Abstract. In this work we present the concept and design of an ultrathin (λ/22) terahertz

Recent advances in designing time-reversal-invariant photonic topological insulators have been extended down to the deep subwavelength scale, by employing synthetic photonic matter made of dense periodic arrangements of subwavelength resonant scatterers. Interestingly, such topological metamaterial crystals support edge states that are localized in subwavelength volumes at topological boundaries, providing a unique way to design subwavelength waveguides based on engineering the topology of bulk metamaterial insulators. While the existence of these edge modes is guaranteed by topology, their robustness to backscattering is often incomplete, as time-reversed photonic modes can always be coupled to each other by virtue of reciprocity. Unlike electronic spins which are protected by Kramers theorem, photonic spins are mostly protected by weaker symmetries like crystal symmetries or valley conservation. In this paper, we quantitatively studied the robustness of subwavelength edge modes originating from two frequently used topological designs, namely metamaterial spin-Hall (SP) effect based on C6 symmetry, and metamaterial valley-Hall (VH) insulators based on valley preservation. For the first time, robustness is evaluated for position and frequency disorder and for all possible interface types, by performing ensemble average of the edge mode transmission through many random realizations of disorder. In contrast to our results in the previous study on the chiral metamaterial waveguide, the statistical study presented here demonstrates the importance of the specific interface on the robustness of these edge modes and the superior robustness of the VH edge stated in both position and frequency disorder, provided one works with a zigzag interface.

1 of 5) 1600606 design becomes cumbersome and leads to similar complexities as the TO approach. The challenging requirements on the electromagnetic parameters imposed by TO can be relaxed when designing carpet cloaks [12] based on the quasi-conformal mapping technique. [12,13] In this case, cloaking is achieved by tailoring the material properties around the object to compress the effective volume of the object into an infinitesimally thin sheet over a ground plane. This transformation can be achieved with nonmagnetic materials with an inhomogeneous permittivity. Low refractive index gradient materials have also been explored to improve the impedance matching with free-space. [14] Recently metasurfaces, i.e., artificially engineered ultrathin surfaces, have attracted significant interest, as they provide a powerful way of controlling the electromagnetic wave by manipulating the local amplitude and phase of the field. [15][16][17] As a result, a different approach to cloaking has been recently introduced based on gradient metasurfaces. [18][19][20][21] The cloaking effect is achieved by manipulating the phase of the reflected wave along the boundary of the concealed volume. In this approach, the cloak is composed of an ultrathin gradient metasurface, artificially engineered with subwavelength elements that are tailored to control the phase of the reflected wave and restore the wavefront scattered from the object, emulating a flat ground plane. A metasurface cloak based on this principle has been experimentally demonstrated in optics, [22] showing interesting perspectives for this cloaking technique. Moreover, a recent experimental work on metasurface-based cloaks in the low-band microwave range shows the possibility of restoring not only the amplitudes and phases but also the initial polarization. [23] Although for higher frequencies the fabrication is more challenging, due to the required smaller sizes of the metasurface elements, this method can also find applications in other frequency ranges, such as the emergent millimeter-wave and THz bands. A polarization-independent metasurface-based cloaking device in these frequency ranges may be promising in a variety of sectors such as communication, security, military, space, and biomedical applications. [24,25] Moreover, as mentioned above, a response independent of polarization is critical for real-life applications given that the polarization of the incident wave is usually unknown. In our previous work, [20] we numerically analyzed a terahertz carpet cloak based on a gradient metasurface made of closed ring resonators. We demonstrated that such a cloak is able to restore the wavefront and phase of the reflected beam, reducing the unwanted scattering from a bump. Here, we improve this theoretical design and experimentally verify a cloaking prototype operating at 80 GHz, demonstrating scattering cross-section reduction from a metallic bump over a ground plane within relatively wide range of angles and frequencies for both transverse electric (TE) and transverse magn...

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An all-metallic steerable beam antenna composed of an ε-near-zero (ENZ) metamaterial lens is experimentally demonstrated at 144 GHz (λ 0 = 2.083 mm). The ENZ lens is realized by an array of narrow hollow rectangular waveguides working just near and above the cut-off of the TE 10 mode. The lens focal arc on the xz-plane is initially estimated analytically as well as numerically and compared with experimental results demonstrating good agreement.Next, an open-ended waveguide is placed along the lens focal arc to evaluate the ENZ-lens antenna steerability. A gain scan loss below 3 dB is achieved for angles up to 15º. 2Within the framework of metamaterials, 1,2 artificial materials with ε-near-zero (ENZ) have been increasingly studied over the past few years due to their almost infinite phase velocity property providing interesting effects such as squeezing, tunneling and supercoupling reported initially at microwaves. [3][4][5][6][7] These properties have also been demonstrated recently using metal-dielectric-metal multilayers at near infrared and visible frequencies. 8 hazardous conditions and higher operation power. Moreover, given the concave profile of an ENZlens 28 , the illumination efficiency is improved compared with common dielectric lenses and some metamaterial lenses with convex profiles whose most distant edges are poorly illuminated. 22,29 In this paper, the mechanical beam steering capabilities of an all-metallic plano-concave ENZlens antenna 16,18 are experimentally demonstrated at 144 GHz (D-band of millimeter waves). First, the radiated power is measured at seven different angles (0º, 3º, 6º, 9º, 12º, 15º, and 18º) as a function of the feeder position on the xz-plane. Owing to reciprocity, this is equivalent to raster scanning the xzplane when the ENZ-lens is illuminated obliquely from its planar interface. These results are supported with analytical calculations based on the Huygens-Fresnel principle and numerical simulations using a plane-wave under the proper oblique incidence illumination. Second, the steering capability of the proposed ENZ-lens antenna is experimentally demonstrated by measuring the radiation pattern while shifting an open-ended rectangular waveguide (WR-7) used as a feeder at the experimental foci positions obtained in the first experiment. The experimental and analytical results demonstrate a high gain of 11 dB for 0º at the operational frequency and a gain scan loss below 3 dB for steering up to 15º. 3The fabricated plano-concave ENZ-lens is shown in Fig.1 mm, with a total number of 33  144 narrow hollow waveguides along x-and y-axes, respectively. As it has been described previously metallic waveguides working near cut-off, can emulate an ENZ medium with near zero values of propagation constant and phase advance inside the medium.Therefore, as it has been mentioned previously, if one face of the lens is planar, the other one should be designed with a concave hemi-spherical profile 15,28 instead of convex in order to convert a spherical phase front into a planar pha...