TiN-Au/HfO₂-Au Multilayer Thin Films with Tunable Hyperbolic Optical Response

Hyperbolic metamaterials (HMM) possess significant anisotropic physical properties and tunability and thus find many applications in integrated photonic devices. HMMs consisting of metal and dielectric phases in either multilayer or vertically aligned nanocomposites (VAN) form are demonstrated with different hyperbolic properties. Herein, self-assembled HfO₂-Au/TiN-Au multilayer thin films, combining both the multilayer and VAN designs, are demonstrated. Specifically, Au nanopillars embedded in HfO₂ and TiN layers forming the alternative layers of HfO₂-Au VAN and TiN-Au VAN. The HfO₂ and TiN layer thickness is carefully controlled by varying laser pulses during pulsed laser deposition (PLD). Interestingly, tunable anisotropic physical properties can be achieved by adjusting the bi-layer thickness and the number of the bi-layers. Type II optical hyperbolic dispersion can be obtained from high layer thickness structure (e.g., 20 nm), while it can be transformed into Type I optical hyperbolic dispersion by reducing the thickness to a proper value (e.g., 4 nm). This new nanoscale hybrid metamaterial structure with the three-phase VAN design shows great potential for tailorable optical components in future integrated devices.