Enhancement of longitudinal EM waves in a graded Epsilon Near Zero (ENZ) materials

Abstract

Low permittivity (low epsilon) materials are now attracting wide attention due to potential novel applications in optics and radio communications. Longitudinal electromagnetic waves play important role in plasma, in surface plasmon polaritons in anisotropic materials, in space-charge waves in semiconductor materials. In this paper, we have shown that in Epsilon Near Zero (ENZ) materials with inhomogeneous space charge and epsilon distribution (spatially graded), the longitudinal EM waves are amplified. Governing equation system was developed for modeling of holographic gratings recording in semiconductors in ENZ spectral region. Wave equation contains terms related to longitudinal components with ε in denominator, that suggest strong enhancement of longitudinal waves in ENZ region. It is shown, that Drude model predict coupling of two waves of unequal intensities resulting in a spatial and temporal self-modulation of the transmission grating phase. This self-phase modulation leads to the transient energy transfer (TET-effect) that enhances weaker beam, and therefore may be responsible for coherent noise amplification, and may be used for optical limiting. New effect of transient homogeneous holographic current generation is predicted, that is due to self-phase modulations. This transient current may be considered as a direct manifestation of longitudinal low frequency space-charge waves, generated by nonlinear coupling of high-frequency EM waves during dynamic grating recording. Similar effects of TET and holographic current are described for reflection grating (Lippmann-Denisyuk geometry). The described in a paper effect of diffraction bleaching (optical analog of Bormann effect) may have practical application, as it reduces absorption in ENZ materials.