Architecturally Tailorable Order-Disorder Transition in Ag/Si Layered Hyperbollic Metamaterials

Hyperbolic Metamaterials (HMMs) are structured optical nanomaterials that exhibit, due to their intrinsic anisotropy, optical topological transitions (OTTs)[1], leading to exceptional optical behaviour such as an unbounded density of optical states or to super collimation. This has been demonstrated in applications like improved efficiency in solar energy, which require high-temperature stability.<br/>In recent years, there has been a growing interest in the stability of multilayered HMMs under high-temperature conditions. However, most of these studies have focused on refractory metal-dielectric pairs with applications in the near-IR, while the stability of the metal-semiconductor HMMs showing OTTs in the visible has gathered little attention. In our work, we examine the evolution of Ag-Si layered HMMs as function of thermal treatments. Silver was chosen over other commonly used metals in plasmonics (eg. Au or Al) because thermodynamic considerations point out at higher thermal stability.<br/>The samples were produced by magnetron sputtering and annealed in ultra-high vacuum at temperatures ranging from 200 to 800C. The optical characterization involved Reflectometry measurements with polarized and unpolarized light and was assisted by transfer matrix calculations and finite element modelling. The structural and chemical characterization was carried out by FIB-SEM, XRD and RBS.<br/>We demonstrate that thermal annealing of the HMM induces a multilayer instability. In a first regime, the expected HMM behaviour is observed, with broadband low reflectivity in agreement with our calculations. This optical regime persists even at 300C and although structural degradation can already be measured at that temperature. This initial degradation can be understood as enhanced roughness, and the measured lower reflectance is in agreement with previous numerical studies [3]. At higher temperatures, a second regime involving an order-disorder transition is found. Silver agglomerates inducing phase separation, and optically the system becomes an effective reflector. Finally, in the third regime, the Ag-to-Si ratio starts changing due to silver evaporation. We examine the different regimes of the multilayer instability and the different phenomena that may influence it.<br/>Furthermore, we show that changing the stacking order in unit cell of the HMM directly impacts the onset and pathway of the instability, as well as the transition to the third regime. This behaviour has been found to hold for systems that differ in the bilayer thickness, composition and number of stacked layers, opening the door to enhance the stability of HMMs for applications in harsh environments.<br/>[1] Krishnamoorthy <i>et al</i>. <i>Science</i> <b>336</b>, 205–209 (2012).<br/>[2] Baranov <i>et al.</i> <i>Nat. Mater.</i> <b>18</b>, 920–930 (2019).<br/>[3] Andryieuski et al. <i>Opt. Express, OE</i> <b>22</b>, 14975–14980 (2014).