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Cataloged from PDF version of article. Thesis (M.S.): Bilkent University, Department of Materials Science and Nanotechnology, İhsan Doğramacı Bilkent University, 2021. Includes bibliographical references (leaves 43-48). Vertical-cavity surface-emitting lasers (VCSEL) are the ideal light sources for optical data communication and 3D sensing due to their small size, low power consumption, high-speed modulation, and low cost. The key to meeting the ever-increasing demand for higher efficiency and modulation devices is scaling down the cavities to near- or sub-wavelength sizes. The current state-of-the-art commercial oxide-VCSEL technology has been very successful for micro-scale cavity diameters (>3 µm), but its processing approach is not appropriate for further miniatur-ization. Improvement in the performance of oxide-VCSELs can be achieved with smaller oxide aperture diameters. However, the high thermal resistance induced by the oxide layer significantly degrades the device performance, especially for the smaller sizes, making this method unreliable for scaling. In this work, we investigated a lithographically defined VCSEL (Li-VCSEL) method in which the transverse photonic and electrical confinement can be enabled by epitaxial growth and lithography. Transverse optical confinement is achieved by introducing an intracavity phase-shifting mesa that provides the confinement by index guiding. Numerical simulation results show high-quality factors even for submicron sizes, which is promising for the realization of submicron size single emitter and high-density array lasers. The fabrication steps include the epitaxial growth of the bottom semiconductor DBR and the cavity, defining the phase-shifting mesa us-ing optical lithographic processes, and the deposition of the top dielectric DBR using thin film deposition techniques. We demonstrated room-temperature lasing around 980 nm from Li-VCSELs with mesa diameters ranging from 0.75 µm to 2.0 µm under continuous-wave optical pumping and presented detailed charac-terization of these devices. The results represent a significant step towards the realization of electrically pumped small-size lasers for practical optoelectronics applications. by Abdulmalik Abdulkadir Madigawa M.S.