A series of wavelength‐scale capsule‐shaped metallic cavity structures with InGaAs/InGaAsP multiple quantum wells are fabricated and characterized by photoluminescence measurement for the first time. By introducing cylindrical facets with optimal curvature to a conventional rectangular cavity, the electric field of the resonant mode is pushed effectively into the center of the mesa. As a result, the mode distribution at the metallic sidewalls and the plasmonic losses are reduced dramatically. We experimentally confirm up to fourfold enhancement in the Q factor of the fabricated capsule‐shaped cavity compared with that of a rectangular structure with same dimension. The measured results are consistent with the 3D finite‐difference time‐domain simulation, and validate the effectiveness of the scheme.
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