We present low threshold quantum cascade surface emitting lasers (QCSELs) emitting at wavelengths of 4.5 micrometers or 8 micrometers. To extract the light vertically from the InP-based buried heterostructure laser a second order InGaAs/InP grating is used. Both ridge facets are formed by dry-etching followed by coating a dielectric-metal film. Due to the high reflectivity of the facets, the cavity can be shortened well below 500 micrometers reducing the threshold power to several hundred milliwatts. The proposed device concept allows large-scale fabrication and wafer-level characterization. The results are an important step towards low-cost and low-power consuming quantum cascade lasers for portable MIR gas sensors.
Non-linear properties of buried heterostructure ring quantum cascade lasers have been investigated. Clear symmetry breaking between two counter-propagating modes has been observed with a transition to a emission that has a solitonic characteristics.
The main obstacle towards widespread industrial adoption of THz quantum cascade lasers is the requirement of cryogenic cooling. Room-temperature operation using the conventional GaAs/AlGaAs material system is inherently limited by the optical phonon energy (ELO=36 meV) being close to the laser transition energy (~10-20 meV). In contrast, the ZnO/ZnMgO material system has a significantly higher ELO of 72 meV, pushing the theoretical high-temperature limit far above room temperature. At the same time, ZnO comes with it's own challenges, such as significantly broadened energy levels and short upper laser state lifetime.
In the present talk, these considerations will be discussed in the light of non equilibrium Green’s function modelling, which is necessary to correctly treat the strong electron-phonon scattering. In addition, design schemes suitable for m-plane (avoiding internal fields) ZnO QCLs will be presented and analysed, providing pathways towards room temperature THz QCLs.
Recently Zinc Oxide has received a renewed attention for the realization of intersubband devices such as quantum cascade lasers (QCLs). Indeed this material is predicted to be able to tackle the main limitation of current terahertz (THz) QCLs: the limited operation temperature. We report the observation of electronic coupling within ZnO/(Zn, Mg)O asymmetric quantum wells (QWs), first step towards this goal. Samples were grown by molecular beam epitaxy (MBE) with surfaces down to 0.4 nm. X-ray reflectivity (XRR) was used for thickness measurements checking and for the investigation of the interface quality. Atomic resolution scanning transmission electron microscopy (STEM) images reveals that we are able to grow 2 monolayers (MLs) thin (Zn, Mg)O barriers in a reproducible way while keeping abrupt interfaces. Room temperature (RT) photoluminescence (PL) spectra show that QWs are still coupled when separated by a 1.0 nm thick barrier. On the contrary, a 4.0 nm thick barrier allows no coupling. Doped samples were investigated by absorption experiment. Absorption spectra were successfully fitted by a theoretical model, proving a clear electronic coupling in our heterostructures. This demonstration allows us to seriously envision the realization of ZnO based intersubband devices.
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