Runren Zhang

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

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.

Ultra-broadband carpet cloak for transverse-electric polarization

Magnetism is a necessity in constructing macroscopic metamaterial invisibility cloaks that are theoretically designed by transformation optics, but will generally limit the cloaking bandwidth to an impractically narrow range. To meet the broad bandwidth demand, magnetism has been fully abandoned in previous demonstrations of macroscopic carpet cloaking, whose approach, however, cannot apply to a transverse-electric (TE) polarization. To fill this gap, here we experimentally demonstrate an ultra-broadband magnetic carpet cloak for the TE polarization. The cloak is made of non-resonant closed-ring metamaterials with little dispersion and the cloaking performance is confirmed with both time-domain simulation and frequency scanning measurement over a broad bandwidth corresponding to a pulse signal illumination.

Free-space carpet cloak using transformation optics and graphene.

Free-space carpet cloak designed with transformation optics requires materials exhibiting simultaneously anisotropic properties and plasma-like behaviors, but materials that simultaneously meet these requirements are rarely found in nature. The recently discovered graphene has shown unique anisotropic plasma-like behavior benefitting from its natural two-dimensional structure and in-plane ultrahigh electron mobility, and therefore, can be a good candidate for the free-space carpet cloak design. In this Letter, we theoretically propose a novel free-space carpet cloak by using periodically stacking layered graphene for the first time. Simulation results show an object under the graphene-based carpet cloak becomes invisible in the THz frequencies. By exploiting the large tunability of graphenes conductivity, we also demonstrate the working frequency of the designed cloak is continuously tunable in a wide spectrum.

Broadband subwavelength imaging using non-resonant metamaterials

Previous subwavelength imaging using hyperlens is based on negative constitutive parameters that are realized by strongly dispersive materials and work only in a narrow frequency band. Here, we demonstrated that subwavelength imaging can be achieved in a broad frequency band using non-resonant magnetic metamaterials. The metamaterial shows an elliptical dispersion relation and can be fabricated by metallic closed-rings with a broadband magnetic response. With this elliptically dispersive material, most of the evanescent waves with high-k modes can be converted to propagating modes and the subwavelength information is reconstructed. Both simulation and experiment results show that this kind of metalens can achieve a broadband subwavelength imaging effect.

A meta-substrate to enhance the bandwidth of metamaterials

We propose the concept of a meta-substrate to broaden the bandwidth of left-handed metamaterials. The meta-substrate, which behaves like an inhomogeneous magnetic substrate, is composed of another kind of magnetic metamaterials like metallic closed rings. When conventional metamaterial rings are printed on this kind of meta-substrate in a proper way, the interaction of the metamaterials units can be greatly enhanced, yielding an increased bandwidth of negative permeability. An equivalent circuit analytical model is used to quantitatively characterize this phenomenon. Both numerical and experimental demonstrations are carried out, showing good agreement with theoretical predictions.

Magnetized Plasma as a Versatile Platform for Switching

We study the magneto-permittivity effect in a magnetized plasma with appropriately designed parameters. We show that at frequencies near the plasma frequency, magneto-optical activity plays an important role to manipulate and control the wave propagations in the magnetized plasma. Such a unique feature can be utilized to establish sensitive magnetic field switching mechanism, which is confirmed by detailed numerical investigations. Switching by magnetic field based on magnetized plasma is flexible and compatible with other optical system; moreover it is applicable to any frequency by tuning the plasma density. For these reasons, our work shows the possibility for developing a new family of high frequency and ultrasensitive switching applications.

Photonic transport in a graphene van der Waals homojunction

Layered structures of two-dimensional (2D) materials stacked by van der Waals interactions such as homojunctions and heterojunctions have shown a number of microelectronic applications in the realm of ultrathin and highly flexible devices. Since van der Waals interaction is the dominant strength among these layered structures and can induce the anisotropic behavior of incident electromagnetic waves or photons, a novel concept of photonic transport can be proposed based on the physical picture of transferring both far-field and near-field components of an object to a certain distance and rebuilding a subwavelength image. Herein, through delicately designing and engineering the layered structures based on graphene, we have achieved a flexible graphene van der Waals homojunction for subwavelength imaging. Such an approach is the first time an optical lens from the atomic level has been designed and constitutes a further important step towards practical applications of subwavelength imaging.

Subwavelength resolution using metamaterials with different dispersion relations

The microscope is one of the most important instruments in the nature science, which allows the researchers to “see” small things beyond naked eyes. However, since high spatial frequency information carried by evanescent waves decay exponentially during propagation in free space, the resolution of a conventional microscope is limited due to the diffraction of light which is called Abbe-Rayleigh limit. In the past decades, the concepts of superlens and hyperlens have been proposed to overcome the diffraction limit. Here, we demonstrate a cylindrical hyperlens works for TE polarized waves. The metamaterial forming this hyperlens is based on S-string resonators architecture, which shows two components of the constitutive parameters negative. We also demonstrate a dielectric hyperlens approach to overcome the diffraction limit at ultraviolet frequencies. The hyperlens is designed basing on layered grapheme and layered h-boron nitride, both of which show significant anisotropy and are ideal for hyperlens implementation. Further more, metamaterial without a hyperbolic dispersion relation can be utilized to realize subwavelength imaging as well. We propose a kind of broadband subwavelength imaging using non-resonant metamaterials. The metamaterial is fabricated by closed-rings structure which shows an elliptical dispersion relation with a broadband magnetic response. We also experimentally achieve a broadband deep subwavelength resolution with singular media. The meta-lens is made of subwavelength metal/air layers which exhibit singular medium property over a broad band. Our work shows subwavelength resolution can be achieved using metamaterials with different dispersion relations. It may provide a new perspective to design the lens and make subwavelength imaging into application.

Broadband 3D metamaterial carpet cloak

We propose and demonstrate ultra-broadband three dimensional carpet cloaking for full polarization. A non-resonant metamaterial, which exhibits broadband magnetic and electric anisotropy with little dispersion, is used as a main building block in the cloak design. Nearly perfect cloaking performance is confirmed over a broad bandwidth with frequency scanning measurement. In particular, the experimental result shows that phase is well preserved by the cloak.

Te-polarized microwave broadband carpet cloak

Invisibility cloaks have attracted wide interest recently because of their possible realization. As is one of types of invisibility cloaks, carpet cloaks were first presented by Pendry in 2008. However, most of the realized carpet cloaks in microwave region can only conceal the metal bump under TM-polarized incident wave because non-magnetism is required in the realization of the carpet cloak. Here, we demonstrate a Te-polarized microwave broadband carpet cloak based on the Tranformation Optics. Closed-rings, which are used to construct cloaking layer, can provide anisotropic permeability with little dispersion in a large frequency range required in our design. The cloaking behavior is confirmed in our measurements from 2-7 GHz.