Semiconductor laser array devices capable of emitting mid- to long-wavelength infrared (i.e., 4-12 μm) radiation are provided. The devices include a quantum cascade laser (QCL) structure comprising one or more active cores; an optical confinement structure; a cladding structure, and a plurality of laterally-spaced trench regions extending transversely through the cladding and optical confinement structures, and partially into the QCL structure. The trench regions define a plurality of laterally-spaced interelement regions separated by element regions in the laser array device. The element regions are characterized by a non-uniform structure across their widths. As a result of this structural non-uniformity, array modes composed of coupled first-order lateral modes of the element regions are preferentially suppressed relative to array modes composed of coupled fundamental lateral modes of the element regions.