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...

In this work,we report on observation of Dyakonov plasmons at an interface with a hyperbolic metamaterial in the mid-IR.The hyperbolic metamaterial is implemented as a CMOS-compatible high aspect ratio grating structure with aluminium-doped ZnO (AZO) ridges grown by atomic layer deposition in deep trench silicon matrix.The dispersion of Dyakonov plasmons is characterized by the attenuated total reflection method in the Otto configuration.We demonstrate that Dyakonov plasmons propagate in a broad range of directions (a few tens of degrees) in contrast to the classical Dyakonov surface waves (about one tenth of degree).The obtained results provide useful guidelines for practical implementations of structures supporting Dyakonov plasmons in the mid-IR.