Three-dimensional scattering matrix method is proposed to investigate mode characteristics for metal-coated nanocavities, with the vertical waveguide structure of an active region confined by upper and lower cladding layers. For a nanocavity with radius of 800 nm, factors of well-confined modes with wavelength around 1550 nm first decrease with the increase of the metallic layer thickness due to the metallic absorption and the increase of radiation loss as the metallic layer thickness is less than 10 nm, and then rise with the increase of the metallic layer. However, for a weak confined nanocavity with a radius of 500 nm, the mode factor increases with the metallic layer thickness first, reaches a maximum value at an optimal metallic thickness, then decrease with the further increase of the metallic layer. For nanocavities confined by a thick metallic layer, the factors approach constants limited by the metallic absorption. However, mode field patterns, including the vertical field distributions, are affected by the metallic layer, which not only influences the metallic layer absorption but also the optical confinement factor in the active region.
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