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Abstract
Being invisible at will has fascinated humanity for centuries and it has become more tangible with the development of metasurfaces, which have demonstrated the extraordinary ability of wavefront manipulation. However, state-of-the-art invisibility cloaks typically work in a deterministic system with a limited bandwidth and small incident angle ranges. Here, by integrating the phase-change material of Ge2Sb2Te5 and the wavefront tailoring functionality of a reflective metasurface, we have achieved a unique carpet cloak that is endowed with broadband invisibility from 6920 to 8220 nm, fully concealing objects over a wide angular span of ±25° and a prominent radar cross-section reduction. Furthermore, the central cloaking wavelength can be continuously tuned with Ge2Sb2Te5 film under different intermediate phases by precisely controlling external stimuli, which will provide a flexible and encouraging way to achieve active features once fabricated. Simulation results also show that the cloaking bandwidth can be significantly extended by triggering Ge2Sb2Te5 from the amorphous to crystalline states. Importantly, the hybrid metasurface can realize switching of “ON” and “OFF” states in terms of cloaking features by converting Ge2Sb2Te5 from the amorphous to the crystalline state. To the best of our knowledge, this is the first metasurface carpet cloak that utilizes the phase-change material of Ge2Sb2Te5 to achieve ultra-broadband, wide-angle, continuously tunable and switchable cloaking with low profiles, light weights, and easy access. This design of a reconfigurable cloak is expected to find potential applications in various areas such as vehicle cloaking, illusions and so on.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rendering an object invisible with an invisibility cloak is a long-standing dream for humanity. An invisibility cloak can suppress the scattering field and reconstruct the polarization, amplitude, and phase of the transmitted/reflected light as if the object did not exist. Such a scenario was nearly inconceivable until the proposal of transformation optics [1] and metamaterials [2]. A transformation optics (TO)-based cloak achieves invisibility through precise design of its constitutive parameters so as to guide light around the hidden objects, but it is a major challenge to experimentally produce bulky materials with both inhomogeneous and anisotropic profiles [3–5]. Afterward, concepts of carpet cloak generated from scattering cancellation [6–8] and quasi-conformal mapping [9–12] of electromagnetic (EM) space were put forward. Scattering cancellation-based cloaks realize stealth by suppressing the dominant multipolar scattering orders, but they are faced with dilemmas, such as large size, a lateral shift of the scattered wave and so on, which renders the hidden objects easily detectable [13]. As an alternative, quasi-conformal mapping in which an inverse transformation of the permittivity and permeability leads to spatially variable refractive index profiles, can be applied to cloak with release in the inherent constraint of full cloaking technique [9]. However, it is still too volumetric to realize in practice for its high profile. More recently, a novel strategy, infrared camouflage technology, has been demonstrated to achieve camouflage /stealth [14,15]. Infrared camouflage conceals the infrared signature of objects by controlling either the surface emittance or the surface temperature of an object, and thus renders them invisible to potential threats with IR detectors. Though infrared camouflage has been rapidly developed and shown many significant advantages, it remains a challenging issue, as the thermal radiation of an object is proportional to the fourth power of temperature.
Metasurface, referring to thin planar arrangement of artificial meta-atoms in the subwavelength scale, has exhibited versatile abilities in manipulating the phase, amplitude, and polarization of EM waves [16–20]. With diverse functionalities and ultrathin thickness, many novel applications based on metasurface have been proposed and experimentally demonstrated, such as flat lenses [21,22], spin-to-orbit conversion [23,24], holograms [25,26], cloaking [27,28], illusion [29,30], to name a few. Among them, cloaking is always one of the most attractive subjects of study. Metasurfaces have opened up a new paradigm for carpet cloaks that conceal objects by mimicking the reflection behaviors of the ground plane. Metasurface-based carpet cloak can relax the limitations of cloaks based on bulky metamaterials and inherits all merits of metasurfaces, such as low-profile (ultrathin thickness) and low-cost (simple material parameters and easy fabrication processes) cloak designs, completely eliminated lateral shifts of the reflected waves [31–33] and so forth. So far, quite a few ultrathin metasurface carpet cloaks have been witnessed from microwave/terahertz to even visible regimes, showing the high flexibility and feasibility of this cloaking technique. Nevertheless, due to the resonant phase dispersion of the meta-atom and the pre-defined phase distribution for specific wavelength in the metasurfaces, most of the current metasurface carpet cloaks work in a deterministic system and operate only for single/dual wavelengths within a small incident angle range. Dynamic-controlled, ultrabroadband and wide-angle metasurface carpet cloaks are more urgent considering practical applications.
Recent studies have shown that reconfigurable metasurfaces, endowed with the dynamic wavefront manipulation, are expected to break the performance bottleneck of conventional metasurfaces and realize dynamic-controlled properties [34–37]. True reconfigurability—that is, complete change of the optical response—therefore becomes challenging, as it requires arbitrarily changing the shape of individual elements of the structure, dynamically controlling the local dielectric environment, or controlling the optical properties of the functional material itself. Currently, the reconfigurable concepts can be fulfilled via the hybridization optical metasurfaces with MEMS [38], elastic platforms [39], graphene [40,41], liquid crystals [42,43], phase-change materials (PCMs) [44–46] and transparent conducting oxide [47]. Among them, PCMs, particularly the germanium antimony telluride alloy Ge2Sb2Te5 (GST) due to its significant changes in optical properties upon exposure to external stimuli, offers an appealing approach to introduce true reconfigurability. Moreover, exotic virtues including low losses in infrared spectral ranges, multi-level intermediate states and non-volatile (latching) transition of GST provide additional degrees of freedom for tunability and flexibility. By integrating GST with plasmonic metasurfaces, changes in optical properties induced by such a phase transition can provide the means to control the resonant dispersion, and thus the hybrid metasurface can be exploited to realize reconfigurable photonic devices. To date, GST-based metasurfaces have been demonstrated to various kinds of reconfigurable photonic devices [44–46]. However, no investigation on tunable or switchable metasurface carpet cloaks based on GST has been reported up to now.
In this work, for the first time, we propose a general principle to integrate plasmonic metasurfaces and the phase-change material of Ge2Sb2Te5 into a unique hybrid carpet cloak that works in the infrared region. The designed metasurface carpet cloak is wrapped on a triangular metallic bump. The key functionalities of wavefront tailoring with metasurfaces and phase transition of GST are combined together to achieve dynamic-controlled invisibility. Numerical simulations demonstrate that the proposed design is capable of reducing the unwanted scattering waves and restoring the scattering wavefront as the mirror-reflection of a metallic flat plate in broad spectra from 6920 to 8220 nm. It is also found that the good cloaking performances of our scheme can be maintained at a wide angular span of ±25°. Furthermore, the cloak has a prominent radar cross-section (RCS) reduction and the 3 dB RCS reduction bandwidth is approximately 35.43%. Benefited from the unique phase transition of GST, the central cloaking wavelength of our design can be continuously tuned (from 7620 to 8120 nm) as the crystallization level of GST increases and the cloaking bandwidth can be significantly extended from 8220 to 8720 nm without altering the structure. Importantly, the hybrid metasurface can realize switching of “ON” and “OFF” states in terms of cloaking features when λ = 7620 nm by converting Ge2Sb2Te5 from the amorphous to crystalline state. Our results provide a feasible route to realize broadband, wide-angle, continuously tunable and switchable cloaking effect, showing a great potential application in stealth, EM camouflage and illusion field [48–51].
2. Simulation and methods
Figure 1(a) shows the schematic configuration of the proposed metasurface carpet cloak, which consists of subwavelength-scale nanoantennas that can provide distinct phase shifts locally to the reflected EM waves. The ultrathin metasurface functions as a mantle wrapped on the hidden object which can be considered as a triangular bump on the flat ground plane. The tilt angle of the triangular bump is θ = 15°. When the incident light impinges onto the bump, unwanted phase shift Δφ will be produced due to different profiles between the bump and ground plane, leading to distorted scattering fields of the reflected wave. To eliminate the distortions and render the phase of the scattered light at each point on the cloak be the same as that of light reflected from a flat mirror, the metasurface carpet cloak is required to compensate for the following phase advance/delay due to the bump [52]:
where k0 represents the free space wave vector, α is the incident angle with respect to the ground. The additional π term represents the phase jump induced by a reflecting mirror. hi = (i−1/2) p sinθ, denoting the height between the center point of the unit cell i (i=1,2,3,…,n) and the ground. The meta-atoms designed with local phase shift Δφ should realign the scattered wavefront. For the two sides of the bump, the inclination angles have opposite signs, thus the deflection angles are also reversed. Either side of the triangular bump is arranged with n unit cells from bottom to top, and p represents the center distance between two adjacent units. In our design, the metasurface carpet cloak includes 24 unit cells (n=24).
Fig. 1. (a) Schematic illustration of the metasurface cloak based on phase-change material of Ge2Sb2Te5. It can eliminate the strong scatterings caused by the triangular bump and renders the electromagnetic wave appear as if it is reflected from a flat ground. Top left inset: the side view of the cloak. Bottom left inset: the unit cell of the proposed metasurface, where p = 3 µm, the thickness of Ge2Sb2Te5 and MgF2 are 400 and 100 nm, respectively. The calculated reflection phase (b) and amplitude (c) of the meta-atom in a 2D parameter space spanned by a and b under normal incidence when λ = 7620 nm. The marks of white circles in (b) represent the nanoantennas with special geometry, which are selected to construct the metasurface carpet cloak. The insets in (b) and (c) denote the reflection phase and amplitude of the selected nanoantennas with different sizes as λ = 7620 nm, respectively, indicating the full-phase covering and efficient reflectance.
The schematic of the meta-atom adopted here is depicted at the bottom inset of Fig. 1(a), which is a prototypical metal-insulator-metal (MIM) configuration [53]. The insulator layers consisting of GST and MgF2 films are sandwiched between the bottom gold mirror and the top array of subwavelength gold nanoantennas, where the GST layer with a thickness of 400 nm functions essentially as an active element that changes the local dielectric circumstance, and the MgF2 film with 100 nm thickness is deposited on top of GST layer to protect it against oxidation in the atmosphere. Furthermore, the MgF2 layer also serves as a refraction index matching layer between high-index GST and low-index air to enhance plasmon resonances. The period p of the unit cell is set to be 3000 nm. The length a and width b of gold nanoantennas are scanned from 100 to 2900 nm for the selection of designs, and the thickness of antennas (d = 100 nm) is chosen to be as thin as possible to reduce the fabrication difficulty. The whole thickness of each meta-atom is 700 nm, only approximately 1/11 of the operating wavelength (7620 nm) in the free space.
All calculations are performed by using the versatile and commercially available software of Comsol Multiphysics. Floquet excitation ports and periodic boundary conditions are used to optimize the meta-atom. For the simulation of the designed metasurface carpet cloak, a perfectly matched layer (PML) is set around the model and a plane wave is incident vertically (α = 0°) or obliquely (α > 0°), as shown in Fig. 1(a). The permittivities of MgF2 and GST films used in the simulations are obtained from the experimental data in Ref. [45]. The metal substrate is replaced by a perfect electrical conductor (PEC) boundary in the simulations, leading to the incident electric fields to be completely blocked and thus the penetrated electric field to be essentially zero. The complex permittivity of gold nanoantennas is described by Drude model [45,54]:
3. Results and discussions
The calculated phase shifts and reflectance in a 2D parameter space spanned by meta-atom dimensions a and b at normal incidence upon λ = 7620 nm are mapped out in Figs. 1(b) and 1(c) for the selection of designs. Based on Eq. (1) that determines the ideal phase distribution, 24 different meta-atoms with phase shifts covering 0 ∼ 2π while preserving the efficient reflectance are chosen as the building blocks of the metasurface carpet cloak. The specific value of the geometric parameter for each meta-atom employed in the designed cloak is indicated with white circles in Fig. 1(b).
To validate the design, full-wave simulations of the proposed metasurface cloak were performed upon a linear y-polarized light incidence. Bared bump and metallic ground plane were also considered for comparison purposes. First, the normal-incidence cases were investigated. Figures 2(a)–2(c) show the reflected electric field distributions at the designed wavelength of 7620 nm. One can clearly see that strong scattering as well as substantial wavefront and phase distortions occurs for the bared triangular bump and the reflected waves are split into two beams along the direction about ± 30° [see Fig. 2(a)]. However, by wrapping the bared bump with the designed metasurface [see Fig. 2(b)], strong scattering disappears and the wavefront is completely reconstructed as if the incident wave is reflected by a flat metal ground [see Fig. 2(c)]. As a result, the bump/hidden object is invisible to the incident wave. In the farfield scattering patterns as shown in Figs. 2(d)–2(f), the results of bared bump, cloaked bump and metallic ground resemble to their corresponding reflected electric field distributions, i.e., the original two scattering peaks of the bared bump are strongly suppressed and rejoined to one central peak after wrapped by the designed metasurface, identical to the case of metallic ground. The above analysis has fully proved that the metallic bump wrapped by the designed metasurface is capable of emulating the EM profile of the flat ground plane at the wavelength 7620 nm upon normal incidence of light.
Fig. 2. Simulated electric field distributions in yoz plane and farfield radiation when λ = 7620 nm. (a, d) A bared bump in normal incidence. (b, e) A cloaked bump that recovers the wavefront of the scattering field. (c, f) A ground plane for comparison purpose. The corresponding 3D farfield radiation patterns are also provided in the insets of (d)-(f). The black arrows show the direction of the incidence.
Small oblique incidence cases (α = 15° and α = 25°) upon λ = 7620 nm were subsequently investigated, as shown in Fig. 3. From the reflected electric field distributions [see Figs. 3(a), 3(c), 3(e) and 3(g), 3(i), 3(k)], one can observe that the designed cloak can still cancel significantly the scattering caused by the bared bump and reconstruct the wavefront of the reflected electric field as that of the ground plane even under the oblique incidence of 15° or 25°, which is probably benefited from the flat slope of Eq. (1) around α = 0°. In the farfield scattering patterns [see Figs. 3(b), 3(d), 3(f) and 3(h), 3(j), 3(l)], we can also find that the original two scattering peaks of the bared bump occurring at the angle of −15° and 45°, or −5° and 55° corresponding to the respective incidence of α = 15° and 25° are strongly suppressed, and the split beams rejoin again at the angle of 15° or 25° after covering the bared bump with the designed metasurface, identical to the cases of the metallic ground plane. Therefore, the objects wrapped by the designed metasurface cloak can be nearly perfectly hidden even for oblique detection when λ = 7620 nm.
Fig. 3. Simulated electric field distributions in yoz plane and the farfield radiations at oblique incident angle of 15° (a-f) and 25° (g-l) when λ = 7620 nm. (a, b, g, h) A bared bump. (c, d, i, j) A cloaked bump. (e, f, k, l) A ground plane for comparison purpose. The corresponding 3D farfield radiation patterns are depicted in the insets of (b, d, f, h, j, l). All the results indicate our cloak can tolerate incident angles at least within 25°.
The great challenge of practical mantle design lies in its cloaking bandwidth due to the strong dispersive response of metasurfaces. Broadband cloaking is always of great interest. To further investigate the operation bandwidth of our metasurface cloak, the farfield radiation patterns were examined under the incident waves from 6020 to 9420 nm. As shown in Fig. 4(a), two directive beams respectively occurring at −30° and 30° for the bared bump are produced during the whole wavelength range. However, for the cloaked bump as shown in Fig. 4(b), the unwanted scattering fields are considerably suppressed and the reconstructed wavefront of reflected waves resembles to that of the ground plane from 6920 (λ1) to 8220 nm (λ2) [see Fig. 4(c)]. The farfield radiation patterns of λ1 and λ2 are then specially extracted, as shown in Figs. 4(d)–4(e), further confirming the effective and broadband stealth performance of the designed cloak. To elucidate the mechanism of broadband cloaking performance, Fig. 4(f) shows the reflection phase dispersion of the carpet cloak as a function of the height from the ground (h) upon normal incidence. One can find that the phase distributions contributed by meta-atoms at 7620 nm completely coincide with those calculated by Eq. (1), indicating the perfect cloaking response. As can be seen from Eq. (1), the phase distribution on the metasurface cloak not only depends on the incident angle α, but also varies linearly with the incident wavelength λ. At the excitation waves of λ1 and λ2, although the simulated phases produced by meta-atoms mildly fluctuate around their corresponding calculated ideal ones, our metasurface can still function as an excellent cloak due to the robustness of phase dispersion of metasurface units [55]. However, as the excitation wavelength moves further away from the operating one (λ = 7620 nm), the implemented reflection phases at different positions will deviate from the corresponding target ones by different degrees, as a result of which the metasurface cannot provide the required phase compensation, and thus deform the cloaking effect.
Fig. 4. (a-c) Farfield radiation patterns of the bared bump, cloaked bump, and metallic ground under normal incidence versus the excitation wavelength from 6020 to 9420 nm. The white lines marked by λ0, λ1 and λ2 donate the central operation wavelength of 7620 nm, the lower and upper limits of the stealth bandwidth of our metasurface cloak, respectively. (d-e) Respective farfield radiations for all three cases at λ1 and λ2. (f) The reflection phase dispersion of the carpet cloak as a function of the height from the ground (h) upon normal incidence, where the solid lines represent the theoretical phase values derived from Eq. (1), the dotted lines donate the simulated phase values.
To quantitatively evaluate the cloaking performance, the reduced total RCS of the metasurface cloak in two typical incident cases of vertical (α = 0°) and oblique incidence (α = 15° and α = 25°) are calculated. The reduced total RCS is defined as the ratio between the total RCS of the cloaked σcloaked, tot and bared bump σbared, tot [33].
Fig. 5. Reduced total RCS for normal incidence and oblique incidence of 15° and 25°. These results are calculated from the simulated farfield radiation patterns.
All of the above virtues can render our cloak a well-behaved paradigm, but if beyond that, some exotic virtues can be induced/excitated, which will render the whole design more appealing. Ge2Sb2Te5 can be switched repeatedly between amorphous (aGST) and crystalline states (cGST) by an appropriate thermal [56], optical [57,58] or electrical stimulus [59]. In particular, the phase transition of GST between the amorphous and crystalline phases is gradual rather than abrupt, enabling the tunable features. During the phase transition, the GST film can be assumed to be at intermediate phase composed of different proportions of amorphous and crystalline molecules. Based on the effective medium theories [60] and the Lorentz-Lorenz equation, the effective permittivity of GST εeff (λ) at any crystallization level is estimated as [61,62]
Fig. 6. Simulated electric field distributions in yoz plane (a-e) and farfield radiations (f-j) for the cloaked bump with different crystallization levels of GST (m = 0.2, 0.4, 0.6, 0.8, 1.0) under normal incidence.
Simultaneously, we investigate the influence of the different crystallization levels of GST on cloaking bandwidth of the designed metasurface, as depicted in Fig. 7. One can clearly observe that the cloaking bandwidths are well maintained within a range of 1200 ∼1300 nm as the crystallization level of the GST increases from 0 to 0.6. In detail, the cloaking wavelength bands of 6920∼8220 nm, 7120∼8320 nm, 7220∼8420 nm and 7220∼8520 nm are achieved when m = 0 [also see Fig. 4(b)], 0.2, 0.4, and 0.6, respectively. Specially, the cloaking bandwidth decreases sharply as the GST crystallization fractions are beyond 60% (the cloaking bands of 7420∼8420 nm and 7920∼8720 nm are corresponding to the cases of m = 0.8 and m = 1.0, respectively), mainly because the GST gets more losses at higher crystallization fraction. The above analysis confirms again that the designed metasurface cloak can realize continuously dynamic control of stealth within the ultrabroadband wavelength range of 6920∼8720 nm by precisely adjusting the GST phase states.
Fig. 7. Farfield radiation patterns of the cloaked bump with different crystallization levels of GST (m = 0, 0.2, 0.4, 0.6, 0.8, 1.0) under normal incidence versus the excitation wavelength from 6920 to 8820 nm. The white lines marked by λc, λmin and λmax donate the central operation wavelength, the lower and upper limits of the stealth bandwidth of our metasurface cloak, respectively.
In addition to the continuous tunability for cloaking, the use of GST in our design can support some other extraordinary degrees of freedom for dynamic control. The farfield radiation patterns of the cloaked design with cGST under normal incidence are displayed in Fig. 8(a) [also see Fig. 7(f)]. It can be found that the application of the cGST-based metasurface carpet cloak also suppresses the scatterings considerably and makes the radiation patterns similar to those of the ground plane within the wavelength range from 7920 (λ3) to 8720 nm (λ4). Consequently, the bump is perfectly concealed. The 2D and 3D farfield radiation patterns for λ3 and λ4 provided in Figs. 8(b)–8(c) further confirm the perfect cloaking performance of the cGST-based metasurface carpet cloak. Notably, by switching GST from amorphous to crystalline states, the cloaking bandwidth of the cloaked design is significantly extended from 8220 nm to 8720 nm, primarily attributed to the increased permittivity of GST and the varied resonant dispersion. More interestingly, when the GST is in the amorphous state, the scheme can realize perfect cloaking performance upon λ = 7620 nm [see Fig. 2(b) and 2(e)], while inducing the amorphous GST to convert into the crystalline state, the mismatch of phase dispersions invalidates stealth of the cloaked design at λ = 7620 nm, as shown in Fig. 8(d). That is to say, the ingenious cloaked design can realize cloaking switching of “ON” and “OFF” states near the optimal wavelength of 7620 nm by switching aGST to cGST, which will push the design to be a more versatile device.
Fig. 8. (a) Farfield radiation patterns of the cloaked bump under normal incidence versus the excitation wavelength from 6020 to 9420 nm when Ge2Sb2Te5 is converted into the crystalline state. The white lines marked by λ0, λ3 and λ4 donate the optimal wavelength of 7620 nm, the lower and upper limits of the stealth bandwidth of cGST-based metasurface cloak, respectively. (b-d) Respective farfield radiations of cGST-based metasurface cloak at λ3, λ4 and λ0. The corresponding 3D farfield radiation patterns are also provided in the insets of (b)-(d).
4. Conclusion
In this study, for the first time, we have designed an ultrathin metasurface carpet cloak based on phase-change material of Ge2Sb2Te5 in infrared region. By elaborately tailoring the local phase compensation of the metasurface that is wrapped on the bump, the carpet cloak can effectively suppress the unwanted scattering waves and successfully reconstruct the scattering wavefront as the mirror-reflection of a metallic flat plate at the operating wave of 7620 nm within a wide angle range of ±25°. Additionally, the robustness of the dispersion properties of metasurface units renders our carpet cloak obtain excellent cloaking performance over 1300 nm (from 6920 to 8220 nm), 17.1% of the bandwidth. Simulation results also suggest that the cloak has a remarkable RCS reduction and a relatively wide 3 dB RCS reduction bandwidth (approximately 35.43%) owing to the fully continuous meta-atom and phase distributions. Foremost, due to the phase transition of GST, our design functions as an active paradigm that not only continuously tunes the central cloaking wavelength, but also significantly extends the cloaking bandwidth while maintaining perfect stealth response without changing its configurations. More interestingly, simulation results also show the ingenious stealth design can realize cloaking switching of “ON” and “OFF” states near the optimal wavelength of 7620 nm by switching aGST to cGST, which has yet been reported. Especially, other cloaking performance of the scheme, such as full-polarization and full-azimuth invisibility, can be further realized by endowing the nanoantenna shapes and the spatial distribution of meta-atoms with perfect rotational symmetry. We believe that the concept of our scheme paves a feasible way to realize broadband, wide-angle, large RCS reduction and active cloaking effect in the infrared regime, and will find great potential applications in stealth, camouflage, and illusion fields.
Funding
National Natural Science Foundation of China (12004347, 61704156); Henan Provincial Science and Technology Research Project (202102310535, 212102310255); Natural Science Foundation of Henan Province (212300410414); Aeronautical Science Foundation of China (2019ZF055002); Key Scientific Research Project of Colleges and Universities in Henan Province (20B140017).
Disclosures
The authors declare no conflicts of interest.
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