Figure 3.

(A) Optical properties of cobalt as tabulated in Ref. 22: real (n) and imaginary (k) parts of the cobalt refractive index are plotted in the long wavelength IR range. (B) Critical volume fraction of cobalt nanoparticles corresponding to ferrofluid transition to hyperbolic metamaterial phase shown as a function of free space light wavelength. (C,D) Wavelength dependencies of ε_z and ε_xy at α(∞) = 8.2%. While ε_xy stays positive and almost constant, ε_z changes sign around λ = 3 μm. (E) Polarization-dependent transmission spectra of 200 μm - thick ferrofluid sample measured using FTIR spectrometer are consistent with hyperbolic character of ε tensor.