Mode coupling and the control of mode Q factor and threshold gain are analyzed for nanocircular resonators confined by isolation and metallic layers based on solving eigenvalue equation for multiple-layer structure circular resonators. For nanocircular resonators only confined by a metallic layer, the metallic layer can enhance the mode confinement for transverse magnetic (TM) whispering-gallery modes (WGMs) and result in high Q TM WGMs. But transverse electric (TE) WGMs can form hybrid modes of surface plasmon polaritons and dielectric modes, with the mode Q factors limited by the metallic layer absorption. By introducing a low index isolation layer between the resonator and the metallic layer, we can greatly enhance the mode Q factors for TE WGMs. However, the mode coupling between different radial modes and the variation of the optical confinement factor in the active layer can result in the oscillation of the mode Q factor and threshold gain versus the isolation layer thickness. The optimization of the isolation layer thickness is important to enhance the mode Q factor and the optical confinement factor for realizing low threshold gain.
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