Development of THz QC lasers based on quantum posts

Quantum cascade lasers (QCL) provide a compact source of coherent radiation in the MIR and THz spectral range. Optical transitions take place between states within the same band, i.e. subbands arising from size quantization, therefore these lasers are unipolar devices giving also a great amount of opportunities for cavity engineering. Although this concept offers a large freedom in electrical and optical band engineering, this scheme suffers from the use of subbands for lasing action. The wide spectrum gives rise to a variety of non-radiative decay as well as intersubband absorption resulting in high threshold currents and also allows for thermally activated LO-phonon emission. This causes severe limitations in the operational performance with respect to long wavelength emission, wall plug efficiency and high temperature performance. The application of an external magnetic field perpendicular to the growth direction introduces additional in-plane confinement by the formation of discrete Landau levels resulting in quasi zero-dimensional states. From that a strong reduction in threshold current as well as long wavelength operation was experimentally achieved due to the reduction of the in-plane degree of freedom. An alternative way to intrinsically achieve a three-dimensional carrier confinement was proposed several years ago by incorporating quantum dots (QDs) within a quantum cascade (QC) structure. However, the large inter-sub-level distances of InAs or AlGaAs QDs typically larger than 40 meV make them unsuitable for the generation of THz emission. Very recently, the group of Petroff has published so called quantum posts (QPs) which are based on coupled QDs. Such a nanostructure provides three-dimensional carrier confinement with tailorable inter-sub-level transitions in the THz spectral range. QPs are based on the III-V material system and epitaxially grown by MBE, allowing for the integration in QC structures. Hence, we propose to incorporate QPs as active material within quantum well QC structures. The complete localization of carriers leads in general to a high material and differential gain as well as high temperature stability of devices. The injection into and extraction out of these QPs will be maintained by quantum well states which also maintain carrier transport within the whole structure. A planar photonic crystal (PhC) embedded in a double-plasmon waveguide will be designed, in which TM polarized light is confined to realize optical cavities with a high quality factor Q and a small modal volume V m allowing for a large spontaneous emission factor. Furthermore, PhCs offer the advantage of gain enhancement in the photonic flat-band region. From such a scheme one expects a dramatic decrease of the necessary threshold for a possible lasing emission, which would outperform THz QCLs with respect to their poor wall plug efficiency. The thermal activated LO- phonon emission would be strongly suppressed within these structures, therefore opening the way for the realization of compact high temperature THz lasers.