Surface absorption channels in light absorption by a 2D-material-coated particle

Light absorption enhancement in micro- and nanoparticles has garnered considerable attention through coated 2D materials, which are physically homogenized by surface conductivities and surface currents within the electromagnetic boundary conditions. However, the electromagnetic absorption through the surface channel remains unexamined, hindering a deeper understanding of the underlying mechanisms of light absorption. In this work, we analytically derive the effective cross sections of surface absorption for a 2D-material-coated sphere, based on the framework of Mie theory amended by the surface conductivity. Our theoretical analysis confirms the absorption unitarity in wrapped particles, whereby the total absorption is equivalent to the sum of surface and volume absorptions. Considering optical dispersion of a polar interior, we identify a blue shift in the resonance wavelength induced by the 2D coating, which leads to a decrease in material dissipation and thus volume absorption within the particle itself in spite of a large field enhancement inside the particle. Finally, through a realistic case of small graphene-wrapped MgO spheres, we illustrate the dominant role of the surface absorption channel on the mechanism of absorption enhancements.