Broadband, wide-angle, low-scattering terahertz wave by a flexible 2-bit coding metasurface

Xin Yan; Lanju Liang; Jing Yang; Weiwei Liu; Xin Ding; Degang Xu; Yating Zhang; Tiejun Cui; Jianquan Yao

doi:10.1364/OE.23.029128

1. Introduction

Metasurfaces are periodic or quasi-periodic two-dimensional planar arrays of sub-wavelength elements. These materials exhibit several interesting phenomena that cannot be normally found in nature and allow more freedom in manipulating the phase, amplitude, polarization, and pattern of electromagnetic (EM) waves [1–5]. This field is thus the latest research direction on artificial EM materials. Capasso et al. presented a gradient metasurface for achieving anomalous reflection and refraction phenomena, which are in agreement with generalized laws derived from Fermat’s principle in 2011 [6]. Giovampaola and Engheta put forward a new concept of “digital metamaterial” that offers a simple but powerful approach toward achieving various functions in 2014 [7]. Cui et al. also proposed coding, digital, and programmable metamaterials with distinct abilities for manipulating EM waves in 2014 [8]. These special metasurfaces may greatly simplify the design and improve the flexibility of specific device functionalities; hence, metasurfaces are valuable for designing various devices with different frequencies (i.e., from microwaves to the visible region), particularly within the so-called terahertz (THz) gap [9–14].

THz waves belong to a spectrum with numerous important technological applications, such as imaging, radar, military, and security detection, among others. To develop THz application, high-performance broadband transmission devices are in high demand, such as isolator [15], beam scanning devices [16, 17] and plasmonic lens [18, 19]. And these applications also require a low reflection metasurface to effectively control the EM wave energy [20–24]. In these applications, THz radar presents significant advantages over microwave radar systems in terms of remote sensing and homeland security because of its high spatial resolution and extremely wide band [25–29]. Hence, novel designs of low-scattering surfaces are highly desirable. Several designs have been proposed to realize broadband EM wave absorption in the THz range [30–36]. Ye et al. proposed a polarization-insensitive and broadband THz absorber using I-shaped resonators [37], while Liu et al. proposed a broadband THz absorber using multi-layer stacked bars [38]. Unfortunately, most of these THz absorbers are fabricated on a rigid substrate and achieve their functions by using a multi-layer metallic structure; hence, difficulties in conforming with the covered object are observed and the fabrication process is fairly complex [39–41]. Moreover, the EM wave was consumed by the dielectric isolation layer and metallic surface and then transformed into heat radiation. Increases in the object temperature will inevitably increase the possibility of detection with infrared detectors [42–44]. Therefore, designing a new flexible, broadband, nonabsorptive, low-scattering metasurface operating at THz frequencies remains an important challenge.

In this paper, a flexible, wide-angle, broadband low-scattering nonabsorptive metasurface was developed by coding the “00”, “01”, “10” and “11” digital elements at THz frequencies. Unlike conventional metamaterial absorbers, the wideband low-scattering is attributed to the reflection phase differences between the four basic digital elements. The reflection energy from the metasurface is scattered in various directions, resulting in the low intensity and uniform distribution of the reflected wave. Experiments and simulation results show that the proposed metasurface is capable of producing a low-scattering wideband at less than –10 dB from 0.8 THz to 1.5 THz and incident angles of up to 50°. The metasurface demonstrated minimal changes as it was wrapped around a metallic cylinder with different dimensions. These characteristics render the proposed novel way with promising application prospects in THz radar, imaging, and several other applications.

2. The 2-bit coding flexible metasurface design and simulations

The low-scattering flexible metasurface is shown in Fig. 1,this metasurface consists of three layers. The top layer includes metallic structure with a number of double cross lines patterned on the flexible polyimide (PI) film, and the bottom layer is a metallic film with a thickness of 200 nm. We propose the basic digital element particles based on double cross lines owing to the symmetry with six-fold rotational symmetry [45]. To achieve a 2-bit coding metasurface with superior THz scattering properties, the sequences of the “00”,“01”,“10” and “11” digital elements were optimized with particle-swarm optimization algorithm.

Fig. 1 Coding metasurface and double cross metallic line coding particles. (a) Schematic of a 2-bit coding metasurface. (b) Schematic of the whole unit cell. (c) Four basic “00”, “01”, “10”, and “11” digital elements. For the “01” element, w = 8 μm, L = 56 μm, for the “10” element, w = 8 μm, L = 86 μm, for the “11” element, w = 16 μm, L = 120 μm, and D = 120 μm.

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Particle swarm optimization (PSO) is a population-based stochastic optimization algorithm,which originally proposed by Eberhart and Kennedy in 1995 as a simulation of social behavior of bird flocks and fish schooling [46]. And it has been used in several electromagnetic design problems due to its simplicity [47]. During the process optimization, we use a far-field pattern prediction algorithm as an auxiliary module in full-wave simulation [4]. The 2 bit coding metasurface manipulate THz waves through different coding sequences of “00”,“01”,“10” and “11” digital elements. The reflection phase difference of neighboring elements is around 90°. To quantitatively the low scattering characteristic of the coding metasurface, we consider the metasurface contains N × N equal-sized lattices with dimension D. Each lattice is occupied by one of the four elements. The scattering phase is assumed to be $ϕ (m, n)$ . Under the normal incidence of plane waves, the far-field function scattered by the metasurface is expresses as [8]:

f (θ, ϕ) = f_{e} (θ, ϕ) \sum_{m = 1}^{N} \sum_{n = 1}^{N} \exp {- i {ϕ (m, n) + K D \sin [(m - \frac{1}{2}) \cos ϕ + (n - \frac{1}{2}) \sin ϕ]}}

Where

θ

and

ϕ

are the elevation and azimuth angles of an arbitrary direction, respectively, and

f_{e} (θ, ϕ)

is the pattern function of a lattice. The directivity function

D i r (θ, ϕ)

of the metasurface can be given as:

D i r (θ, φ) = 4 π | f {(θ, φ)}^{2} | / \int_{0}^{2 π} | f {(θ, φ)}^{2} | \sin θ d θ d φ

Here, the

f_{e} (θ, ϕ)

term for 2 bit coding metasurface has been eliminated for the 180° reflection phase different between digital elements. Therefore, the coding metasurface can be accomplished by appropriately distributing the basic four digital elements, and each distribution of metasurface will result in a specific scattering pattern.

The proposed lower-scattering metasurface is shown in Fig. 1(a), and the digital elements are randomly arranged to achieve the desired diverse scattering pattern. The PI has a thickness of h = 40 μm with a dielectric constant 3.1and a loss tangent 0.05. The size of a unit cell of the metasurface is 2.4 mm × 2.4 mm, which contains 20 × 20 coding particles. The basic digital element particles are “00”, “01”,“10”, “11” as shown in Figs. 1(c)-1(f). The “00” element was realized using no metallic structure, where as “01”, “10”, and “11” elements were realized using three structures formed from different sizes of metallic double cross lines. The distance between double cross lines is D = 120 μm. For the “01” element, w = 8 μm and L = 56 μm; for the “10” element, w = 8 μm and L = 86 μm; finally, for the “11” element, w = 16 μm and L = 120 μm.

To understand the properties of the basic digital elements, numerical simulation results were performed by CST Microwave Studio, as shown in Fig. 2. Two resonances were observed in the reflection spectrum of the “10” element at 0.7 and 1.5 THz. From simulation results, we obtain the electric-field distributions excited in the double cross lines, as shown in Fig. 2(a), in which the electric fields reach maximums at different positions, showing strong reflections on the double metallic line surface. So the designed metasurface is a nonabsorptive metasurface. The reflection phase difference of neighboring elements for metasurface is around 90° in a wideband of frequencies from about 0.7 THz to 1.5 THz, which results in the reflected energy redistributing in numerous directions, thereby creating a broadband low-scattering metasurface. Moreover, the reflection phase difference of the “00” and “10” elements revealed a small change at incidence angles less than 60°, as shown in Fig. 2(c); this wide-angle characteristics were also observed in the other basic elements. Thus, simulation results show that the designed coding metasurface is insensitive to oblique incident angles during actual circumstances.

Fig. 2 Simulated reflection spectra and reflection-phase differences of different elements. (a) The simulated reflection of “10” elements. (b) The simulated reflection-phase difference of “00,” “01,” “10,” and “11” elements at normal incidence angle. (c) The simulated reflection-phase difference of “00” and “10” elements at different incident angles. (d) The reflection spectra of the 2-bit coding metasurface for TE and TM polarizations at normal angle. (e) The reflection spectra of the 2-bit curved coding metasurface wrapped around a metallic cylinder with the diameter of 8 mm for TE and TM polarizations at normal angle.(f) Schematic of the 1D sequences.

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In order to understand the broadband characteristics of the coding metasurface, the reflection spectra of flat metasurface angle were simulated for TE and TM polarizations at normal incidence, as shown in Fig. 2(d). The reflectivity less than –10 dB were achieved over a wide frequency range from 0.75 THz to 1.45 THz for both polarizations, which is about 63.6% relative to the center frequency. Moreover, a narrow band low reflectivity at about 0.64 THz was observed. For further application, the metasurfacewas bent around a metallic cylinder with a diameter of 8 mm and studied in terms of its broadband scattering properties. Figure 2(e) shows that the reflectivity less than –10 dB from about 0.75 THz to 1.45 THz can be achieved by a curved metasurface for both polarizations at normal incidence and that a narrow low-reflectivity band can also be observed at about 0.64 THz. the scattering property of the coding metasurface was insensitive to THz wave polarization because of the eight-fold rotational symmetry of the double cross lines. In summary, the simulation results show that broadband reflection characteristics can be achieved for flat and curved metasurface, and it is insensitive to THz wave polarization.

Figures 3(a)–3(d) show the simulated 3D scattering patterns of the 2-bit coding flat metasurface at 0.6, 0.8, 1.4, and 1.6 THz. To compare the scattering properties of the designed metasurface and the metallic plate quantitatively, the scattering patterns of the metasurface on the xoy-plane were simulated at 0.4, 0.6, 1.4 and 1.6 THz, as shown in Figs. 3(e)–3(l). A scattering peak in the backward direction was evident for the metallic plate. By contrast, no significant elevation of side lobes was observed for the coding metasurface at 0.8 and 1.4 THz. As the existence of several beams over a wideband frequency range can be inferred from these results, the scattered energy in each beam must be low according to the law of energy conservation.

Fig. 3 Scattering patterns of the flat 2-bit coding metasurface for normal incidence. Three-dimensional scattering patterns of the metasurface at (a) 0.4, (b) 0.6, (c) 1.4, and (d) 1.6 THz. Scattering patterns of the metasurface on the xoy-plane at (e) 0.4, (f) 0.6, (g) 1.4, and (h) 1.6 THz. Scattering patterns of the metallic plate on the xoy-plane at (i) 0.4, (j) 0.6, (k) 1.4, and (l) 1.6 THz.

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Above scattering behaviors can also be interpreted by simple theory. By treating each of these elements as a dipole radiation source, the far-field radiation in the top half space is given by, where A is the complex scattering amplitude vector. The overall electric field at a far field position r can be expressed as the sum of the sources (neglecting the factor the $e^{- i ω t}$ ):

E = \sum_{n} A_{n} e^{i k \cdot (r - r_{n}^{'})} = \sum_{n} A_{n} e^{i (\frac{2 π \sin θ}{λ} (x - x_{n}^{'}) + \frac{2 π \cos θ}{λ} z)}

where n indicates the nth source and

x_{n}^{'}

is the position of the nth source, as shown in Fig. 2(f). This may be caused by the assumption of spherical radiation of the sources. This equation implies that anomalous behaviors come from the superposition of radiation from each “00”, “01”, “10”, and “11” digital element. The broadband of the low-scattering coefficient of the coding metasurface was realized by optimizing the sequences of the basic elements, and the proposed metasurface was constructed with a single metallic structure layer. Thus, the proposed metasurface is easier to fabricate than other multi-layer metasurface.

3. Metasurface fabrication and experiments

The proposed flexible 2-bit THz coding metasurface was fabricated through a lift-off process. First, the liquid polyimide (viscosity, 3600 centipoise) was spin-coated on a silicon substrate to form an about 8 μm-thick PI film layer after curing for 5 h. A 40 μm-thick PI layer was fabricated by repeating the processes above. Second, the lift-off technique was used to obtain a metallic layer pattern. Third, the sample was rinsed in HF for about 15 min and then carefully peeled off from the silicon substrates. Finally, a 200 nm-thick gold layer was deposited by using an electron beam evaporator on the other side of the PI film. A microscopic image of part of the sample is shown in Fig. 4(a), and the size of the entire sample is 2 inch, as shown in Fig. 4(b).

Fig. 4 Measurement results of the 2-bit THz coding metasurface over a wide frequency band under different incidence angles. (a) Microscopic image of a sample portion. (b) The whole coding metasurface sample. Measured reflection spectra of the flat metasurface for (c) TE and (d) TM polarizations. Reflection coefficient maps of the flat metasurface for (e) TE and (f) TM polarizations. Reflection coefficient maps of the curved metasurface wrapped around a metallic cylinder with the diameter of 37 mm for (g) TE and (h) TM polarizations.

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The scattering features of the coding flat and curved metasurfaces were measured by using a reflection system through fiber-coupled THz time domain spectroscopy system. A major feature of this system is that the THz emission and detection are arranged in the optical guide respectively. The angle between them can be adjusted by rotating the two optical guides. Reflection was normalized to that of a gold mirror with the same sample size. Reflection spectra of the flat 2-bit coding metasurface under TE and TM polarizations are shown in Figs. 4(c) and 5(d), respectively. Reflectivity values less than –10 dB were achieved from 0.8 THz to 1.5 THz, which is about 60.8% relative to the center frequency for both TE and TM polarizations. Wideband characteristics are maintained when the incident angle is less than 50°. Low reflectivity was also obtained at 0.64 THz, and the reflectivity of this resonance frequency decreased as the incident angle increased.

Fig. 5 Simulated results for the curved 2-bit THz coding metasurface wrapped around a metallic cylinder. Far-field patterns of the metasurface at (a) 0.6, (b) 0.8, (c) 1.4, and (d) 1.6 THz. Metallic cylinder with a diameter of 4 mm. Magnetic field distributions H_z for a metallic cylinder and the curved metasurface at 0.6 (e, f) THz and (g, h) 1.4 THz. (e, g) Only metallic cylinder. (f, h) Metasurface wrapped around a metallic cylinder with a diameter of 1 mm.

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In order to characterize the flexibility of our designed coding metasurface, the sample was bent and wrapped around a metallic cylinder with a diameter of 37 mm. Figures 4(e) and 5(f) show the reflection maps at various incident angles for TE and TM polarizations. Results clearly indicate that the reflectivity of the broadband frequency range is nearly unity for incidence angles up to 50°. Low-reflection characteristics could still be achieved over a wide frequency range and the reflection feature changed only minimally for the curved metasurface. These results imply that our flexible coding metasurface can conform with other objects, thereby satisfying the requirements of practical applications. A comparison of the actual experiments with simulation results reveals good agreement with each other.

We also studied the scattering characteristics of a flexible 2-bit coding metasurface wrapped on a metallic cylinder with a diameter of 4 mm. Figures 5(a) and 5(b) demonstrate the far-field patterns of the scattering THz waves. The far-field patterns obtained are remarkably similar to those of the flat coding metasurface but have more uniform intensities. To enhance the characteristics of the curved coding metasurface, we also simulated the magnetic field distributions H_z for a metallic cylinder only and a metasurface wrapped around a metallic cylinder with a diameter of 1 mm at 0.6 and 1.4 THz, as shown in . The magnetic fields H_z for the metallic cylinder only at 0.6 and 1.4 THz are shown in Figs. 5(e) and 5(g), respectively; here, strong back-scattering characteristics were observed. By contrast, the back-scattering field of the curved metasurface became very weak at 0.6 and 1.4 THz, as shown in Figs. 5(f) and 5(i), respectively.

The scattering spectrum maps of 2-bit coding for flat and curved metasurfaces were measured under TE and TM polarizations at an incidence angle of 13° as shown in Fig. 6. Figures 6(a) and 6(b) show the experimental scattering spectra of the curved metasurface wrapped around a metallic cylinder with a diameter of 37 mm. Here, the scattering coefficients are less than −10 dB over a wide frequency range from 0.75 to 1.5 THz. These measurements results indicate that the proposed 2-bit coding metasurface is a wideband low-scattering metasurface. From Figs. 6(c) and 6(d) we also know that the broadband low-scattering characteristics of the flat metasurface also are consistency with those of the curved metasurface. Compared with that of the flat metasurface, the scattering distribution in the broadband frequency range of the curved metasurface is more uniform. So this proposed metasurface can be conformal to the curved objects.

Fig. 6 Measurement results of the flat and curved THz 2-bit coding metasurface over a wide frequency under an incidence angle of 13°. Scattering coefficients maps of the curved metasurface wrapped around a metallic cylinder with a diameter of 37 mm for (a) TE and (b) TM polarizations. Scattering coefficient maps of the flat metasurface for (c) TE and (d) TM polarizations.

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4. Summary

In summary, we proposed and fabricated a new broadband, wide-angle, low-scattering, and polarization-insensitive 2-bit coding nonabsorptive metasurface based on an eightfold symmetric double cross metallic line structure with special THz properties. The mechanism of low THz scattering was achieved by redirecting EM energies to all directions through optimization of the arrangement of basic “00”, “01”, “10” and “11” digital elements. The measured wideband scattering characteristics of metasurface were consistent with the simulation results. The superior properties of the coding metasurface can be realized not only on flat surfaces but also on curved metallic cylinders with different dimensions. These results major benefit from the broadband scattering of wide angle and flexible bending capacity of design coding metasurface.

The proposed low scattering coding metasurface may be potentially applied to stealth technology, imaging and so on. Both numerical simulations and experimental results demonstrate that the reflection energy of the metasurface is scattered in various directions from 0.8 THz to 1.5 THz and incident angles of up to 50°, so in each direction of the energy for scattering is small based on the energy conservation principle. Consequently, the radar cross section of the metasurface is reduced in broadband frequencies and wide angles. Moreover, flexible metasurface was easy conformal to any curved surface, which obviously may broaden its practical applications in stealth applications. And the diffuse reflection radiation characteristics of the coding metasurface are diverse under different frequencies, which can be applied in compressive sensing imaging technology as a single-channel aperture. The coding metasurface can be control the radiation beams with simpler approach by the coding sequences of digital particles, so this present work provides a potential method to control THz waves flexibly and introduces new opportunities for developing novel THz devices.

Acknowledgments

Xin Yan and Jing Yang contributed equally to this work. This work is supported by the National Natural Science Foundation of China (NSFC) (Grant No. 61271066), the China Postdoctoral Science Foundation (Grant No. 2015M571263), the Science and Technology Development Program of Shandong Province (Grant No. 2013GGA04021), and the High Education Science Technology Program of Shandong Province (Grant No. J15LN36). The authors thank Hou-Tong Chen for theoretical guidance and Hao Qian for his enthusiasm and drawing of the illustrations.

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Broadband, wide-angle, low-scattering terahertz wave by a flexible 2-bit coding metasurface

Abstract

1. Introduction

2. The 2-bit coding flexible metasurface design and simulations

3. Metasurface fabrication and experiments

4. Summary

Acknowledgments

References and links

Cited By

Figures (6)

Equations (3)

Optics Express