Optics of subwavelength gradient nanofilms

Propagation and tunneling of light through subwavelength photonic barriers, formed by dielectric layers with continuous spatial variations of the dielectric susceptibility across the film are considered. Effects of giant heterogeneity-induced non-local dispersion, both normal and anomalous, are examined by means of a series of exact analytical solutions of the Maxwell equations for gradient media. Generalized Fresnel formulae, showing a profound influence of the gradient and curvature of dielectric susceptibility profiles on the reflectance/transmittance of periodic photonic heterostructures, are presented. Depending on the cutoff frequency of the barrier, governed by the technologically managed spatial profile of its refractive index, propagation or tunneling of light through it is examined. Non-attenuative transfer of electromagnetic energy by evanescent waves, tunneling through dielectric gradient barriers characterized by real values of the refractive index decreasing into the interior of the medium, is shown. Scaling of the results obtained for different spectral ranges of visible, IR and THz waves is illustrated. The potential of gradient optical structures for the design of miniaturized filters, polarizers and frequency–selective interfaces of subwavelength thickness is considered.