Temperature-Controlled Chameleonlike Cloak

Invisibility cloaking based on transformation optics has brought about unlimited space for reverie. However, the design and fabrication of transformation-optics-based cloaks still remain fairly challenging because of the complicated, even extreme, material prescriptions, including its meticulously engineered anisotropy, inhomogeneity and singularity. And almost all the state-of-the-art cloaking devices work within a narrow and invariable frequency band. Here, we propose a novel mechanism for all-dielectric temperature-controllable cloaks. A prototype device was designed and fabricated with ${\mathrm{SrTiO}}_3$ ferroelectric cuboids as building blocks, and its cloaking effects were successfully demonstrated, including its frequency-agile invisibility by varying temperature. It revealed that the predesignated cloaking device based on our proposed strategy could be directly scaled in dimensions to operate at different frequency regions, without the necessity for further efforts of redesign. Our work opens the door towards the realization of tunable cloaking devices for various practical applications and provides a simple strategy to readily extend the cloaking band from microwave to terahertz regimes without the need for reconfiguration.

Popular Summary

Invisibility cloaks, which can ideally render an object invisible to all electromagnetic waves, have long captured popular imagination. Implementation of such cloaks is possible thanks to, for example, metamaterials, whose precise shape and orientation can manipulate incident radiation. But practical cloaks are currently limited to very specific situations, only working within a narrow and invariable range of frequencies. It is difficult to design and obtain materials that can cloak a volume of any shape from any viewing angle and over a wide frequency band. A cloak whose frequency can be actively tuned by means of an external field might offer a possible solution. The electromagnetic properties of the cloak have to anisotropically change in order to reach the required values for different frequencies. Here, we propose a simple design methodology for such a temperature-controllable cloak and experimentally verify its feasibility.

Like chameleons changing their colors to match their surroundings, our device possesses a cloaking effect with working frequencies controllable by an external stimulus—in this case, temperature. We tested this theory with a prototype device fabricated from millimeter-sized ${\mathrm{SrTiO}}_3$ ferroelectric cuboids. The cloak successfully rendered a small aluminum slab mostly invisible to 11.50-GHz radiation at a temperature of $25°\mathrm{C}$. By varying the temperature from $15°\mathrm{C}$ to $40°\mathrm{C}$, we were able to alter the invisibility frequency over a range of about 500 MHz.

By scaling the dimensions of the cuboids, our cloaking strategy could operate at different frequency regions without the necessity of redesign. This work opens the door toward the implementation of tunable cloaking devices for various practical applications and provides a simple strategy to readily extend the cloaking band from microwave to terahertz regimes.

Figure 1

Schematic of the chameleonlike all-dielectric cloak. The cloak can conceal the object under different temperatures, and the invisible frequency can be adjusted from lower frequency (lower temperature $T_1$) (a), to higher frequency (higher temperature $T_2$) (b). The cloak is constructed from dielectric cuboids, which can realize the required anisotropic electromagnetic parameters. A TE wave polarized along the $z$ direction is incident from the left. A quarter-wave separation between the conducting quadrangular and the inner cloak boundaries is designated to simulate a perfect magnetic conductor boundary. The inset shows unit cells used to calculate the effective electromagnetic parameters. (c) Two methods to adjust the working frequency of the cloak. The first is by scaling the size, and the second is by changing the temperature of the dielectric cuboids.

Figure 2

Mechanism of the temperature-tunable cloak. (a) The temperature dependence of intrinsic permittivity and effective permittivity of the dielectric cuboids. At different temperatures $T_1$ and $T_2$, the cloak works at different frequencies, and the corresponding characteristic curves are shown in (b) and (c), respectively. When the temperature is changed, the resonance frequencies ${\omega }_{0,x}$ and ${\omega }_{0,y}$ are kept proportional, which can be expressed as ${\omega }_{0,x}=\rho {\omega }_{0,y}$. Meanwhile, the same property applies to the working frequency ${\omega }_i({\overline{\mu }}_i)$ and resonance frequencies ${\omega }_{0,i}$ in the $i$ direction ($i=x$, $y$); i.e., ${\omega }_i({\overline{\mu }}_i)={\overline{\rho }}_i{\omega }_{0,i}$, in which ${\overline{\rho }}_i$ is the proportionality constant. In other words, the working frequencies in the $x$ and $y$ directions are proportional. The same logic applies when the temperature is changed from $T_1$ to $T_3$, so even though the permeabilities ${\mu }_x$ and ${\mu }_y$ both change with the temperature, they can still meet the required invisible parameters at another working frequency.

Figure 3

Full-wave simulation of the designed cloak. (a) Simulated model. (b) Scattering cross sections of the conducting quadrangular with or without the cloak at different frequencies. The calculated electric-field distribution of the conducting quadrangular without the cloak at 11.45 GHz (c), with the cloak at a nonworking frequency of 12.00 GHz, (d) and with the cloak at a working frequency of 11.45 GHz (e).

Figure 4

Experimental measurement of the fabricated cloak. (a) Fabricated chameleonlike cloak and electric-field measurement system. (b) ${\mathrm{S}}_{21}$ of the dielectric cuboid with H polarized along the $y$ direction. The frequency of the resonance peak increases with the increase of temperature. Measured electric-field distributions at different temperatures and frequencies: (c) $25°\mathrm{C}$, 10.00 GHz; (d) $15°\mathrm{C}$, 11.18 GHz; (e) $25°\mathrm{C}$, 11.50 GHz; (f) $40°\mathrm{C}$, 11.72 GHz.

Figure 5

Numerical demonstration of the frequency-tunable mechanism of the cloak by scaling the dimensions of the cuboids. The permittivity of the cuboids is set to 200 with different sizes. (a) The same size as the cuboids in the experiment, with a working frequency of 12.6 GHz. (b) Scaling down the size to $1/10$, with a working frequency of 126 GHz. (c) Scaling down the size to $1/100$, with a working frequency of 1.26 THz.