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Register a new userTime-variant metasurfaces enable tunable spectral bands of negative extinction
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Added by
Maxim Shcherbakov R.
Publication date
2019
fields
Physics
and research's language is
English
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We demonstrate that rapidly switched high-Q metasurfaces enable spectral regions of negative optical extinction.
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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.
We introduce the concept of nonlinear graphene metasurfaces employing the controllable interaction between a graphene layer and a planar metamaterial. Such hybrid metasurfaces support two types of subradiant resonant modes, asymmetric modes of structured metamaterial elements (metamolecules) and graphene plasmons exhibiting strong mutual coupling and avoided dispersion crossing. High tunability of graphene plasmons facilitates strong interaction between the subradiant modes, modifying the spectral position and lifetime of the associated Fano resonances. We demonstrate that strong resonant interaction, combined with the subwavelength localization of plasmons, leads to the enhanced nonlinear response and high efficiency of the second-harmonic generation.
We uncover and identify the regime for a magnetically and ferroelectrically controllable negative refraction of light traversing multiferroic, oxide-based metastructure consisting of alternating nanoscopic ferroelectric (SrTiO$_2$) and ferromagnetic (Y$_3$Fe$_2$(FeO$_4$)$_3$, YIG) layers. We perform analytical and numerical simulations based on discretized, coupled equations for the self-consistent Maxwell/ferroelectric/ferromagnetic dynamics and obtain a biquadratic relation for the refractive index. Various scenarios of ordinary and negative refraction in different frequency ranges are analyzed and quantified by simple analytical formula that are confirmed by full-fledge numerical simulations. Electromagnetic-waves injected at the edges of the sample are propagated exactly numerically. We discovered that for particular GHz frequencies, waves with different polarizations are characterized by different signs of the refractive index giving rise to novel types of phenomena such as a positive-negative birefringence effect, and magnetically controlled light trapping and accelerations.
We experimentally demonstrate fabrication of tunable high contrast periodic fishnet metasurfaces with 3 um period on 200 nm thick Ge2Sb2Te5 films sputted onto glass and sapphire substrates using direct laser writing technique. We find that the use of sapphire substrate provides better accuracy of metasurface segments due to high thermal conductivity. The advantages of the demonstrated method consist in its simplicity, rapidity, robustness, and the ability of tuning of fabricated structures. This is of crucial importance for the creation of robust and tunable metasurfaces for applications in the field of telecommunications and information processing.
Metasurfaces represent a new paradigm in artificial subwavelength structures due to their potential to overcome many challenges typically associated with bulk metamaterials. The ability making very thin structures and change their properties dynamically make metasurfaces an exceptional meta-optics platform for engineering advanced electromagnetic and photonic metadevices. Here, we suggest and demonstrate experimentally a novel tunable metasurface capable to enhance significantly the local image quality in magnetic resonance imaging (MRI). We present a design of the hybrid metasurface based on electromagnetically-coupled dielectric and metallic elements. We demonstrate how to tailor the spectral characteristics of the metasurface eigenmodes by changing dynamically the effective permittivity of the structure. By maximizing a coupling between metasurface eigenmodes and transmitted and received fields in the MRI system, we enhance the device sensitivity that results in a substantial improvement of the image quality.
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