Quantum-cascade vertical-cavity surface-emitting lasers (QC VCSELs)  combine features
of VCSELs in respect of low threshold current, high quality of output beam, possible high speed modulation and fabrication of two dimensional phase-coupled arrays and quantum cascade lasers (QCLs) due to their emission in a broad range of infrared radiation up to about 100 m.
In those structures vertical resonance and stimulated emission of photons is possible due to embedding QCs in the stripes of a monolithic high-refractive-index contrast grating (MHCG). Unipolar QCs provide flexibility in the number of the active regions used in the structure, leading to designs with distributed active regions enabling efficient stimulated emission. The expected high performance of QC VCSELs relies on sophisticated designing of MHCG and active regions which takes into account distributions of the QC VCSEL modes. Spatial distributions of modes are highly unintuitive and anticipation of them requires the use of numerical methods solving fully vectorial Maxwell eigenvalue problem.
In this article, we present the principles of QC VCSELs designing illustrated by examples of optimization of a structure emitting at the wavelength of 9 µm. Particularly, we demonstrate optimization of the MHCGs, the resonant cavities and the numbers of active regions in QC VCSELs. In this contribution, optimal designs with respect to minimal threshold current and maximal output power are presented.
 T. Czyszanowski: Quantum Cascade Vertical Cavity Surface Emitting Laser, IEEE Photon. Technol. Lett. vol.29, pp. 351-354, 2018