Epitaxial TiN/MgO multilayers with ultrathin TiN and MgO layers as hyperbolic metamaterials in visible region

Multilayers with metal-dielectric stacks are considered as an effective approach to produce hyperbolic metamaterials (HMMs). However, fabricating ultrathin metal layers with high crystallinity for low-loss HMMs is very challenging. In this work, TiN has been applied as an alternative candidate to the noble metals, and TiN/MgO multilayers with varied numbers of layers and layer thicknesses have been designed. The microstructure study reveals that the thickness of TiN nanolayer is controlled as thin as 2.9 nm, 1.26 nm and 0.9 nm by varying the total number of layers, and the epitaxial quality of the multilayer films is high even with such ultrathin layers. Extraordinary optical properties have been achieved, such as plasmonic resonance in visible range, as well as strong optical anisotropy. In addition, the real part of dielectric permittivity (ε′) shows opposite optical anisotropy in different wavelength ranges, i.e. ε∥′>0, ε⊥′<0 in the range of 350 nm–450 nm, and <0, ε⊥′>0 under higher wavelength, which is a novel property realized in a multilayer system.