and are aligned with each other. The semiconductor mesa has an end face extending in a direction of a second axis intersecting the first axis. The first embedding semiconductor region is disposed in the first recess so as to embed the end face of the semiconductor mesa.

A quantum cascade laser includes a laser structure including a substrate and a semiconductor laminate region, the substrate having a principal surface that includes first to third areas that extend in a direction of a first axis, the substrate having first and second end faces arranged in the direction of the first axis, the semiconductor laminate region having first and second mesas and a semiconductor mesa which are arranged on the first to third areas, respectively; and a first semiconductor film disposed on the third area, side faces of the first and second mesas, and an end face of the semiconductor mesa. The laser structure includes a first region including the semiconductor mesa, and a second region including the first and second mesas. The second region includes a recess defined by the third area, the side face of the first mesa, and the side face of the second mesa.

Methods of passivating at least one facet of a multilayer waveguide structure can include: cleaning, in a first chamber of a multi-chamber ultra-high vacuum (UHV) system, a first facet of the multilayer waveguide structure; transferring the cleaned multilayer waveguide structure from the first chamber to a second chamber of the multi-chamber UHV system; forming, in the second chamber, a first single crystalline passivation layer on the first facet; transferring the multilayer waveguide structure from the second chamber to a third chamber of the multi-chamber UHV system; and forming, in the third chamber, a first dielectric coating on the first single crystalline passivation layer, in which the methods are performed in an UHV environment of the multi-chamber UHV system without removing the multilayer waveguide structure from the UHV environment.

According to embodiments, a semiconductor laser comprises a semiconductor layer stack, which comprises an active zone for generating radiation. The semiconductor laser also comprises a first resonator mirror, a second resonator mirror, and an optical resonator, which is arranged between the first and second resonator mirrors and extends in a direction parallel to a main surface of the semiconductor layer stack. A reflectance R1 of the first resonator mirror is wavelength-dependent, so that R1 or a product R of R1 and the reflectance R2 of the second resonator mirror in a wavelength range decreases from a target wavelength .sub.0 of the laser to .sub.0+ from a value R0, wherein is selected as a function of a temperature-dependent shift in an emission wavelength.

A very strong selection mechanism is provided in a tunable vertical cavity surface emitting laser (VCSEL) by manipulating the laser threshold to be different for TE and TM polarization by a employing a subwavelength grating in the laser cavity. The laser selects the polarization with the lowest threshold. The grating does not diffract and does not add loss to the cavity. It works by creating a large birefringence layer between the semiconductor and air sub-cavities of the full VCSEL. Multilayer stack calculations show that this results in a lower threshold for the TM polarization over the TE. This subwavelength grating layer, in one embodiment, replaces the AR coating on the semiconductor surface.