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Single-mode emission from vertical-cavity surface-emitting lasers (VCSELs) with low-index defects is studied numerically. For analyzing optical, thermal, and electrical characteristics of the structures, a comprehensive numerical model is adopted. The nonuniform loss distribution of high-index regions of 2-D square photonic lattice is investigated as a means to improve single-mode behavior of 2-D photonic lattice VCSEL (PhL-VCSEL) with low-index defect. A new structure for higher performance low-index defect-based PhL-VCSEL using a concave configuration of InGaP spacer layer is proposed. Simulation results demonstrates that using the new structure decreases threshold current and the width of the far-field pattern, weakens spatial hole burning effects and increases the modal loss of the competing modes with fundamental defect mode. Therefore, the high-power single-mode operation of the laser is enhanced. Using specific concave configuration considerably increases both threshold current of the first competing side mode and optical output power.