An all‐dielectric metasurface exhibiting a strong toroidal resonance is theoretically designed and experimentally demonstrated as an angular‐dependent resonant polarization beam‐splitter in the ...microwave K‐band. The metasurface is fabricated by embedding a square periodic array of high‐permittivity ceramic cuboid resonators in a 3D‐printed substrate of polylactic acid. It is demonstrated that by properly selecting the resonator geometry and by tuning the angle of incidence through mechanical rotation, the metasurface can switch between a polarization beam splitting and bandpass or bandstop operation. Such performance is achieved by exploiting the highly asymmetric Fano spectral profile of the toroidal resonance and the very low (high) dispersion of the associated p‐(s‐) polarized mode resulting from the resonant toroidal dipole mode's field profile, as evidenced by both full‐wave and band structure simulations. Theoretically infinite extinction ratios are achievable for polarization beam splitting operation with very low insertion losses and adjustable bandwidth. The experimental demonstration of such a compact, all‐dielectric metasurface expands the research portfolio of resonant metasurfaces toward not only the investigation of the intriguing physics of toroidal modes but also to the engineering of functional millimeter‐wave components for polarization control, for instance, in the context of 5G wireless communication networks.
Toroidal resonances are an elusive class of electromagnetic excitations, rarely found in natural materials. Here, strong toroidal dipole resonances in all‐dielectric metasurfaces composed of high‐permittivity, low‐loss ceramic resonators are demonstrated. By harnessing their particular polarization‐dependent properties, toroidal metasurfaces for angular‐dependent resonant polarization beam splitting in the microwave K‐band are engineered and experimentally demonstrated.
This letter presents an alternating-direction implicit finite-difference time-domain scheme for the efficient study of plasmonic systems. The material dispersion is described by generalized modified ...Lorentzian terms and it is implemented via the auxiliary differential equations technique employing an order reduction. The computational domain is backed by a properly designed convolution perfectly matched layer. The efficiency of the proposed method is validated in benchmark examples and its unconditional stability is evidenced by the Fourier method.
A rectangular metallic leaky waveguide loaded with liquid crystals (LC) and operating in its fundamental TE mode at 1 THz is proposed to mitigate the well-known trade-off between directivity and ...tunable angular range of dynamic beamscanning antennas. The radiating aperture consists of a partially reflecting surface (PRS) realized through a one-dimensional array of longitudinal slots etched on one of the metallic walls of the waveguide to efficiently couple with the propagating TE leaky mode. The antenna performance is evaluated by defining suitable figures of merit that take into account the beamscanning feature and the gain peak. These figures of merit are evaluated for different combinations of the antenna design parameters. After optimization, a tunable angular range of about 28° is reached, while maintaining a gain of around 7 dBi. A leaky-wave analysis of a simplified model of the structure allows to design the antenna without resorting to computationally expensive optimization processes. More rigorous models are then considered to accurately analyze the LC dynamics and the radiating properties of PRS. The three-dimensional structure is finally validated through full-wave simulations, showing a remarkable agreement with the theoretical predictions obtained with the simplified model.
A long-range surface plasmon polariton variable optical attenuator based on available nematic liquid crystals and polymers is proposed and theoretically investigated. It is demonstrated that the ...electro-optic control of the nematic molecular orientation is capable of tuning the level of index asymmetry of an Au stripe waveguide and the key properties of the fundamental long-range plasmonic mode, such as modal profile and propagation losses. By proper structural design and material selection, plasmonic in-line intensity modulators are designed, which exhibit very low power consumption, extinction ratios in excess of 30 dB, and insertion losses as low as 1 dB for a device length in the millimeter range. Such active plasmonic elements are envisaged to be used in interchip photonics bus interconnects.
Compact polarization control elements based on index-guiding soft-glass photonic crystal fibers infiltrated with nematic liquid crystals are proposed and thoroughly studied. The nematic director ...profiles at the fiber's cross section are consistently calculated by solving the coupled electrostatic and elastic problem, in the context of an analysis on the tunability of liquid-crystal-infiltrated photonic crystal fibers. The fiber's dispersive properties and light propagation in the proposed polarization controller are studied by means of a fully anisotropic finite-element-based beam propagation method. The electrically induced evolution of the state of polarization is mapped on the Poincaré sphere. Efficient polarization conversion is demonstrated, with a crosstalk of -50 dB, for a total device length of 4.65 mm and a maximum applied voltage of 150 V. Crosstalk values lower than -20 dB are achieved over a 30 nm window. The proposed devices are envisaged as compact all-in-fiber dynamic polarization controllers.
The transmission electrode technique has been recently proposed as a versatile method to obtain various types of liquid-crystal (LC) lenses. In this work, an equivalent electric circuit and new ...analytical expressions based on this technique are developed. In addition, novel electrode shapes are proposed in order to generate different phase profiles. The analytical expressions depend on manufacturing parameters that have been optimized by using the least squares method. Thanks to the proposed design equations and the associated optimization, the feasibility of engineering any kind of aspheric LC lenses is demonstrated, which is key to obtain aberration-free lenses. The results are compared to numerical simulations validating the proposed equations. This novel technique, in combination with the proposed design equations, opens a new path for the design and fabrication of LC lenses and even other types of adaptive-focus lenses based on voltage control.
A whispering-gallery mode nanophotonic laser cavity having as active medium a transition-metaldichalcogenide (TMD) bilayer is examined. The proposed system is analysed and designed utilizing a strict ...and rigorous computational framework based on the coupled-mode theory. Our framework is capable of accurately and efficiently handling the gain properties of two-dimensional materials, such as contemporary TMD monolayers, multilayers, and heterostructures. The presented lasing cavity exhibits an adequately low pump threshold and light emission in the order of milliwatts is predicted. Exploiting the capabilities of the developed framework, we were in position to efficiently design the cavity as well as to estimate quantitative lasing parameters such as the pumping threshold and the lasing frequency.
Thanks to their lower losses and sharper resonances compared to their metallic counterparts, all-dielectric metasurfaces are attracting a quickly growing research interest. The application of such ...metasurfaces in the field of refractive index sensing is extremely attractive, especially due to the expected high performance and the simplicity of the sensing element excitation and readout. Herein, we report on an all-dielectric silicon metasurface based on complementary split-ring resonators (CSRRs) optimized for refractive index sensing. A quasi-bound state in the continuum (quasi-BIC) with an ultra-high quality factor can be excited in the near-infrared (NIR) window by violating the structure symmetry. By using the three-dimensional finite element method (3D-FEM), a refractive index sensor for biomedical applications with an ultra-high figure of merit (FoM > 100,000 RIU−1) has been designed, exploiting the quasi-BIC resonance. The proposed design strategy opens new avenues for developing flat biochemical sensors that are accurate and responsive in real time.
A single‐layer, all‐dielectric metasurface exhibiting a strong toroidal resonance in the low‐atmospheric loss radio window of the subterahertz W‐band is theoretically proposed and experimentally ...demonstrated. The metasurface is fabricated on a high‐resistivity floating‐zone silicon wafer by means of a single‐process, wet anisotropic etching technique. The properties of the toroidal mode of both the constituent dielectric elements and the metasurface are rigorously investigated by means of the multipole decomposition technique and full‐wave simulations. The experimental demonstration of such a compact, all‐silicon metasurface opens new venues of research in the investigation of toroidal modes and the engineering of functional millimeter‐wave components, which can be scaled to terahertz and higher frequencies of the electromagnetic spectrum.
Toroidal resonances are hard to observe in natural materials. Here, an all‐silicon metasurface that exhibits a strong toroidal dipole resonance in the subterahertz W‐band is designed and experimentally demonstrated. Such compact, all‐silicon metasurfaces provide new possibilities for the engineering of functional millimeter‐wave components.
A new type of nematic liquid-crystal infiltrated photonic bandgap-guiding fiber for single polarization or high-birefringence guidance is proposed. Numerical studies demonstrate that modal ...birefringence can be tuned by proper selection of the structural and material parameters as well as by the application of an external electric field in conjunction with the specific liquid-crystal anchoring conditions
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