The majority of plasmonic and metamaterials research utilizes noble metals such as gold and silver which commonly operate in the visible region. However, these materials are not well suited for many applications due to their low melting temperature and polarization response at longer wavelengths. A viable alternative is aluminum doped zinc oxide (AZO); a high melting point, low loss, visibly transparent conducting oxide which can be tuned to show strong plasmonic behavior in the near-infrared region. Due to it’s ultrahigh conformality, atomic layer deposition (ALD) is a powerful tool for the fabrication of the nanoscale features necessary for many nanoplasmonic and optical metamaterials. Despite many attempts, high quality, low loss AZO has not been achieved with carrier concentrations high enough to support plasmonic behavior at the important telecommunication wavelengths (ca. 1550 nm) by ALD. Here, we present a simple process for synthesizing high carrier concentration, thin film AZO with low losses via ALD that match the highest quality films created by all other methods. We show that this material is tunable by thermal treatment conditions, altering aluminum concentration, and changing buffer layer thickness. The use of this process is demonstrated by creating hyperbolic metamaterials with both a multilayer and embedded nanowire geometry. Hyperbolic dispersion is proven by negative refraction and numerical calculations in agreement with the effective medium approximation. This paves the way for fabricating high quality hyperbolic metamaterial coatings on high aspect ratio nanostructures that cannot be created by any other method.
The ability to stimulate, track, and record biological processes with as many data channels as possible is central to decoding complex phenomena in the body. For example, many biological processes involve small mechanical cues that can help drive chemical reactions and/or initiate responses to external stimuli. However, to measure these nanomechanical events, specialized tools are required that can not only achieve piconewton force resolution, but be able to record from multiple sites while maintaining a small footprint to allow embedded or intracellular measurements. This is challenging for state-of-the-art instruments such as atomic force microscopes or optical traps due to the difficulty in multiplexing, their size, and feedback mechanisms. Here we describe a new nanofiber-optic platform that can detect sub-piconewton forces by monitoring far-field scattering signals of plasmonic nanoparticles moving within the near-field. To provide mechanical resistance to the nanoparticles, and allow quantitative forces to be extracted, compressible polymer claddings have been designed that have tunable spring constants and chemical compositions. The transduction mechanism is demonstrated both on detecting local contact forces acting on the nanoparticles as well as acoustic waves propagating in the medium. Because of the small cross-sectional areas (< 1 um2) and long lengths (> 1 mm), these nanofibers can also be inserted deep into tissue to locally excite and collect signals from single cells (e.g., neurons) with minimal invasiveness. Experiments focused on stimulating and recording from brain tissue will be discussed.