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Register a new userTime-varying Huygens meta-devices for parametric waves
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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.
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Metasurfaces are an enabling technology for complex wave manipulation functions, including in the terahertz frequency range, where they are expected to advance security, imaging, sensing, and communications technology. For operation in transmission, Huygens metasurfaces are commonly used, since their good impedance match to the surrounding media minimizes reflections and maximizes transmission. Recent theoretical work has shown that Huygens metasurfaces are non-optimal, particularly for large angles of refraction, and that to eliminate reflections and spurious diffracted beams it is necessary to use a bianisotropic metasurface. However, it remains to be demonstrated how significant the efficiency improvement is when using bianisotropic metasurfaces, considering all the non-ideal features that arise when implementing the metasurface design with real meta-atoms. Here we compare concrete terahertz metasurface designs based on the Huygens and Omega-type bianisotropic approaches, demonstrating anomalous refraction angles for 55 degrees, and 70 degrees. We show that for the lower angle of 55 degrees, there is no significant improvement when using the bianisotropic design, whereas for refraction at 70 degrees the bianisotropic design shows much higher efficiency and fidelity of refraction into the designed direction. We also demonstrate the strong perturbations caused by near-field interaction, both between and within cells, which we compensate using numerical optimization.
This paper proposes a novel technique for the design of miniaturized waveguide filters based on locally resonant metamaterials (LRMs). We implement ultra-small metamaterial filters (Meta-filters) by exploiting a subwavelength (sub-lambda guiding mechanism in evanescent hollow waveguides, which are loaded by small resonators. In particular, we use composite pin-pipe waveguides (CPPWs) built from a hollow metallic pipe loaded by a set of resonant pins, which are spaced by deep subwavelength distances. We demonstrate that in such structures, multiple resonant scattering nucleates a sub-lambda mode with a customizable bandwidth below the induced hybridization bandgap (HBG) of the LRM. The sub-lambda guided mode and the HBG, respectively, induce pass- and rejection- bands in a finite-length CPPW, creating a filter whose main properties are largely decoupled from the specific arrangement of the resonant inclusions. To guarantee compatibility with existing technologies, we propose a unique subwavelength method to match the small CPPW filters to standard waveguide interfaces, which we call a meta-port. Finally, we build and test a family of low- and high-order ultra-compact aluminum CPPW filters in the Ku-band (10-18GHz). Our measurements demonstrate the customizability of the bandwidth and the robustness of the passband against geometrical scaling. The 3D-printed prototypes, which are one order of magnitude smaller and lighter than traditional filters and are also compatible with standard waveguide interfaces, may find applications in future satellite systems and 5G infrastructures.
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.
Graded metasurfaces exploit the local momentum imparted by an impedance gradient to transform the impinging wavefront. This approach suffers from fundamental limits on the overall conversion efficiency and it is challenged by fabrication limitations on the maximum spatial resolution. Here, we introduce the concept of meta-gratings, which enables arbitrary wavefront engineering with unitary efficiency and significantly lower demands on the fabrication precision. We employ this concept to design reflective metasurfaces for wavefront steering without limitations on efficiency. A similar approach is envisioned for transmitted beams and for arbitrary wavefront transformation, opening promising opportunities for highly-efficient metasurfaces for extreme wave manipulation.
Invariance under time translation (or stationarity) is probably one of the most important assumptions made when investigating electromagnetic phenomena. Breaking this assumption is expected to open up novel possibilities and result in exceeding conventional limitations. For that, we primarily need to contemplate the fundamental principles and concepts from a nonstationarity perspective. Here, we revisit one of those concepts: The polarizability of a small particle, assuming that its properties vary in time. We describe the coupling of the induced dipole moment with the excitation field in a nonstationary, causal way, and introduce a complex-valued function, called temporal complex polarizability, for elucidating a nonstationary Hertzian dipole under time-harmonic illumination. This approach can be extended to any subwavelength particle having electric response. In addition, we also study the polarizability of a classical electron through the equation of motion whose damping coefficient and natural frequency are changing in time. We theoretically derive the effective permittivity corresponding to time-varying media (comprising free or bound electrons) and explicitly show the differences with the conventional macroscopic Drude-Lorentz model. This paper will hopefully pave the road towards the understanding of nonstationary scattering from small particles and the homogenization of time-varying materials, metamaterials, and metasurfaces.
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