Wave-front Transformation with Gradient Metasurfaces

Estakhri, Nasim Mohammadi; Alù, Andrea

doi:10.1103/physrevx.6.041008

Cited by 257 publications

(153 citation statements)

References 59 publications

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“…For this purpose, we use a synthesis method based on generalized Snell's laws which intuitively allows for arbitrary control of reflected and transmitted waves through local phase shifts at the metasurface. It should be mentioned that this synthesize method suffers in terms of power efficiency due to spurious scattering and it has been recently found that the ideal required phase shift differs from the prescription of the generalized Snell's law [77][78][79]. However, due to the 2π phase span provided by the proposed design paradigm, it can be used flexibly in more sophisticated synthesis methods to improve the efficiency of functionality.…”

Section: Wavefront Engineering Of Generated Frequency Harmonicsmentioning

confidence: 99%

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

2018

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Modulation of metasurfaces in time gives rise to several exotic space-time scattering phenomena by breaking the reciprocity constraint and generation of higher-order frequency harmonics. We introduce a new design paradigm for time-modulated metasurfaces, enabling tunable engineering of the generated frequency harmonics and their emerging wavefronts by electrically controlling the phase delay in modulation. It is demonstrated that the light acquires a dispersionless phase shift regardless of incident angle and polarization, upon undergoing frequency conversion in a timemodulated metasurface which is linearly proportional to the modulation phase delay and the order of generated frequency harmonic. The conversion efficiency to the frequency harmonics is independent of modulation phase delay and only depends on the modulation depth and resonant characteristics of the metasurface, with the highest efficiency occurring in the vicinity of resonance, and decreasing away from the resonant regime. The modulation-induced phase shift allows for creating tunable spatially varying phase discontinuities with 2π span in the wavefronts of generated frequency harmonics for a wide range of frequencies and incident angles. Specifically, we apply this approach to a time-modulated metasurface in the Teraherz regime consisted of graphene-wrapped silicon microwires. For this purpose, we use an accurate and efficient semi-analytical framework based on multipole scattering. We demonstrate the utility of the design rule for tunable beam steering and focusing of generated frequency harmonics giving rise to several intriguing effects such as spatial decomposition of harmonics, anomalous bending with full coverage of angles and dual-polarity lensing. Furthermore, we investigate the angular and spectral performance of the time-modulated metasurface in manipulation of generated frequency harmonics to verify its constant phase response versus incident wavelength and angle. The nonreciprocal response of the metasurface in wavefront engineering is also studied by establishing nonreciprocal links with large isolations via modulationinduced phase shift. The proposed design approach enables a new class of high-efficiency tunable metasurfaces with wide angular and frequency bandwidth, wavefront engineering capabilities, nonreciprocal response and multi-functionality.

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Section: Wavefront Engineering Of Generated Frequency Harmonicsmentioning

confidence: 99%

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

2018

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“…[27] see also Ref. [36] where almost perfect absorption is demonstrated by exciting a single near-field mode).…”

Section: Introductionmentioning

confidence: 96%

“…However, in this case the wave impedance of a scattered wave does not equal to the wave impedance of an incident wave. It makes the efficiency of the anomalous reflection (refraction) to decrease significantly when the angle between the incident and reflected (refracted) wave increases (as well as the impedance mismatch) [31,35,36]. The outcome is even worse when it comes to the conversion of a propagating wave into a surface wave using the recipe provided by the generalized Snell's law.…”

Section: Introductionmentioning

confidence: 99%

“…Recently, Tcvetkova et al have for the first time rigorously studied the problem of conversion of an incident plane wave into a surface wave with a growing amplitude [37] by means of a reflecting anisotropic metasurface (described by tensor surface parameters). The incident plane wave and the surface wave had orthogonal polarizations in order to avoid interference resulting in the requirement of "loss-gain" power flow into the metasurface [35,36]. Unfortunately, the anisotropic metasurface with the required impedance profile is difficult to realize, and no experimental results are available.…”

Section: Introductionmentioning

confidence: 99%

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Omega-bianisotropic metasurface for converting a propagating wave into a surface wave

et al. 2019

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Although a rigorous theoretical ground on metasurfaces has been established in the recent years on the basis of the equivalence principle, the majority of metasurfaces for converting a propagating wave into a surface wave are developed in accordance with the so-called generalized Snell's law being a simple heuristic rule for performing wave transformations. Recently, for the first time, Tcvetkova et al. [Phys. Rev. B 97, 115447 (2018)] have rigorously studied this problem by means of a reflecting anisotropic metasurface, which is, unfortunately, difficult to realize, and no experimental results are available. In this paper, we propose an alternative practical design of a metasurface-based converter by separating the incident plane wave and the surface wave in different half-spaces. It allows one to preserve the polarization of the incident wave and substitute the anisotropic metasurface by an omega-bianisotropic one. The problem is approached from two sides: By directly solving the corresponding boundary problem and by considering the "time-reversed" scenario when a surface wave is converted into a nonuniform plane wave. We develop a practical three-layer metasurface based on a conventional printed circuit board technology to mimic the omega-bianisotropic response. The metasurface incorporates metallic walls to avoid coupling between adjacent unit cells and accelerate the design procedure. The design is validated with full-wave three-dimensional numerical simulations and demonstrates high conversion efficiency. arXiv:1905.04334v1 [physics.app-ph]

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“…Most recently, a theoretical scheme for gradient (spatially dispersive) metasurfaces, which offers a perfect control of refracted/reflected waves (i.e., 100% power efficiency), was introduced in the seminal studies in Refs. [3,19,21]. Based on that scheme, metasurface designs several interesting applications involving wavefront control were proposed (see e.g., Refs.…”

Section: Introductionmentioning

confidence: 99%

Simultaneous Perfect Bending and Polarization Rotation of Electromagnetic Wavefront Using Chiral Gradient Metasurfaces

2020

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We introduce chiral gradient metasurfaces that allow perfect transmission of all the incident wave into a desired direction and simultaneous perfect rotation of the polarization of the refracted wave with respect to the incident one. Besides using gradient polarization densities which provide bending of the refracted wave with respect to the incident one, using metasurface inclusions that are chiral allows the polarization of the refracted wave to be rotated. We suggest a possible realization of the proposed device by discretizing the required equivalent surface polarization densities realized by proper helical inclusions at each discretization point. By only using a single optically thin layer of chiral inclusions, we are able to unprecedentedly deflect a normal incident plane wave to a refracted plane wave at 45°with 72% power efficiency which is accompanied by a 90 • polarization rotation. The proposed concepts and design method may find practical applications in polarization rotation devices at microwaves as well as in optics, especially when the incident power is required to be deflected.

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Wave-front Transformation with Gradient Metasurfaces

Cited by 257 publications

References 59 publications

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

Omega-bianisotropic metasurface for converting a propagating wave into a surface wave

Simultaneous Perfect Bending and Polarization Rotation of Electromagnetic Wavefront Using Chiral Gradient Metasurfaces

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