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Heat-assisted magnetic recording is a technology to improve recording density of hard disk drives. The authors' group previously proposed a device, in which a Au nano-antenna as a near-field transducer was attached to a GaAs ring resonator as a light source. The ring resonator should stably oscillate with a single mode to generate near-field light at the tip of the nano-antenna with sufficient energy density. A non-axisymmetric ring resonator structure was proposed this time to improve the energy density and laser oscillation stability. When the decenter between the inner and outer circles of the ring resonator increased, the resonance frequency increased and eigenmodes with higher radial mode order disappeared, which leads to stable laser oscillation. Moreover, when the decenter was 200 or 250 nm, the normalized energy density at the tip of nano-antenna increased 1.2–2.4 times compared with an axisymmetric ring resonator structure while keeping the quality factor to more than 5000. [ABSTRACT FROM AUTHOR]