Presently quantum-cascade (QC) lasers enable emission at the wavelengths ranging from infrared to terahertz making them ideal light source for the distant detection of harmful gases and free-space optical communication. In those applications, requirements for the lasers include: narrow, single-fundamental-mode operation, low-divergent emitted beam, low threshold current and high speed modulation. Those properties are inherently owned by vertical-cavity surface-emitting lasers (VCSELs). However, when a QC is embedded into conventional vertical cavity, stimulated emission is impossible, because of the absence of the vertical electromagnetic wave component, which makes fundamentally impossible fabrication of QC VCSELs in their conventional design.
We propose a design of QC VCSEL in which top DBR mirror is replaced with a monolithic high-refractive-index contrast grating (MHCG). QCs are embedded within the MHCG stripes where the vertical component of the electromagnetic field is induced, enabling stimulated emission from the QCs.
Using a three-dimensional, fully vectorial optical model combined with an electrical model and gain model we discuss the distribution of the optical field, threshold current, emitted optical power and wall-plug efficiency of a 9 micro m AlInAs/InGaAs/InP QC VCSEL. Our anticipation shows that threshold current of QC VCSELs can be as low as 0.09 mA and the wall-plug efficiency at the level of 4%. We consider methods of current injection to active regions as well as methods of current and optical confinement.
The fabrication possibility of QC VCSELs opens new perspectives for merging the advantages of VCSELs with those of QCLs.