Tailoring optical properties of surface charged dielectric nanoparticles based on an effective medium theory

We propose an effective-medium theory (EMT) for the surface charged dielectric nanoparticles (CDNPs) in the long wavelength limit, in which a homogenous CDNP is demonstrated to be equivalent to a conventional absorbing neutral particle of the same size but with different constitutive parameters (effective permittivity εe and effective permeability μe). It is found that while the surface charge induces negligible change of magnetic permeability in particle, it gives rise to a significant change of electric permittivity. The change in permittivity depends on the charge, the particle size, and the working frequency but is independent of the constituent material. In infrared frequencies, both the real and imaginary parts of the particle permittivity may be changed considerably by surface charging. At higher frequency, the surface charge can lead to a remarkable decrease in the real part of the permittivity while keeping its imaginary part nearly unchanged. Therefore, based on EMT we can tailor the optical properties of CDNPs by optimizing their parameters, allowing for many exotic phenomena, such as vanishing scattering efficiency, great enhancement of light absorption efficiency, and surface charge induced surface plasmon resonance.