AbstractControlling the lasing mode, emission direction, threshold, and quality factor of whispering-gallery mode lasing is important for practical applications such as optical interconnections, on-chip communications, trace detection, high-density storage, etc. In order to simultaneously control the mode and emission direction and to achieve a high-quality factor in a low-threshold whisper-gallery mode laser, such as a GaN floating microdisk, a novel fabrication design of a microdisk with a vertical slit is proposed. To demonstrate proof of concept, we experimentally measure whispering-gallery mode lasing spectra of microdisks with and without a slit. Our findings suggest that the disks can indeed operate in whispering-gallery mode, and the slit is able to change the optical path in the microcavity without breaking lasing resonance. The slit in the microdisk can also influence the lasing mode, quality factor, and directional emission. Therefore, our study provides a feasible way to control whispering-gallery mode lasing properties.