July 16, 2015

Broadband surface-wave transformation cloak - Molding acoustic, electromagnetic and water waves with a single cloak

Two experiments demonstrating that a cylindrical cloak formerly introduced for linear surface liquid waves works equally well for sound and electromagnetic waves. This structured cloak behaves like an acoustic cloak with an effective anisotropic density and an electromagnetic cloak with an effective anisotropic permittivity, respectively. Measured forward scattering for pressure and magnetic fields are in good agreement and provide first evidence of broadband cloaking. Microwave experiments and 3D electromagnetic wave simulations further confirm reduced forward and backscattering when a rectangular metallic obstacle is surrounded by the structured cloak for cloaking frequencies between 2.6 and 7.0 GHz. This suggests, as supported by 2D finite element simulations, sound waves are cloaked between 3 and 8 KHz and linear surface liquid waves between 5 and 16 Hz. Moreover, microwave experiments show the field is reduced by 10 to 30 dB inside the invisibility region, which suggests the multi-wave cloak could be used as a protection against water, sonic or microwaves.

The cloak could have potential applications in telecommunications (protection against mobile phone radiations) and soundproof devices (humans are most sensitive to sound waves of frequencies between 2 and 5 kHz). The design could also be scaled down in order to achieve cloaking at optical wavelengths, what would require an analysis of cloak’s dispersion.

Since the cloak works both in acoustic and microwave domains, it might offer new opportunities to drive and tune one field with the other one: for instance if the fluid within which pressure waves propagate is no longer air but some gas plasma or liquid electrolyte, we might be able to tune the density profile by microwave signals. On larger scales, control of sound and elastic waves could be used in anti-earthquake designs if protection is achieved via cloaking with seismic metamaterials34, that is for frequencies below 50 Hz. Finally, we note that preliminary numerical simulations suggest our cloak should also work in the context of management of thermal flux.

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.

Abstract - Broadband surface-wave transformation cloak
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.



Broadband surface-wave transformation cloak

scientists at Zhejiang University in Hangzhou, China, Nanyang Technological University, Singapore, and Massachusetts Institute of Technology created (so-called invisibility) cloaks based on specifically-designed nonmagnetic anisotropic, or directionally dependent, metamaterials that achieve nearly ideal transmission efficiency over a broadband frequency range.




● Overcome the challenge of momentum mismatch by adopting strict transformation optics with anisotropy in the design – a strategy that can also work for ultrasharp corners and bumps

● Realized broadband performance by employing an area-preserving coordinate transformation, which can produce non-magnetic constitutive parameters for a surface plasmon polariton (SPP –an electromagnetic excitation existing on the surface of an appropriate metal) wave cloak

● Experimentally demonstrated SPP cloak performance by designing a layered metamaterial composed of microwave ceramic plates and low-permittivity foam with subwavelength periodicity (in which the periodicity of the metamaterial is much smaller than the wavelength of the electromagnetic wave being cloaked)

The key result being reported is that unlike topological electromagnetic surface states, in the new approach phase is preserved when surface waves are perfectly guided by the cloaks. "Sharp bending of surface waves was previously achieved only in topological electromagnetic edge states," Zhang tells Phys.org. "Because the required materials generally are magnetic, it suffers from narrow bandwidth. In our work, however, the use of anisotropic non-magnetic materials and transformation optics ensure the phase preservation of surface waves."

Chen says that the scientists plan to extend their experimental demonstration from microwaves to higher frequencies, including infrared and visible light. "This may push our work much closer to practical application. Moreover, scientists have extended the concept of transformation beyond electromagnetic fields to other types of physical fields, such as heat, diffusive light, acoustics, and static fields. "No matter which kind of physical field it is, the fundamental point is to control the propagation of the waves and the distribution of the field," Chen concludes. "Our work can therefore be extended to many other areas of research."

SOURCES - PNAS, Nature, Phys.org

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