Invisibility has attracted intensive research in various communities, e.g., optics, electromagnetics, acoustics, thermodynamics, dc, etc. However, many experimental demonstrations have only been achieved by virtue of simplified approaches due to the inhomogeneous and extreme parameters imposed by the transformation-optic method, and usually require a challenging realization with metamaterials. In this Letter, we demonstrate a bilayer thermal cloak made of bulk isotropic materials, and it has been validated as an exact cloak. We experimentally verified its ability to maintain the heat front and its heat protection capabilities in a 2D proof-of-concept experiment. The robustness of this scheme is validated in both 2D (including oblique heat front incidence) and 3D configurations. The proposed scheme may open a new avenue to control the diffusive heat flow in ways inconceivable with phonons, and also inspire new alternatives to the functionalities promised by transformation optics.

/pdf/experimental-demonstration-of-a-bilayer-thermal-cloak-2y897u9sh6.pdf

Experimental demonstration of a bilayer thermal cloak

Invisible cloak has long captivated the popular conjecture and attracted intensive research in various communities of wave dynamics, eg, optics, electromagnetics, acoustics, etc However, their inhomogeneous and extreme parameters imposed by transformation-optic method will usually require challenging realization with metamaterials, resulting in narrow bandwidth, loss, polarization-dependence, etc In this paper, we demonstrate that thermodynamic cloak can be achieved with homogeneous and finite conductivity only employing naturally available materials It is demonstrated that the thermal localization inside the coating layer can be tuned and controlled robustly by anisotropy, which enables an incomplete cloak to function perfectly Practical realization of such homogeneous thermal cloak has been suggested by using two naturally occurring conductive materials, which provides an unprecedentedly plausible way to flexibly realize thermal cloak and manipulate heat flow with phonons

/pdf/homogeneous-thermal-cloak-with-constant-conductivity-and-556l6hfmm2.pdf

Homogeneous Thermal Cloak with Constant Conductivity and Tunable Heat Localization

A cloak that can hide living creatures from sight is a common feature of mythology but still remains unrealized as a practical device. To preserve the wave phase, the previous cloaking solution proposed by Pendry and colleagues required transformation of the electromagnetic space around the hidden object in such a way that the rays bending around the object inside the cloak region have to travel faster than those passing it by. This difficult phase preservation requirement is the main obstacle for building a broadband polarization-insensitive cloak for large objects. Here we propose a simplified version of Pendry’s cloak by abolishing the requirement for phase preservation, as it is irrelevant for observation using incoherent natural light with human eyes, which are phase and polarization insensitive. This allows for a cloak design on large scales using commonly available materials. We successfully demonstrate the cloaking of living creatures, a cat and a fish, from the eye.

/pdf/ray-optics-cloaking-devices-for-large-objects-in-incoherent-336757jq73.pdf

Ray-optics cloaking devices for large objects in incoherent natural light

Invisible cloak has long captivated the popular conjecture and attracted intensive research in various communities of wave dynamics, e.g., optics, electromagnetics, acoustics, etc. However, their inhomogeneous and extreme parameters imposed by transformation-optic method will usually require challenging realization with metamaterials, resulting in narrow bandwidth, loss, polarization-dependence, etc. In this paper, we demonstrate that thermodynamic cloak can be achieved with homogeneous and finite conductivity only employing naturally available materials. It is demonstrated that the thermal localization inside the coating layer can be tuned and controlled robustly by anisotropy, which enables an incomplete cloak to function perfectly. Practical realization of such homogeneous thermal cloak has been suggested by using two naturally occurring conductive materials, which provides an unprecedentedly plausible way to flexibly realize thermal cloak and manipulate heat flow with phonons.

http://phononics.tongji.edu.cn/userfiles/files/paper/Baowen%20Li/bwli_2013_sr.pdf

Homogeneous Thermal Cloak with Constant Conductivity and Tunable Heat

We present the first experimental realization and verification of a three-dimensional stand-alone mantle cloak designed to suppress the total scattering of a finite-length dielectric rod of moderate cross-section. Mantle cloaking has been proposed to realize ultralow-profile conformal covers that may achieve substantial camouflage, transparency and high-performance non-invasive near-field sensing. Here, we realize and verify a mantle cloak for radio-waves. We report an extensive campaign of far- and near-field free-space measurements demonstrating that conformal cloaks can indeed produce strong scattering suppression in all directions and over a relatively broad bandwidth of operation.

/pdf/demonstration-of-an-ultralow-profile-cloak-for-scattering-p0n7pjczk3.pdf

Demonstration of an ultralow profile cloak for scattering suppression of a finite-length rod in free space

We experimentally demonstrated an alternative approach of invisibility cloaking that can combine technical advantages of all current major cloaking strategies in a unified manner and thus can solve bottlenecks of individual strategies. A broadband cylindrical invisibility cloak in free space is designed based on scattering cancellation (the approach of previous plasmonic cloaking), and implemented with anisotropic metamaterials (a fundamental property of singular-transformation cloaks). Particularly, nonsuperluminal propagation of electromagnetic waves, a superior advantage of non-Euclidian-transformation cloaks constructed with complex branch cuts, is inherited in this design, and thus is the reason of its relatively broad bandwidth. This demonstration provides the possibility for future practical implementation of cloaking devices at large scales in free space.

/pdf/experimental-demonstration-of-a-free-space-cylindrical-cloak-3yphc6lqil.pdf

Zhengliang Huang

Papers

Experimental demonstration of a free-space cylindrical cloak without superluminal propagation.