High-Order Hilbert Curves: Fractal Structures with Isotropic, Tailorable Optical Properties

Fractals are promising candidates as nonperiodic, nonresonant structures exhibiting a homogeneous, isotropic, and frequency-independent effective optical response. We present a comprehensive optical investigation of a metallic Hilbert curve of fractal order <i>N</i> = 9 in the visible and near-infrared spectral range. Our experiments show that high-order fractal nanostructures exhibit a nearly frequency independent reflectance and an in-plane isotropic optical response. The response can be simulated in the framework of a simple effective medium approximation model with a limited number of parameters. It is shown that high-order Hilbert structures can be considered as a “transparent in-plane metal”, the dielectric function of which is modified by the filling factor <i>f</i>, hence creating a tunable conductive effective metal with tailorable plasma frequency and variable reflectance without going through an insulator-to-metal transition

Niobium as Alternative Material for Refractory and Active Plasmonics

The development of stable compounds for durable optics is crucial for the future of plasmonic applications. Even though niobium is mainly known as a superconducting material, it can qualify as an alternative material for high-temperature and active plasmonic applications. We utilize electron beam lithography combined with plasma etching techniques to fabricate nanoantenna arrays of niobium. Tailoring the niobium antenna geometry enables precise tuning of the plasmon resonances from the near- to the mid-infrared spectral range. Additionally, the hydrogen absorptivity as well as the high-temperature stability of the antennas have been investigated. Further advantages of niobium such as superconductivity make niobium highly attractive for a multitude of plasmonic devices ranging from active and refractory perfect absorbers/emitters to plasmon-based single photon detectors