Anti-reflection coatings for epsilon-near-zero materials
Epsilon-near-zero (ENZ) materials are materials that have a spectral region where the permittivity is below zero. This leads to unconventional optical behaviour, ranging from light propagation through arbitrary apertures, wavefront shaping, and extreme enhancement of nonlinear optical effects. These materials form one of the main pillars of our research work and we have previously shown strong optical nonlinearities and flexible ENZ materials. Of particular interest here is that these physical effects are related to the ENZ p[roerty, rather than the material in which the ENZ effect is realized. We can achieve the ENZ condition in three main ways, natural materials where the real part of the permittivity goes from positive to negative, e.g. metals or transparent conductive oxides, multilayer stacks consisting of metals and dielectrics, or through photonic-crystal-like structures.
Just as the desired effects of the ENZ condition are independent of the realization, so are some of the main challenges. One particular challenge is that the near-zero permittivity leads to a strong impedance mismatch with air or other surrounding materials and thus light is mostly reflected at the interface of the ENZ material.
Together with the Synthetic optics group here in St Andrews we have now addressed this issue, developing an Anti-reflection coating that reduces reflection and increases transmission through a multilayer-stack ENZ material. The coating and underlying ENZ material consist of alternating metal and silica layers, with the thickness of the layers tailored for the AR coating.
This work, published in the journal Optical Materials Express paves the way toward applications of ENZ materials for both linear and nonlinear optics.