Reconfigurable Metasurface Cloak for Dynamical Electromagnetic

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Reconfigurable metasurface cloak for dynamical electromagnetic illusions Cheng Huang, Jianing Yang, Xiaoyu Wu, Jiakun Song, Mingbo Pu, Changtao Wang, and Xiangang Luo ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01114 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Reconfigurable metasurface cloak for dynamical electromagnetic illusions Cheng Huang,† Jianing Yang,†,‡ Xiaoyu Wu,† Jiakun Song,† Mingbo Pu,† Changtao Wang† and Xiangang Luo*,†

†

State Key Laboratory of Optical Technologies on Nano-Fabrication and

Micro-Engineering, Institute of Optics and Electronics, Chinese Academy of Sciences, P. O. Box 350, Chengdu 610209, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

The first two authors contributed equally to this work.

ABSTRACT: Considerable attentions have been focused on the realization of carpet cloaks in the past decade. Most of the current carpet cloaks can perform only one electromagnetic (EM) shape for concealing inside objects, and dynamical control of arbitrary EM virtual shapes remains a significant challenge. Here, a desirable method is proposed to construct a reconfigurable carpet cloak using tunable metasurface technology. By controlling the active elements loaded on the metasurface, the surface phase distribution can be dynamically changed to make the cloak generate the predesigned scattering field. This reconfigurable metasurface cloak is experimentally realized at the microwave frequency, which demonstrates that it cannot only reduce unwanted scattering for imitating the reflection of a flat metallic plane, but also mimic other different EM virtual shapes by reconfiguring the phase distribution. To the best of our knowledge, this is the first carpet cloak that achieves dynamical illusions by switching the EM shapes without changing its physical shape. The proposed

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technology could be also extended to the terahertz and even optical domain. KEYWORDS: carpet cloak, tunable metasurface, phase control, dynamical illusions, virtual shaping

Invisibility has existed in the realm of science fantasy for mankind over several centuries. With advent of metamaterials and the emergences of concepts such as transformation optics (TO)1 and scattering cancellation,2 the possibility of cloaking object has become a technological reality, and the corresponding research has been rapidly developed to be one of the most fascinating topic in material science and electromagnetic (EM) community.1-10 TO proposed by Pendry et al.1 and Leonhardt 3 offers a powerful method to control the EM waves, leading to the birth of the first invisibility cloak.5 However, the TO-based cloak is severely dependent on precise design of inhomogeneous and anisotropic profiles, and the fabrication of volumetric metamaterials is also too complicated using the available techniques. The above constraints limit the popularization of such cloak in the real applications. Another simple approach is the plasmonic cloaking based on scattering cancellation proposed by Alù.2 In this frame, only a thin patterned metamaterial is required to construct the cloak, in which the anti-phase response of the scattering wave radiated by the object to be cloaked and the outer meta-structure is produced and then the total scattering is dramatically reduced. This cloak is obviously simple and more robust, whereas it is restricted to the size of the concealed objects. Afterwards, the concept of the carpet cloak was proposed to release the inherent constraint of full cloaking technique,4


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which exploits the quasi-conformal mapping to minimize the anisotropy in a two dimensional coordinate transformation. In this case, the carpet cloak is utilized to hide an object on a reflective surface, but it is still too volumetric to realize in practice for its high profile. Recently, the advent of metasurfaces provides promising new venues for researchers to control the scattering wave due to its strong ability in the local phase modulation of the field,11-20 and so far several ultra-thin carpet cloaks have been realized in the microwave and even visible regimes.21-25 Benefited from surface phase compensation of the introduced metasurfaces, a bump can reduce the unwanted scattering and restore the mirror-reflection of a flat metallic plane. Compared with the traditional TO method, using metasurface can make the carpet cloak ultrathin and easy to be designed without involving the complex material parameters although the metasurface cloaks can only realize unidirectional invisibility. As far as we know, most of the carpet cloaks realized so far only perform the EM shape of a metallic flat plate. Compared with the single EM scattering property realized by the carpet cloak, using it to achieve arbitrary or multiple EM virtual shapes remains a significant challenge. Here, we propose a novel reconfigurable carpet cloak based on the tunable metasurface technology,26-33 which can dynamically adjust its EM virtual shapes for realizing illusions. Although there have been several works on the design of the reconfigurable cloaks,34-36 none of them is aimed at the generation of the multiple EM virtual shapes.


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In our design, the tunable metasurface can realize the dynamical and continuous modulation of reflection phase. Through special design of the surface phase profile, the carpet cloak can reshape the scattering wave into predetermined wavefronts for generating different EM virtual shapes. Therefore, it is difficult for the observer to identify the real physical shape from various scattering signatures. Experimental demonstration is performed at the microwave frequencies, which shows that our reconfigurable cloak can not only mimic a metallic flat plate, but also perform a good dynamical illusion performance by switching to different EM shapes. This new concept could be developed for potential applications in EM camouflage and illusion field.37, 38 

RESULTS AND DISCUSSION

Concept and Analytical Design. The operation principle of the proposed reconfigurable carpet cloak is schematically illustrated in Figure 1a. The cloaked bump provides a triangular space for concealing objects inside. When light illuminates a metallic bump, the wavefront of the reflected wave would be distorted due to the production of an unnecessary phase shift. The proposed cloak composed of many tunable phase-shifting elements could effectively compensate the phase difference between the wave reflected by the reference flat plane and the boundary of the bump, thus reconstructing the wavefront of the scattering wave as if light was incident on a flat metallic plate. In this case, the cloak performs an EM shape of a metallic flat plate. By further reconfiguring the surface phase distribution through a


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programmable voltage source, the cloaked bump could behave other different EM shapes to delude the observers. Figure 1b shows the design theory of this reconfigurable carpet cloak. When light impinges onto a bump with a height of f(x) along -z direction, the reflection phase of the bare bump will be

bump    2k0 * f ( x)

(1)

where k0 is the wave vector in free space. The metasurface cloak will introduce another reflection phase of φcloak, bringing in an equivalent height of g(x). So the total reflection phase of the cloaked bump can be expressed as

Total  bump   cloak    2k0 * g ( x )

(2)

In the previous passive cloak, g(x) is a constant that is generally set to be zero. At the case of g(x) = 0, φTotal = π and thus the cloaked bump will mimic a metallic flat plate. Interestingly, g(x) is a variable in our design since it can be dynamically changed by tuning the reflection phase of the proposed metasurface. According to the predesigned EM shapes of g(x), the corresponding φTotal can be calculated and then the tunable metasurface is utilized to construct the surface phase profile of the cloaked bump, based on the metasurface-assisted law of reflection and refraction (MLRR).13 Figure 2a shows the design of the reconfigurable carpet cloak using the tunable metasurface. The metallic bump with the tilt angle of 13° is fully covered by our metasurface cloak. The voltage-controlled varactors embedded into each meta-particle are employed for controlling the reflection phase. The tunable metasurface is divided into 2×n regions, and all the meta-particles share the same capacitance configuration


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within the identical region. The capacitance values in each region from top to bottom side of the bump are described as C1, C2… Cn (at the left side of the bump) and Cn+1, Cn+2 … C2n (at the right side of the bump), respectively. We can independently control the capacitance value at each region to make our cloak generate different surface phase distributions. The designed meta-particle is composed of two semicircle patches linked by a microstrip across two varactors, as displayed in Figure 2b. All the metallic patterns are etched on top side of a 0.254 mm thick Rogers 5880 substrate with relative constant of 2.2 and loss tangent of 0.0009. The bottom side of the substrate is separated from a metallic plate by a 4.5 mm thick foam spacer. The model of the selected varactor diode is SMV1405 from Skyworks Solutions Inc., and its capacitance tuning range is from 0.6 pF to 2.67 pF.39 The radius of the semicircle patch is 10 mm, and the size of the microstrip is 14 mm × 1 mm. The period of the meta-particle is 36 mm（~ λ/4, λ is the working wavelength in free space). The equivalent circuit model is adopted to explain how this meta-particle modulates the phase of electromagnetic wave. The metallic part can be modeled as inductor, and L1 and L2 are respectively used to describe the inductance from the metallic semicircle patches and the ground plane. The two series varactors are represented by the tunable capacitor C. The whole meta-particle behaves as the termination load of the transmission Z eff ( ) 

 

line,

and

its

jwL2 ( jwL1  1/ jwC  R) . jwL2  ( jwL1  1/ jwC  R)

Z eff  Z 0 Z eff  Z 0

load

impedance

can

be

deduced

as

Therefore, the reflection coefficient is calculated as

, where Z0 is the free-space wave impedance. By tuning the capacitance

value of the varactors, the surface impedance of our metasurface would be changed,


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thus resulting in the phase variation of the reflected wave. Figure 2c shows the reflection characteristic of the proposed meta-particle that is investigated by a commercial software of CST Microwave Studio.40 When tuning the capacitance value from 0.6 pF to 2.67 pF, the reflection phase of this meta-particle illuminated by an x-polarized wave has obvious variation and its varying range exceeds 320°around 2.4 GHz, satisfying most of the phase compensation requirement of the cloaked bump. The reflection loss is less than 3 dB within the phase-tuning range, implying the high reflectivity of the designed metasurface. It is still noted that the large phase-tuning range remains even at the oblique incident angle of 30°.

Numerical Analysis of the proposed cloak. In order to investigate the cloaking performance of the proposed carpet cloak, the metasurface composed of 1×72 unit cells is utilized in the full-wave simulation with periodic boundary condition set to y sides, which can reduce calculation volume and keep the calculation accuracy. We first verify the capability of our reconfigurable cloak to emulate the flat metallic plate. When an x-polarized light normally impinges onto the cloaked bump, the required phase compensation values can be calculated according to equation 1-2, and thus the corresponding capacitance values at each region is acquired based on the simulated phase response of the meta-particle. Table S1 lists the capacitance configurations of the metasurface cloak. The simulated electric field distribution of the cloaked bump are presented in Figure 3 which also includes simulation results of the flat metallic plate and bare bump as comparison. For the bare bump under normal illumination, the


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backward scattering wave is mainly divided into two directions, which is obviously different from the mirror reflection of the flat metallic plate (see Figures 3a and 3b). When using the tunable metasurface to complete the surface phase compensation, the wavefront of the scattering field for the cloaked bump is restored as if light was incident on a flat metallic plate (see Figure 3c). The scattering patterns of the above three cases are also investigated. It is found in Figure 3d that the bare bump distorts the scattering field and generates two scattering peaks in the far field, while the flat metallic plate produces only one peak at 0°position, as seen in Figure 3e. The cloaked bump can successfully restore the phase distortion using the employed metasurface and thus realize a similar scattering property with the flat metallic plate. As Figure 3f shows, there is also one scattering peak occurring at normal, and in the frequency band of 2.35 - 2.45 GHz, the cloaked bump behaves almost the same scattering performances. It should be also pointed out that due to the dispersion of the phase accumulated by light during propagation and the building metasurface block, there are some relatively high sidelobes in the scattering patterns of the cloaked bump at 2.35GHz and 2.45 GHz. If we redesign the phase compensation at 2.35 GHz or 2.45 GHz, the undesired high sidelobes can be reduced by changing the capacitance configuration. To further demonstrate the dynamical illusion performance of the proposed reconfigurable carpet cloak, we adopt four typical examples to show that the designed cloaked bump has the ability to emulate other different EM virtual shapes which can be dynamically switched by controlling the capacitance distribution of varactors on


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the surface of the bump. As seen in Figure 4a, the scattering signature of the cloaked bump is similar with that of a metallic bump with a tilt angle of 5.8°(see Figure 4b), so that the observer would mistake the physical shape of the cloaked bump as a lower metallic bump. The corresponding scattering patterns given in Figure 4c also show that the cloaked bump can change the scattering peak positions of the bare bump and making them shifted from ±26°to ±11.6°, realizing the same scattering property as its intimated real object. Through changing the capacitance configuration of the varactors, the EM shape of the cloaked bump can be also switched to a higher metallic bump (θ=18.8°), as seen in Figures 4d and 4e. From their scattering patterns displayed in Figure 4f, it is still found that the positions of two scattering peaks produced by the bare bump are tuned from ±26 to ±37.6° by our reconfigurable cloak, which is in accordance with the case of the metallic bump with a tilt angle of 18.8°. In addition, we have studied other non-triangle bump shapes generated by our carpet cloak. From the analysis of the simulated field distributions and scattering patterns, it reveals that the cloaked bump is able to mimic the geometrical contour of the trapezoid bump (see Figures 4g-4i) and the wedge bump (see Figures 4j-4l) without changing its physical shape. Thus the advantage of the proposed reconfigurable metasurface cloak has been fully validated with the generation of the multiple EM shapes. Compared with the passive cloak, our cloak can dynamically alter its scattering signatures in the predesigned procedure, achieving illusion performance. Therefore, the observer cannot distinguish, and the real physical shape of our cloak cannot be accurately


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identified. The detailed capacitance configurations for the above four examples are respectively listed in Table S2-S5 of the Supporting Information. Realization of the reconfigurable metasurface cloak. In order to verify the dynamical illusion performance of our reconfigurable carpet cloak in experiment, we fabricated the sample composed of 8×16 unit cells by the printed circuit board technique, as shown in Figure 5 which also includes the measurement system. It is seen that the sample is located in the central stage of the measurement system, and two S-band horn antennas respectively utilized as the transmitter and receiver are placed on two brackets which can rotate along the sample center. Due to the finite size of the horn antennas, the incidence and reflection angles are fixed as 6°for making an approximation of the normal incidence. The distance between the source and the sample is about 30 λ, which is far enough to avoid the near field effect. We will validate three kinds of EM shapes of our cloak, which are the flat metallic plate, the metallic bump with a tilt angle of 5.8°and the metallic bump with a tilt angle of 18.8°, respectively. Limited by the horn size and measurement conditions, the receiving horn can only acquire the scattering signatures within the angular range of 6°~ 85°, when the sample is illuminated by the incoming wave from the transmitting horn. In the experiment, the reflection phase of the designed metasurface should be first measured for different bias voltages. As Figure S1 shows, the phase tuning range is over 310° when varying the bias voltages from 0V to 30V. Based on the relationship of the reflection phase and bias voltage, we can determine the voltage configurations of the cloaked bump for the corresponding EM virtual shape. Then the scattering patterns of


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the bare bump, physical object and cloaked bump are respectively measured and compared at the above three cases. There will be the distinctions of the scattering patterns between the bare bump and physical object due to their different geometrical shapes. If the cloaked bump restores its scattering signatures to be similar to that of its intimated real object at each case, the dynamical illusion performance is validated. For the bare bump, both full-wave simulation and experimental results show that there is an obvious reflection peak around the angle of 30°, as seen in Figures 6a and 6b, respectively. Its scattering peak position is different from that of the flat metallic plate under illumination with the incidence angle of 6º. After introducing the reconfigurable cloak to cover the bump, the scattering peak of the cloaked bump is restored to be at the angle of 6°and its scattering pattern is highly similar to that of the flat metallic plate, which means that the cloaked bump has successfully emulated the EM shape of the flat metallic plate at this case. Benefiting from the switchable functionality of the tunable metasurface, the cloaked bump can also generate other EM shape for realizing illusions. As Figures 6c and 6d show, it is obvious that the scattering behavior of the bare bump with the tilt angle of 5.8°can be restored by the cloaked bump. The main and secondary scattering peak positions of the cloaked bump are well matched with those of the intimated real object. At the third case, the cloaked bump is also demonstrated to emulate the EM shape of the bare bump with the tilt angle of 18.8°, as displayed in Figures 6e and 6f. The measured scattering patterns of the cloaked bump and its intimated object exhibit a good agreement. However, it should be pointed out that there are some deviations in the scattering field intensity


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between the simulated and measured results, especially for the case of bare bump. It may be due to the fact that the experimental environment has a little difference from the simulated environment. For instance, the ideal infinite uniform plane wave in the simulation is difficult to be realized by the horn antenna in the experiment, which could influence the scattering field distribution. Part of the unexpected reflection wave energy may be redirected to the receiving horn by the bare bump, causing some scattering dips in the measurement. In addition, the DC bias lines on and around the metasurface cloak, which are not considered in simulations, may also affect the scattering patterns. Nevertheless, the above measured results have clearly verified that the metallic bump covered by our reconfigurable cloak is able to generate multiple EM shapes, which can be used for achieving dynamical EM illusions. The detailed voltage configurations of the cloaked bump at the above three cases are respectively listed in Tables S6-S8 of the Supporting Information. Note that, our metasurface cloak could be also designed as other different physical shapes to cover the targeted object. As long as the metasurface unit can be individually controlled, the dynamical EM illusion effect will be expected through the surface phase modulation of the cloak. Furthermore, to achieve a reconfigurable cloak at any polarization, one can seek to design an isotropic tunable meta-particle, but the more complicated bias network would be a technical challenge that should be carefully considered. 

CONCLUSIONS We proposed a reconfigurable carpet cloak and verified its dynamical illusion

performance. Our cloak has an ultrathin profile (~λ/26), which is realized by using the


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tunable metasurface whose reflection phase distribution can be manipulated flexibly. Full-wave simulation results have shown that the triangle bump covered by our reconfigurable cloak can not only generate the mirror-reflection property that often realized in the previous passive cloak, but also dynamically emulate other objects for illusions. The proposed reconfigurable carpet cloak has been experimentally verified at the microwave frequencies. By tuning the varactor configurations on our metasurface through the programmable voltage source, the cloaked bump can successfully reconfigure its scattering patterns for imitating the three predesigned real objects. This is the first reconfigurable carpet cloak that achieves the dynamical illusion effect by switching the various EM shapes without changing its physical shape. Although the current cloak is limited by its narrow bandwidth, it may be solved by broadband achromatic metasurface technology.41,42 More broadly, The above concept could be also extended to the terahertz and optical frequencies, where modern technologies, such as phase change material29, 32 and graphene33, may be feasible for controlling and tailoring EM scattering shapes. In general, our technology opens up an unprecedented avenue to the scattering wave control, which may find a variety of real-world potential applications in EM cloaking or illusion devices. 

METHODS

Reflection Phase and Scattering Pattern Measurement. The arch-shape method is adopted to measure the reflection phase of the tunable metasurface and the scattering signatures of the triangular bump with and without the metasurface cloak. Two S-band horn antennas connected to a vector network are utilized as the transmitter and


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receiver, respectively. The transmitting horn has a fixed position and its incident angle is 6°, while the receiving horn can move along the sample center and its receiving angular range is 6°~85°. In the first step, the receiving angle was set to be 6° for measuring the reflection phase of the tunable metasurface as a function of the bias voltage. Then the receiving horn was set to rotate along the sample center with a step size of 2.5°, and the scattering signals of the sample under test was obtained by recording the reflection amplitude at each of the rotating angles. The dynamical tunability of the cloaked bump was completed by a 16-way DC voltage source which can realize independent control of the varactors at each region.


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Figure 1. (a) Schematic illustration of the reconfigurable carpet cloak using tunable metasurface technology. When light impinges onto the cloaked bump, multiple virtual electromagnetic (EM) shapes will be generated by simply changing the surface phase distributions through the tunable metasurface. The dynamical switching among the various EM virtual shapes is implemented through a computer-controlled multi-way DC voltage source. (b) Equivalent EM shape of g(x) realized by the reconfigurable carpet cloak. g1(x), g2(x) and g3(x) respectively represent the three typical examples, wherein g1(x) describes the EM shape of a reference flat plane.


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Figure 2. (a) Design of the reconfigurable carpet cloak using the tunable metasurface. (b) Schematic of the proposed metasurface unit with its equivalent circuit model. The geometric parameters of the unit cell are optimized as h1=0.254, h2=4.5, m=14, n=1 and p=36 in mm. (c) Reflection magnitude and phase as a function of the capacitance value for different incident angles at 2.4 GHz.

Figure 3. Full-wave simulation results of electric-field distributions on xoz plane and scattering patterns of the bare bump, flat metallic plate and cloaked bump under normal incidence. (a-c) Electric-field distributions of bare bump, flat metallic plate and cloaked bump at 2.4 GHz. (d) Scattering pattern of the bare bump at 2.4 GHz. (e) Scattering pattern of the flat metallic plate at 2.4 GHz. (f) Scattering patterns of the cloaked bump at 2.4GHz , 2.35 GHz, and 2.45 GHz.


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Figure 4. Full-wave simulation results of electric-field distributions and scattering patterns of four cases. (a-c) The first case: (a-b) Electric field distribution of the cloaked bump and its imitated object that is a metallic bump with a tilt angle of 5.8°at 2.4 GHz. (c) The corresponding scattering patterns. (d-f) The second case: (d-e) Electric field distribution of the cloaked bump and its imitated object that is a metallic bump with a tilt angle of 18.8°at 2.4 GHz. (f) The corresponding scattering patterns. (g-i) The third case: (g-h) Electric field distribution of the cloaked bump and its imitated object that is a trapezoid metallic bump at 2.4 GHz. (i) The corresponding scattering patterns. (j-l) The forth case: (j-k) Electric field distribution of the cloaked bump and its imitated object that is a wedge metallic bump at 2.4 GHz. (k) The corresponding scattering patterns. The black and red vertical lines show the scattering peak positions of the bare bump and cloaked bump, respectively. The scattering signature of the cloaked bump is identical to that of the corresponding real object at the above four cases, indicating that the cloaked bump has successfully imitated the EM shapes of these four real objects.


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Figure 5. Photograph of the measurement system and the fabricated reconfigurable metasurface carpet cloak.


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Figure 6. Simulated and measured scattering patterns in far field. The scattering signatures within the angular range of 6°~ 85° are shown at 2.4 GHz for different samples under illumination with the incident angle of 6°. (a, b) Simulated and measured results in the first case where a metallic flat plate is emulated. (c, d) Simulated and measured results in the second case where a bare bump with the tilt angle of 5.8°is emulated. (e, f) Simulated and measured results in the third case where a bare bump with the tilt angle of 18.8°is emulated. The metasurface cloak is wrapped over a metallic bump with the tilt angle of 13°.


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

ASSOCIATED CONTENT

Supporting Information is available free of charge on the ACS Publications website at DOI: The detailed capacitance configuration of the reconfigurable carpet cloak for the five different EM virtual shapes (Table S1-S5) in simulation; the detailed voltage configuration of the reconfigurable carpet cloak for three different EM virtual shapes (Table S6-S8) in experiment; the measured reflection phase responses of the fabricated metasurface for different bias voltages (Figure S1) (PDF). 

AUTHOR INFORMATION

Corresponding Author *

E-mail: [email protected]

Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS

This work was sponsored by the National Basic Research (973) Program of China under Grant No. 2013CBA01700 and the National Natural Science Foundation of China under Grant Nos. 61475160, 61605213 and 61775218.



REFERENCES

(1). Pendry, J. B.; Schurig, D.; Smith, D. R., Controlling Electromagnetic Fields. Science 2006, 312, 1780-1782. (2). Alù, A.; Engheta, N., Achieving Transparency with Plasmonic and Metamaterial Coatings. Phys. Rev. E 2005, 72, 016623.


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FOR TABLE OF CONTENTS USE ONLY TABLE OF CONTENTS FIGURE

Reconfigurable metasurface cloak for dynamical electromagnetic illusions Cheng Huang,† Jianing Yang,†,‡ Xiaoyu Wu,† Jiakun Song,† Mingbo Pu,† Changtao Wang† and Xiangang Luo*,†

The reconfigurable metasurface cloak is able to generate multiple electromagnetic (EM) virtual shapes for dynamical illusions without changing its physical shape, and all these EM shapes can be switched by simply changing voltage configuration of a programmable voltage source.


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Reconfigurable Metasurface Cloak for Dynamical Electromagnetic

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