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In recent years, with the continuous progress of micro/nano fabrication technique, the interaction of material and electromagnetic wave in the subwavelength scale has attracted widespread attention. Electromagnetic metamaterial is artificial material composed of building blocks whose feature size is much smaller than the working wavelength, with the electromagnetic properties that does not exist in natural materials. As an important branch of electromagnetic metamaterials, hyperbolic metamaterials become the focus of research for their unique characteristic to control near-field waves. By changing the size and arrangement of the components of hyperbolic metamaterials, the excitation intensity and direction of the surface plasmons (SPs) in them can be modulated, so that the unique dispersion curves can be achieved. Hyperbolic metamaterials have been used in many fields, such as subwavelength imaging, light localization and enhanced spontaneous emission. Hyperbolic metasurface is a new type of planar metamaterials with hyperbolic dispersion relationship and has many similarities in theory and applications with hyperbolic metamaterial. Compared with the bulk hyperbolic matematerials, hyperbolic metasurfaces exhibit more excellent performances because the large reduction in the longitudinal dimension limits the propagation of the electromagnetic waves in the two-dimensional plane.
In this review, starting with hyperbolic metamaterials, we introduce their basic theory of dispersion equation and isofrequency surface, and then describe the method of realizing hyperbolic dispersion from two different structures: metal-dielectric multilayer structure and metal nanowire structure. Effective medium theory to calculate the effective dielectric tensor and the choice of real materials are also presented. At the end of this section, we briefly introduce the typical applications of hyperbolic metamaterials, including optical negative refraction and hyperlens imaging. The latter part of the review is about hyperbolic metasurface and we begin with the introduction of the basic theory and isofrequency curve of hyperbolic metasurface. The difference is that in addition to the introduction of ordinary hyperbolic dispersion, we also stress the special case of near the topological transition point, where the dispersion curve is almost flat and the transmission of electromagnetic wave is almost diffraction-free. Then we list the natural hyperbolic materials that can be used to fabricate hyperbolic metasurface, including uniaxial and two-dimensional materials. The artificial method of using graphene to achieve any topological structure on the plane is illustrated. In analogy with the hyperbolic metamaterial, the negative refraction effect and the hyperlens imaging in hyperbolic metasurface are also introduced. In addition, intriguing properties of hyperbolic metasurfaces and their potential applications are described. Finally, we point out the restrictions of the hyperbolic metamaterials and metasurfaces and the prospect of future applications.
Isofrequency surfaces of the TM-polarized wave in hyperbolic metamaterials [13].
(a) Schematic of the metal-dielectric multilayer structure, ε, μ, d represent the permittivity, permeability and film thickness respectively [20]. (b) The real part of the permittivity of a semiconductor multilayer structure, ε⊥, ε|| represent the permittivity that are perpendicular and parallel to the film, respectively [15]. (c) Schematic diagram of the nanowire array structure, d is the period of the nanowire array and r is the radius of the metal nanowire [16].
(a) Hyperbolic dispersion relationship in nanowire array structure [17]. (b) Simulated negative refraction of hyperbolic metamaterials [16]. (c) Schematic of a silver nanowire hyperbolic metamaterial as well as scanning electron microscopy images showing the top and side views of the nanowires. The scale bars indicate 500 nm [18].
(a) Schematic of the planar and cylindrical multilayer hyperbolic structures and their dispersion properties [47]. (b) Schematic of hyperlens and its simulation results (up) and possible realizations of hyperlens——concentric metallic layers alternate with dielectric layers or radially symmetric "slices" alternate in composition between metallic and dielectric (down) [6]. (c) Left: Schematic of the hyperlens and the incident wavelength is 365 nm, Right: The beyond-diffraction image of the word "ON" [7].
(a)~(c) Color maps show the z-component of the electric field excited by a z-directed dipole (black arrow) located above the surface. The insets present the isofrequency contour of each metasurface topology[12]. (d) In the case that the loss cannot be ignored [58].
(a) Ez field component of surface plasmons excited by a z-oriented electric dipole located above homogeneous metasurfaces defined by various conductivity tensors. The insets show possible realizations of the different metasurface topologies using pristine or nanostructured graphene layers [57]. (b) The imaginary part of the effective electric conductivity along y (up) and x (down) direction versus ribbon width W [57].
(a) Schematic of metasurface based on grating structure [74]. (b) The dispersion relationship of the grating metasurface at different wavelengths. The grating period is 120 nm, the width is 60 nm and the height is 80 nm [74]. (c) The simulated negative refraction of the grating metasurface with 500 nm incident wave. The grating period is 100 nm, the width is 40 nm and the height is 100 nm [74]. (d)~(f) Propagation of surface plasmons along the metasurface at three different wavelengths of (d) 500 nm, (e) 543 nm and (f) 633 nm [74].
Image of SPPs refraction at different wavelengths. The grating period is 150 nm, the width is 90 nm and the height is 80 nm [75].
(a) Schematic of the non-diffraction transmission of the monolayer graphene [12]. (b) The normalized z component of the electric field at the observation lines in Figure (a) [12]. (c) The method of adding substrate and bias to transform graphene to hyperbolic metasurface [5]. (d) Simulated SPPs propagation on the graphene metasurface in Figure (c) [5]. (e) Schematic of the planar magnifying hyperlens [79].
(a) Schematic of the coupling of a dipole emitter to graphene SPPs [80]. (b) Field confinement at different angles of propagation [58]. (c) SER in logarithm scale versus the conductivity components of the structure [58].