Diffraction Phenomena in Time-Varying Metal-Based Metasurfaces

This paper presents an analytical framework for the analysis of time-varying metal-based metamaterials. Specifically, we particularize the study to time-modulated metal-air interfaces embedded between two different semi-infinite media that are illuminated by monochromatic plane waves of frequency ω0. The formulation is based on a Floquet-Bloch modal expansion, which takes into account the time periodicity of the structure (Ts = 2π/ωs) and integral-equation techniques. It allows us to extract the reflection and transmission coefficients as well as to derive nontrivial features about the dynamic response and dispersion curves of time-modulated metal-based screens. In addition, the proposed formulation has an associated analytical equivalent circuit that gives a physical insight into the diffraction phenomenon. Similarities and differences between space- and time-modulated metamaterials are discussed via the proposed circuit model. Finally, some analytical results are presented to validate the present framework. Good agreement is observed with numerical computations provided by a self-implemented finite-difference time-domain (FDTD) method. Interestingly, the present results suggest that time-modulated metal-based screens can be used as pulsed sources (when ωs ω0), beam formers (ωs ∼ ω0) to redirect energy in specific regions of space, and analog samplers (ωs ω0).Spanish Government ID2020-112545RB-C54 RTI2018-102002-A-I00Junta de Andalucia B-TIC-402-UGR18 A-TIC-608- UGR20 PYC20-RE-012-UGR P18.RT.4830Leonardo Grant of the BBVA FoundationBBVA Foundatio

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing three major spectroscopic programs. The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky Way stars at high resolution and high signal-to-noise ratios in the near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is obtaining spatially resolved spectroscopy for thousands of nearby galaxies (median $z\sim 0.03$). The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas distributions between $z\sim 0.6$ and 3.5 to constrain cosmology using baryon acoustic oscillations, redshift space distortions, and the shape of the power spectrum. Within eBOSS, we are conducting two major subprograms: the SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray AGNs and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey (TDSS), obtaining spectra of variable sources. All programs use the 2.5 m Sloan Foundation Telescope at the Apache Point Observatory; observations there began in Summer 2014. APOGEE-2 also operates a second near-infrared spectrograph at the 2.5 m du Pont Telescope at Las Campanas Observatory, with observations beginning in early 2017. Observations at both facilities are scheduled to continue through 2020. In keeping with previous SDSS policy, SDSS-IV provides regularly scheduled public data releases; the first one, Data Release 13, was made available in 2016 July

Broadband parasitic modeling of diodes in the millimeter-wave band

International audienceThis paper presents the extraction of an equivalent circuit model for PIN diodes and varactors in the millimeter waves. This circuit model is handy for new communication applications involving, for example, electronic beam reconfigurability, where diodes are commonly used. For parameter extraction, the proposed model includes the effects of the device, the pads, and the gap underneath the device. From the measurement at various bias points (varactor) or states (PIN diode), it is possible to extract an equivalent circuit to properly model the behavior of these devices when using them to design reconfigurable elements. The results are validated experimentally, obtaining an excellent agreement between the measurements and the equivalent circuit models in a large frequency band up to 67 GHz

Capacity in Weibull Fading with Shadowing for MIMO Distributed System

Distributed systems are becoming the tool to reach high transmission rates in wireless environments. However, these systems are exposed to macroscopic and microscopic fading because they have the nodes (antennas) spatially distributed. This letter proposes a simple model to compute distributed multiple-input multiple-output capacity in composite Weibull shadowing fading in order to account for both effects, macroscopic (shadowing) and microscopic (Weibull channel)

Metal-Only Reflect-Transmit-Array Unit Cell with Polarization-Dependent Performance

International audienceA metal-only reflect-transmit-array unit cell whose operation is dependent on the incident polarization is presented. The proposed unit cell is based on 3D geometry which allows a high level of independent phase tuning for orthogonal and linear incident polarizations. In our unit cell, the horizontal polarization is reflected while the vertical polarization is transmitted. The modification of the reflected phase is done by the length of a metal block located at the end of the slits where the horizontal polarization propagates. The modification of the transmitted phase is changed by the depth of the corrugations implemented in the slit that supports the propagation of the vertical polarization. The results obtained in transmission mode show an impedance matching below -15 dB with a linear phase response from 30 GHz to 50 GHz. For the reflected polarization, there is almost total reflection with phase performance that is also linear along the frequency

Modelling Partially-Dielectric Unit Cells for OAM Transmitarrays

International audienceThe presented study introduces a semi-analytical circuit model (CM) to fully characterize a dielectric unit cell structure made by blocks instead of applying the effective medium theory. A fitting strategy is used to a obtain a better understanding of the properties of the unit cell. Then, the approach is used to develop a semi-analytical model, showing a good agreement with the scattering parameters extracted from a full-wave simulator. By tuning the dielectric material, our model can describe the behaviour of unit cells with different permittivities and electrical sizes. Finally, as an application for the analyzed dielectric unit cells, transmitarrays to generate orbital angular momentum (OAM) waves are designed and 3D-printed. The measured results show good performance for different OAM modes at 31 GHz