High aspect ratio free-standing Al-doped ZnO (AZO) nanopillars and nanotubes were fabricated using a combination of advanced reactive ion etching and atomic layer deposition (ALD) techniques. Prior to the pillar and tube fabrication, AZO layers were grown on flat silicon and glass substrates with different Al concentrations at 150-250 °C. For each temperature and Al concentration the ALD growth behavior, crystalline structure, physical, electrical and optical properties were investigated. It was found that AZO films deposited at 250 °C exhibit the most pronounced plasmonic behavior with the highest plasma frequency. During pillar fabrication, AZO conformally passivates the silicon template, which is characteristic of typical ALD growth conditions. The last step of fabrication is heavily dependent on the selective chemistry of the SF 6 plasma. It was shown that silicon between AZO structures can be selectively removed with no observable influence on the ALD deposited coatings. The prepared free-standing AZO structures were characterized using Fourier transform infrared spectroscopy (FTIR). The restoration of the effective permittivities of the structures reveals that their anisotropy significantly deviates from the effective medium approximation (EMA) prognoses. It suggests that the permittivity of the AZO in tightly confined nanopillars is very different from that of flat AZO films.

Optical surface waves, highly localized modes bound to the surface of media, enable manipulation of light at nanoscale, thus impacting a wide range of areas in nanoscience. By applying metamaterials, artificially designed optical materials, as contacting media at the interface, we can significantly ameliorate surface wave propagation and even generate new types of waves. Here, we demonstrate that high aspect ratio (1 to 20) grating structures with plasmonic lamellas in deep nanoscale trenches, whose pitch is 1/10 -1/35 of a wavelength, function as a versatile platform supporting both surface and volume infrared waves. The surface waves exhibit a unique combination of properties, such as directionality, broadband existence (from 4 µm to at least 14 μm and beyond) and high localization, making them an attractive tool for effective control of light in an extended range of infrared frequencies. Main text 2Optical surface waves (SWs) arise at the interface of two dissimilar media with different types of permittivity or permeability 1 . Research on SWs has intensified in the last decade due to their unique properties of surface sensitivity, field localization, unusual dispersion and polarization properties at the nanoscale, stimulating the development of surface photonics 2 . The most studied SWs are surface plasmon-polaritons supported at the interfaces between metals and dielectrics 3 , which enable effective nanophotonic devices for sensing 4 , nano-guiding 5 , and imaging 6 based on near-field techniques. A newly emerging alternative is Dyakonov surface waves existing at the interfaces between anisotropic and isotropic dielectrics 7-10 . Up to present, various types of SWs have mostly been investigated individually. However, we can obtain new features by combining traits from various types of surface waves. This is where metamaterials, an artificially engineered materials and structures 11-13 , can play an essential role because in order to combine different SWs unprecedented and extreme optical parameters are often required. One example of such combined SWs on metamaterial structures are Dyakonov plasmons (DPs) 14,15 , a combination of surface plasmons and Dyakonov waves supported at the boundaries of hyperbolic metamaterials (HMMs) 16 . The diagonal components of the HMMs' permittivity tensors are of different signs, giving rise to hyperbolic iso-frequency contours in the k (wavevector) space accompanied by singularities in the density of optical states in an ideal lossless case. Natural material equivalents of HMMs are often referred to as indefinite media 17,18 . Characteristically, HMMs and their two-dimensional analogues of metasurfaces possess a unique combination of properties including unusually high wavevectors, optical density of states, and anisotropy. These feature lead to a wide variety of HMM potential applications such as broadband enhancement in the spontaneous emission for a single photon source 19,20 , sub-wavelength imaging 21 , sensing 22,23 , thermal engineering 19,20,24 , and steering of opti...

Mid-infrared spectroscopy offers unique sensing schemes to detect target molecules thanks to the absorption of infrared light at specific wavelengths unique to chemical compositions. Due to the mismatch of the mid-infrared light wavelength on the order of micron and nanometer size molecules, the interaction between them is typically weak, resulting in small signatures of absorption. Plasmonics can play an important role, enhancing photon−matter interactions by localization of light in small volumes or areas. Thus, it enables the increase of light absorption by molecules providing higher sensitivity. Here, we demonstrate the enhancement of infrared absorption in plasmonic trench structures that function as hyperbolic metamaterials. The metamaterial is composed of plasmonic trenches made of aluminum-doped zinc oxide. We use a 5 nm thick silica layer as a model analyte conformally coated around the plasmonic trenches, which absorbs light with wavelengths around 8 μm. The enhanced absorption is achieved by the interaction of bulk plasmon modes propagating in the trenches with the analyte silica layer on the pronounced extended surface area of the trench structure. Such plasmonic nanotrench structures may serve as a highly sensitive bio-and chemo-sensing platform for mid-infrared absorption spectroscopy.