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Register a new userTime-varying metasurfaces for broadband spectral camouflage
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The possibility of making an object invisible for detectors has become a topic of considerable interest over the past decades. Most of the studies so far focused on reducing the visibility by reshaping the electromagnetic scattering in the spatial domain. In fact, by manipulating the electromagnetic scattering in the time domain, the visibility of an object can also be reduced. Importantly, unlike previous studies on phase-switched screens and time-varying metasurfaces, where the effect is narrow band due to the dispersive resonance, for microwave frequency range, we introduce a broadband switchable metasurface integrated with p-i-n diodes. The reflection phase of the metasurface can be changed by approximately {pi} over a fractional bandwidth of 76%. By modulating the metasurface quasirandomly in the time domain, the incident narrow-band signal is spread into a white-noiselike spectrum upon reflection, creating a spectral camouflage. The broadband feature of the proposed time-varying metasurface can provide practical insight for various applications, including radar stealth and ultrawide-band wireless communication.
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We demonstrate that rapidly switched high-Q metasurfaces enable spectral regions of negative optical extinction.
Many new possibilities to observe and use novel physical effects are discovered at so called exceptional points (EPs). This is done by using parity-time (PT) -symmetric non-Hermitian systems and balancing gains and losses. When combined with EP-physics, recently, metasurfaces have shown greater abilities for wave manipulation than conventional metasurface systems. However, the solving process for EPs usually requires the transfer matrix method (TMM) or a parametric sweep, which are both complex and time-consuming. In this Letter, we develop a simple theoretical model, which is based on acoustic equivalent-circuit theory and can find the analytic solutions for EPs directly. As a proof of concept, PT-symmetric acoustic metasurfaces are studied to test the theoretical model, which enables unidirectional antireflection effects at EPs. In addition, finite element method (FEM) simulations are performed to study these EP solutions using the theoretical model for different mediums, wavelengths, angles of incidence, and gain-loss ratios. Our work offers a simple and powerful theoretical tool for designing PT-symmetric metasurfaces at EPs and may also be used for other classical wave systems.
In this paper, a broadband tunable polarization converter based on graphene metasurfaces is proposed. This polarization converter works in the terahertz (THz) frequency region, using the advantage of graphene characteristics to have a tunable frequency response. The designed graphene-shaped periodic structure on top of the substrate is utilized to convert the incident wave polarization to the desired target in a flexible operational band in the THz frequencies. The polarization conversion ratio is more than 0.85 in a wide range of frequencies in the THz band from 4.86 to 8.42 THz (the fractional bandwidth is 54%). The proposed polarization converter is insensitive to the angle of the incident wave up to 40{deg}. Using graphene provides a tunable frequency response without changing the geometry of the designed structure.
Huygens metasurfaces have demonstrated almost arbitrary control over the shape of a scattered beam, however, its spatial profile is typically fixed at fabrication time. Dynamic reconfiguration of this beam profile with tunable elements remains challenging, due to the need to maintain the Huygens condition across the tuning range. In this work, we experimentally demonstrate that a time-varying metadevice which performs frequency conversion can steer transmitted or reflected beams in an almost arbitrary manner, with fully dynamic control. Our time-varying Huygens metadevice is made of both electric and magnetic meta-atoms with independently controlled modulation, and the phase of this modulation is imprinted on the scattered parametric waves, controlling their shapes and directions. We develop a theory which shows how the scattering directionality, phase and conversion efficiency of sidebands can be manipulated almost arbitrarily. We demonstrate novel effects including all-angle beam steering and frequency-multiplexed functionalities at microwave frequencies around 4 GHz, using varactor diodes as tunable elements. We believe that the concept can be extended to other frequency bands, enabling metasurfaces with arbitrary phase pattern that can be dynamically tuned over the complete 2pi range.
We propose the optical trapping of Rayleigh particles using tailored anisotropic and hyperbolic metasurfaces illuminated with a linearly polarized Gaussian beam. This platform permits to engineer optical traps at the beam axis with a response governed by nonconservative and giant recoil forces coming from the directional excitation of ultra-confined surface plasmons during the light scattering process. Compared to optical traps set over bulk metals, the proposed traps are broadband in the sense that can be set with beams oscillating at any frequency within the wide range in which the metasurface supports surface plasmons. Over that range, the metasurface evolves from an anisotropic elliptic to a hyperbolic regime through a topological transition and enables optical traps with distinctive spatially asymmetric potential distribution, local potential barriers arising from the momentum imbalance of the excited plasmons, and an enhanced potential depth that permits the stable trapping of nanoparticles using low-intensity laser beams. To investigate the performance of this platform, we develop a rigorous formalism based on the Lorentz force within the Rayleigh approximation combined with anisotropic Greens functions and calculate the trapping potential of nonconservative forces using the Helmholtz-Hodge decomposition method. Tailored anisotropic and hyperbolic metasurfaces, commonly implemented by nanostructuring thin metallic layers, enables using low-intensity laser sources operating in the visible or the IR to trap and manipulate particles at the nanoscale, and may enable a wide range of applications in bioengineering, physics, and chemistry.
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