Optical beam quality of an optoelectronic device is improved by suppression of high-order transverse optical modes by their resonant interaction with the continuum of modes in the surrounding regions, such continuum being realized by replacement of one or several layers by layers having a lower refractive index. In particular, selective oxidation of GaAlAs-based vertical cavity surface emitting laser results in (Ga)AlO layers surrounding the aperture and having a lower refractive index than the original (Ga)AlAs layers. The continuum of optical modes originates due to the modification of the optical field in the areas surrounding the aperture caused by the low index insertions positioned to result in enhancement of the optical field in their vicinity. High-order lateral optical modes in the aperture region exhibit larger leakage losses than the fundamental lateral optical mode due to the resonant interaction with the continuum of modes outside the aperture, enabling single-mode lasing from a broad aperture vertical cavity surface emitting laser.