Extremely thin infrared absorbers made of metallo-dielectric core-shell nanospheres

During the first years of Metamaterials, losses were usually considered undesirable for the most of envisioned applications. Oppositely, in 2008 a paper titled “Perfect Metamaterial Absorbers” just took advantage of the losses (N. I. Landy et al., Phys. Rev. Lett., vol. 100, p. 207402, 2008). A perfect absorber was built as a 2D periodical array of resonant particles responding to both the electric and magnetic field. Just at the resonance frequency a narrow band of quasi-perfect absorption appears. An important drawback is that this absorber is polarization dependent and very sensible to the incidence angle. Subsequent papers (X. Liu et al., Phys. Rv. Lett., vol. 104, p. 207403, 2010; J. Hao et al., High performance optical absorber based on a plasmonic metamaterial, Appl. Phys. Lett., vol. 96, p. 251104, 2010) addressed this problem by using as unit cell a metallic cross or a metallic square over a metallic plane. The same plausible explanation was given in all these papers: in order to avoid any reflected power, the effective medium impedance must match to that of vacuum and, in order to reduce the transmitted power, the imaginary parts of ε and μ must be high enough, so that most of the power is dissipated inside the slab.