On-chip resonant leaky-wave coupling of quantum cascade lasers (QCLs) emitting at 8.36 μm has been realized by selective regrowth of interelement layers in curved trenches, defined by dry and wet etching. The fabricated structure provides large index steps (Î"n = 0.10) between antiguided-array element and interelement regions. In-phase-mode operation to 5.5 W front-facet emitted power in a near-diffraction-limited far-field beam pattern, with 4.5 W in the main lobe, is demonstrated. A refined fabrication process has been developed to produce phased-locked antiguided arrays of QCLs with planar geometry. The main fabrication steps in this process include non-selective regrowth of Fe:InP in interelement trenches, defined by inductive-coupled plasma (ICP) etching, a chemical polishing (CP) step to planarize the surface, non-selective regrowth of interelement layers, ICP selective etching of interelement layers, and non-selective regrowth of InP cladding layer followed by another CP step to form the element regions. This new process results in planar InGaAs/InP interelement regions, which allows for significantly improved control over the array geometry and the dimensions of element and interelement regions. Such a planar process is highly desirable to realize shorter emitting wavelength (4.6 μm) arrays, where fabrication tolerance for single-mode operation are tighter compared to 8 μm-emitting devices.