Faxin Yu

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.

A metasurface carpet cloak for electromagnetic, acoustic and water waves.

We propose a single low-profile skin metasurface carpet cloak to hide objects with arbitrary shape and size under three different waves, i.e., electromagnetic (EM) waves, acoustic waves and water waves. We first present a metasurface which can control the local reflection phase of these three waves. By taking advantage of this metasurface, we then design a metasurface carpet cloak which provides an additional phase to compensate the phase distortion introduced by a bump, thus restoring the reflection waves as if the incident waves impinge onto a flat mirror. The finite element simulation results demonstrate that an object can be hidden under these three kinds of waves with a single metasurface cloak.

Broadband surface-wave transformation cloak

Significance Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices. In this work, we introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way for high-performance, large-scale integrated photonic circuits. Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices, but is fundamentally difficult to realize because light momentum must be conserved in a scattering event. A partial realization has been achieved by exploiting topological electromagnetic surface states, but this approach is limited to narrow-band light transmission and subject to phase disturbances in the presence of disorder. Recent advances in transformation optics apply principles of general relativity to curve the space for light, allowing one to match the momentum and phase of light around any disorder as if that disorder were not there. This feature has been exploited in the development of invisibility cloaks. An ideal invisibility cloak, however, would require the phase velocity of light being guided around the cloaked object to exceed the vacuum speed of light—a feat potentially achievable only over an extremely narrow band. In this work, we theoretically and experimentally show that the bottlenecks encountered in previous studies can be overcome. We introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. These cloaks consist of specifically designed nonmagnetic metamaterials and achieve nearly ideal transmission efficiency over a broadband frequency range from 0+ to 6 GHz. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way toward high-performance, large-scale integrated photonic circuits.

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.

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.

A Full-Parameter, Broadband, Homogeneous, and Compact Waveguide Coupler Designed With Transformation Optics

Traditional waveguide couplers are always with large volume, nonneglectable reflection and distortion of beam profile. To provide a possible solution, some transformation optics based strategies have been proposed, however, implementation of the constitutive parameters in these strategies is very challenging. In this letter, we propose a scheme of a full-parameter, broadband, homogeneous, and compact waveguide coupler. The waveguide coupler is designed with linear homogenous coordinate transformation method and with nearly nondispersive metamaterials. Simulation results on the waveguide coupler show nearly perfect beam profile preservation and an almost total transmission in a broad band. Our strategy may find potential applications in microwave and terahertz devices.

Realization of non-linear coherent states by photonic lattices

In this paper, first, by introducing Holstein-Primakoff representation of α-deformed algebra, we achieve the associated non-linear coherent states, including su(2) and su(1, 1) coherent states. Second, by using waveguide lattices with specific coupling coefficients between neighbouring channels, we generate these non-linear coherent states. In the case of positive values of α, we indicate that the Hilbert size space is finite; therefore, we construct this coherent state with finite channels of waveguide lattices. Finally, we study the field distribution behaviours of these coherent states, by using Mandel Q parameter.

Highly Directional Small-Size Antenna Designed with Homogeneous Transformation Optics

Achieving high directivity antenna usually requires a large size antenna aperture in traditional antenna design. Previous work shows that, with the help of metamaterials and transformation optics, a small size antenna can perform as high directivity as a large size antenna, but the material parameters are inhomogeneous and difficult to realize. In this paper, we propose a linear homogeneous coordinate transformation to design the small size antenna. Distinguishing from inhomogeneous transformation, we construct a regular polygon in virtual space and then divide it into several triangle segments. By applying linear homogeneous coordinate transformation, the antenna devices can be greatly compressed without disturbing the radiation patterns by using homogeneous metamaterial substrates. The material parameters of the antenna designed from this method are homogeneous and easy to fabricate. Square and hexagonal antenna structures are numerically demonstrated to illustrate the validity of our methodology.

Electromagnetic invisibility cloaks based on inverse design method

Inspired from the thoughts of forward-design cloaking techniques, invisibility cloaks with inverse designs have experience a period of remarkably fast development in the past few years. In this paper, we report an inverse design method of invisibility cloaks based on the Mie scattering theory with the adoption of the genetic algorithm. With the inverse design, an invisible dipole antenna is demonstrated experimentally and an omnidirectional spherical invisibility cloak design is proposed.