Alternating Nanolayers of Dielectric MgF₂and Metallic Ag as Hyperbolic Metamaterials: Probing Surface States and Optical Topological Phase Transition and Implications for Sensing Applications

Hyperbolic metamaterials (HMMs) possess marvelous electromagnetic properties, which enable a wide variety of applications, such as super-resolution and spontaneous emission. In addition, HMMs have emerged as a plasmonic biosensor platform with extreme sensitivity owing to the higher quality factors of their surface states. When predicting and analyzing these optical properties of HMMs, most of the researchers adopted the effective medium theory (EMT). However, this theory only validated for the long wavelength limit and the infinite stacking layers. To demonstrate the optical topological transition, we fabricated planar one-dimensional HMMs (1D-HMMs) that are composed of alternating MgF2/Ag nanolayers with various filling ratios and mapped the dispersion of their surface states. Strikingly, all our analytic analyses, numerical calculations, and experimental measurements indicated that the "transition point"on the dispersion curve of the surface states of 1D-HMMs did not depend on the intrinsic metal/dielectric properties but depend only on the thickness ratio of the metal nanolayers to the dielectric nanolayers. This outperformed the conventional effective medium theory. The results based on our plasmonic band theory provided a more rigorous interpretation and will benefit the sensing applications of the 1D-HMMs.