Gradient index metamaterial based on slot elements

Abstract

We present a gradient-index (GRIN) metamaterial based on an array of annular slots. The structure allows a large variation of the effective refractive index under normal-to-plane incidence and thus enables the construction of GRIN devices consisting of only a small number of functional layers. Using full-wave simulations, we demonstrate the annular slot concept by means of a 3-unitcell thin GRIN lens for the terahertz (THz) range. In the presented realizations, we achieved an index contrast of ∆n = 1.5 resulting in a highly refractive lens suitable for focusing THz radiation to a spot size smaller than the wavelength.In the last ten years, metamaterials have emerged to be powerful tools for the manipulation of light on the subwavelength scale. From the very first day of metamaterials, the scientific interest has been driven by the possibility of guiding light by tailoring the effective material parameters. In this respect, various concepts have been proposed, ranging from gradient index materials [1][2][3], to invisibility cloaks [4][5][6][7] and other transformation optical structures [8][9][10][11][12]. Most of the concepts have only been experimentally realized in the microwave regime where comparatively simple fabrication techniques allow a large freedom in the metamaterial design. However, when approaching higher frequencies as the THz or the optical regime, the experimental realization becomes more challenging since the standard fabrication methods, such as photo-or electron beam lithography, allow only the fabrication of planar structures with a very limited number of layers. In order to compensate the layer restriction in the high frequency range, it is therefore important to use metamaterials that provide a high refractive index contrast. However, most of the metamaterial structures are very lossy and allow only a moderate change of the refractive index within an acceptable transmission window. In particular, this is the case for resonant elements which are usually associated with high intrinsic losses. One possibility to create a non-resonant element is to choose the operation frequency well below the resonance frequency and, hence, operate in the non-resonant region of the structure. This approach has been applied in Ref. 2 − 4 where the achieved index contrast was reported to be in the range of ∆n = 0.7−0.9. In this paper, we present an alternative approach for a non-resonant, polarization-independent structure which allows a very large variation of the refractive index of about ∆n = 1.5 with high transmission and reasonable bandwidth for practical applications.The unit cell of the proposed metamaterial consists of a 3 µm wide annular slot within a square metal patch as shown in Fig. 1(a). The metal layer is embedded in a cubic dielectric matrix with an edge length of 60 µm. The radius of the annular slot was varied in order to alter the effective index of refraction. We analyzed the electromagnetic properties of the metamaterial by fullwave time-domain simulations (CST Microwave Studio).To consider ...

Terahertz metalenses for evanescent wave focusing and super-resolution imaging

“…However, the curved interface will arouse aberration when used for imaging [2] and the lens can rarely be integrated into compact THz systems. There were different types of metasurfaces [3][4][5][6][7] with the function of adjusting the effective index to realize plane gradient-index (GRIN) metalenses. Using a meta-transmitting array, [8] the initial phase of emerge wave could be arbitrarily adjusted, so as to realize spatial focusing or FT.…”

Terahertz path-length lens composed of oblique metal slit array

“…Meta-atoms can realize metamaterial absorbers [6][7][8][9], antireflection structures [10], and structures with high refractive indices [11] in the terahertz wave band. Various flat metamaterial lenses [12][13][14][15][16] have been proposed because conventional resin lenses are too large for convenient optical element design. Two-dimensional electric fields were measured in [17] to confirm the terahertz focusing effect of an artificial dielectric multilayer lens with metal rectangular chips.…”

Automatic Design of Broadband Gradient Index Metamaterial Lens for Gain Enhancement of Circularly Polarized Antennas

“…Recently, GRIN metamaterial lenses consisting of resonant metamaterials with a positive index of refraction were designed to transform cylindrical or spherical waves into planar waves, yielding antennas with an increased directivity [16][17][18][19][20][21][22][23][24][25]. In particular a highly-sophisticated broadband, dual-linear-polarized, and high-directivity lens horn antenna using GRIN metamaterials, composed of multi-layer microstrip square-ring arrays, was presented in [24].…”