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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Elementaranregungen und Transport in Festkörpern

Light emitters based on intersubband transitions: Quantum Cascade Laser Physics

The Quantum cascade laser (QCL) is a new semiconductor laser type for the mid to far infrared spectral range. It can serve as an attractive laser source for the sensing of trace gases and for wireless optical communication, especially since it became capable of continuous-wave, high-power, room temperature operation.

QCLs are based on intersubband transitions rather than on interband transitions

Conventional semiconductor laser diodes (e.g. DH-Laser) rely on interband transitions, which are radiative recombinations of electrons from the conduction band (CB) and holes from the valence band (VB). Hence a laser diode is limited to wavelengths shorter than the band gap of the materials used.
On the other hand the QCL creates photons by radiative intersubband transitions between quantised electronic states (so called subbands) inside the conduction band. Therefore the QCL is ideally suited for a infrared laser source. The wavelength of this unipolar device can be chosen in a huge range (about 3 to 150 µm so far) simply by the choice of layers thicknesses.

Cascaded creation of photons with many periods

By repeating the active region it is possible to create many photons with one electron. This 'recycling' of electrons, impossible with conventional laser diodes, enables laser output powers in the range of several hundred mW to several W and high quantum efficiencies. Typical QCLs consist of about 20 to 30 periods.

Creating a population inversion

The main challenge in creating a population inversion necessary for lasing is the ultrashort lifetime of excited electronic states inside the quantum wells (QW). The lifetime is limited by the dominant polar LO phonon-electron interaction and typically of the order of 1 ps. Nevertheless a population inversion between two subbands (in the picture: 3 and 2) can be achieved be a proper 'engineering' of the electronic wavefunctions and energy levels. In the simplest case the energetic spacing between subbands 2 and 1 is chosen close to the LO-phonon energy of the actual material (~36 meV for GaAs). The strength of the phonon-electron interaction is stronger for small momentum transfers from the electrons to the lattice and therefore the scattering rate is bigger for the nearly 'resonant' intersubband transition from subband 2 to 1 than for the scattering from subband 3 to 2. Fast depopulation of the lowest subband 1 is accomplished by tunneling into the next period.

QCL device structure

QCLs typically consist of hundreds of layers of semiconductor material with individual thicknesses down to a few monolayers. Such heterostructures are grown using molecular-beam epitaxy (MBE) or gas-source molecular-beam epitaxy (GSMBE).

The upper SEM-picture shows a typical cross section of a QCL device. The photons are created within the active region, which shows a slightly lighter color. It is surrounded by two claddings with a lower refractive index than the active region. Thus a waveguide is built for the laser light. On top an electrical contact made of gold is evaporated and the substrate serves as the back contact (cf. the lower SEM-picture).
The laser cavity is produced by etching the heterostructure down to the substrate and cleavage of that stripes. Thus the cleaved facets serve as mirrors for the laser light.

Typical QC lasers are about 1 cm long and as narrow as a few 10 µm. The lower SEM-picture shows a mounted QCL device. The arrow indicates the actual laser bar.