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The resonant modes found in a modified single-defect two-dimensional photonic crystal slab structure are theoretically and experimentally studied. There exist several modes in the band gap: doubly degenerate (dipole and quadrupole modes) and nondegenerate (hexapole and monopole modes). Among them, the monopole mode specifically attracts our interest because of its nondegeneracy, good coupling with the gain medium, and existence of the intensity minimum at the center of the cavity, which would open up the chance for the electrically driven single-defect laser. The nondegenerate hexapole mode, a special type of whispering gallery mode, has a very high quality factor. We have fabricated two types of modified single-defect lasers, i.e., air-based free-standing and SiO2-based epoxy-bonded structures. Rich lasing actions in both structures are experimentally observed under optically pulsed pumping conditions at room temperature. In the free-standing slab structure, photons are strongly confined in vertical direction, and the lasing operations of all resonant modes with low thresholds are obtained. Especially, the nondegenerate monopole-mode laser is confirmed to have a large spontaneous emission factor of >0.06, estimated by analyzing rate equations. In the SiO2-based slab structure, thermal properties are improved at the expense of vertical losses.