Abstract
Epsilon-near-zero (ENZ) media are an emerging class of nanophotonic materials that engender electromagnetic fields with small phase variation due to their approximately zero permittivity. These quasi-static fields facilitate several unique optical properties, such as subwavelength confinement, arbitrary wavefront control, and enhanced light–matter interactions, which make ENZ materials promising platforms for nanophotonic and plasmonic systems. Here, we report our analysis of single and dimer nanoantennas deposited on an aluminum-doped zinc oxide layer with an ENZ wavelength around 1.5 μm. Using near-field microscopy, far-field spectroscopy, finite-element numerical simulations, and a semi-analytic Fabry–Perot (FP) model, we show that single nanoantennas support highly dispersive plasmonic modes with less than unity effective mode index at wavelengths greater than the ENZ wavelength, which consequently fixes the resonance near the ENZ wavelength of the substrate. Furthermore, we observe a strong reduction in the near-field coupling between dimer nanoantennas via measurements of the resonance shift as a function of gap size. This reduction of near-field coupling allows one to design arrays of independently operating antennas with higher densities and thereby significantly improve the array characteristics, especially when targeting gradient metasurface implementations. Our results demonstrate the use of ENZ materials for increasing the versatility and functionality of plasmonic structures and provide foundational insight into this exotic material phenomenon.
Highlights
Epsilon-near-zero (ENZ) materials shape and control light in extraordinary ways
Permittivity values of gold were taken from the Palik handbook [39], while values of aluminum-doped zinc oxide (Al):ZnO and ZnO were extracted using spectroscopic ellipsometry from 315-nm-thick Al:ZnO and ZnO films deposited via pulsed laser deposition (PLD) onto glass slides
We find that the effective mode index for the nanorod waveguide on the Al:ZnO has a strong negative dispersion and, that it is less than unity for wavelengths past the ENZ wavelength of 1475 nm
Summary
Epsilon-near-zero (ENZ) materials shape and control light in extraordinary ways. Unlike conventional materials, the permittivity ε netic of an ENZ materials, media is implies approximately zero, a refractive index owfhizcehrofor nnon-pmaffiεffig-. An electromagnetic wave will have a nearly constant phase variation inside the ENZ media, alternatively interpreted as a “stretching” of the effective wavelength λeff λ0∕n [1] These static-like modes are the basis for many of the exotic phenomena and applications predicted theoretically and observed experimentally in ENZ materials, such as beam shaping and steering [2,3,4], subwavelength tunneling [5,6,7], and enhanced nonlinear interactions [8,9,10,11,12,13]. Prior plasmon–ENZ work includes studies of single nanorods for resonance wavelength and radiation engineering [32,33], metamaterial split-ring resonators for polariton splitting [34,35], and plasmon-enhanced quantum wells for active terahertz control [36] These studies demonstrate the great potential for plasmon–ENZ systems, but do not provide a thorough analysis or direct observation of the plasmon–ENZ coupling. We demonstrate a strong suppression of near-field coupling between dimer nanorods on an ENZ substrate, which we attribute to the mode characteristics observed in single nanorod antennas
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