Tiancheng Han

Homogeneous Thermal Cloak with Constant Conductivity and Tunable Heat Localization

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.

Full Control and Manipulation of Heat Signatures: Cloaking, Camouflage and Thermal Metamaterials

Thermal camouflage and cloaking can transform an actual heat signature into a pre-controlled one. A viable recipe for controlling and manipulating heat signatures using thermal metamaterials to empower cloaking and camouflage in heat conduction is demonstrated. The thermal signature of the object is thus metamorphosed and perceived as multiple targets with different geometries and compositions, with the original object cloaked.

Manipulating DC currents with bilayer bulk natural materials.

A novel and general method for spatially manipulating DC currents has been proposed and experimentally verified by only using bilayer bulk natural conductive materials. Our approach shows distinctive advantages with respect to homogeneity, isotropy, and independence of complicated microfabrication techniques. Our design scheme can be readily extended to robust manipulations of magnetic fields, thermal heat, elastic mechanics, and matter waves.

Ultra-broadband infrared metasurface absorber.

By using sub-wavelength resonators, metamaterial absorber shows great potential in many scientific and technical applications due to its perfect absorption characteristics. For most practical applications, the absorption bandwidth is one of the most important performance metrics. In this paper, we demonstrate the design of an ultra-broadband infrared absorber based on metasurface. Compared with the prior work [Opt. Express22(S7), A1713-A1724 (2014)], the proposed absorber shows more than twice the absorption bandwidth. The simulated total absorption exceeds 90% from 7.8 to 12.1 um and the full width at half maximum is 50% (from 7.5 to 12.5 μm), which is achieved by using a single layer of metasurface. Further study demonstrates that the absorption bandwidth can be greatly expanded by using two layers of metasurface, i.e. dual-layered absorber. The total absorption of the dual-layered absorber exceeds 80% from 5.2 to 13.7 um and the full width at half maximum is 95% (from 5.1 to 14.1 μm), much greater than those previously reported for infrared spectrum. The absorption decreases with fluctuations as the incident angle increases but remains quasi-constant up to relatively large angles.

Manipulating Steady Heat Conduction by Sensu -shaped Thermal Metamaterials

The ability to design the control of heat flow has innumerable benefits in the design of electronic systems such as thermoelectric energy harvesters, solid-state lighting, and thermal imagers, where the thermal design plays a key role in performance and device reliability. In this work, we employ one identical sensu-unit with facile natural composition to experimentally realize a new class of thermal metamaterials for controlling thermal conduction (e.g., thermal concentrator, focusing/resolving, uniform heating), only resorting to positioning and locating the same unit element of sensu-shape structure. The thermal metamaterial unit and the proper arrangement of multiple identical units are capable of transferring, redistributing and managing thermal energy in a versatile fashion. It is also shown that our sensu-shape unit elements can be used in manipulating dc currents without any change in the layout for the thermal counterpart. These could markedly enhance the capabilities in thermal sensing, thermal imaging, thermal-energy storage, thermal packaging, thermal therapy, and more domains beyond.

Isotropic nonmagnetic flat cloaks degenerated from homogeneous anisotropic trapeziform cloaks

We propose a novel kind of trapeziform cloak requiring only homogeneous anisotropic materials. Large-scale flat cloaks can be degenerated from the general trapeziform cloak with PEC inner boundary, and be realized by isotropic nonmagnetic materials for optical frequencies with controlled index profiles and improved invisibility. With the support of PEC inner boundary, large vehicles and objects of arbitrary shape can be concealed between the PEC and ground, and PEC can be firm by adding pillars in the cloaking space. Full-wave simulations validate the proposed cloaking concept, which is not only based on simple isotropic nonmagnetic materials but also realizable in practice.

Theoretical realization of an ultra-efficient thermal-energy harvesting cell made of natural materials

Three-dimensional devices capable of efficiently harvesting light energy or microwave radiation from arbitrary directions are still challenging to make due to the stringent requirement of inhomogeneous and extreme material parameters. This usually requires the use of metamaterials and results in time-consuming and complicated fabrication, narrow bandwidth performance and huge losses, which prevent these devices from being extended to large-scale energy-related applications. In this paper, we demonstrate that thermodynamic cells harvesting heat energy in three dimensions can be achieved by employing naturally available materials with constant thermal conductivity. Particularly, the thermal-energy harvesting efficiency of the proposed devices is independent of geometrical size and may achieve nearly 100% with tunable anisotropy, much superior to the concentrating devices reported so far. Theoretical analysis and numerical experiments validate the excellent performance of the advanced thermal cells. We further show that such thermal cells can be practically realized by using two naturally occurring conductive materials in a simplified planar geometry, which may open a new avenue for potential applications in solar thermal panels and thermal-electric devices.

Creation of vectorial bottle-hollow beam using radially or azimuthally polarized light

We propose a single-beam generation scheme to obtain a bottle-hollow (BH) beam using a binary phase mask and a focusing lens. The resulting BH beam is shown to possess an open bottle-shaped null intensity region, which has two hollow tube-shaped null intensity regions located on two opposite sides of this bottle. It is found that this scheme works identically under incident illumination with radial or azimuthal polarization. Another advantage of this scheme is that the same binary mask can be employed as a focusing lens with different choices of numerical aperture (NA). Furthermore, we observe that the length of the BH beam is inversely proportional to NA2 while the diameters of both the bottle and hollow regions are inversely proportional to NA; thereby leading to an adjustable BH beam. This BH beam may find attractive applications in noninvasive manipulation of microscopic particles over large distances.

Homogeneous and isotropic bends to tunnel waves through multiple different/equal waveguides along arbitrary directions

We propose a novel optical transformation to design homogeneous isotropic bends connecting multiple waveguides of different cross sections which can ideally tunnel the wave along any directions through multiple waveguides. First, the general expressions of homogeneous and anisotropic parameters in the bend region are derived. Second, the anisotropic material can be replaced by only two kinds of isotropic materials and they can be easily arranged in planarly stratified configuration. Finally, an arbitrary bender with homogeneous and isotropic materials is constructed, which can bend electromagnetic wave to any desired directions. To achieve the utmost aim, an advanced method is proposed to design nonmagnetic, isotropic and homogeneous bends that can bend waves along arbitrary directions. More importantly, all of the proposed bender has compact shape due to all flat boundaries, while the wave can still be perfectly tunneled without mode distortion. Numerical results validate these functionalities, which make the bend much easier in fabrication and application.

An arbitrarily shaped cloak with nonsingular and homogeneous parameters designed using a twofold transformation

An arbitrarily asymmetrical cloak with nonsingular and homogeneous parameters is obtained by dividing an arbitrary N-sided polygonal region into N triangular regions, applying an independent twofold spatial compression, and smoothing the inner boundary of the polygonal cloak. In each triangular region, a small line segment is first stretched along one side of the triangle; this is followed by a second transformation along the other side which is carried out to create a pairing triangle. The inner boundary will thus be continuous and take a shape similar to the outer boundary. The proposed cloak is nonsingular and moreover each region of the cloak is composed of homogeneous materials.