Based on complementary fractal geometry structures, we design a novel infrared quasi-three-dimensional (3D) nanocavity with a localized enhanced field with multiband resonant frequencies. The fractals offer the nanostructure two important characteristics, multiband functionality and a subwavelength effect. The electric field, power flow, and the field intensity distributions are given to indicate the internal mechanism of the localized enhanced field in the nanocavity. Additionally, the effective medium method is established to retrieve the permittivity and impedance of the structure. It is shown that a strongly enhanced localized field is achieved in the nanocavity at two different resonant frequencies by using the finite difference time domain method. The field intensity in the nanocavity is enhanced by a factor of up to 60 times over that of the incident light because of the important contribution of the loss factor in the permittivity. The surface plasmon hybridization is thought to play an important role in the strong localized field enhancement. The multiband property and high localized intensity offer the nanocavity great potential for applications in surface enhanced Raman scattering and other nanoscale novel devices.
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