In this communication we present a concept of a partially reflective surface (PRS) antenna capable of independent beam-scanning and beamwidth dynamic control. The beamwidth is controlled by ...reconfiguring the PRS reflectivity via embedded varactor diodes, while the beam is steered by using a small phased array in the H-plane as a source antenna exciting the Fabry-Pérot cavity. A fully operational prototype is fabricated and measured upon reconfiguration, demonstrating the viability of the concept and good agreement between simulated and measured results. The achieved beamwidth variation is from 18.7° to 22.4°, with the scanning range of 15° and 20° for the narrowest and the widest beamwidth, respectively.
In this communication we propose the concept of a circularly polarized reflectarray (RA) antenna capable of independent beam-switching in both K and Ka bands. The RA unit cell comprises one ...microstrip ring per each operation frequency. Each ring is integrated with six equally spaced series RF micro electro-mechanical systems (RF MEMS) switches, which allows implementing the sequential rotation principle formerly used in circularly-polarized RA for single-frequency operation. A detailed design is proposed, considering the best relative arrangement of the rings corresponding to each frequency, the accurate modeling of the RF MEMS switches, and the full-wave simulation of the full array. The designed RA is implemented on a 4-inch quartz wafer and comprises 109 K-band and 124 Ka-band split-rings. Two prototypes representing two frozen states of the reconfigurable antenna are fabricated and measured. The designed RA can provide {\pm}{120}^{\circ} progressive phase difference in both operation bands exhibiting beam switching to {\pm}{35}^{\circ} and {\pm}{24}^{\circ} off the broad-side in K and Ka bands respectively. The performance of the designed antenna is verified by the agreement of the measured and simulated radiation patterns.
A reflecting cell allowing the dynamic and independent control of the reflection phase of two perpendicular linearly-polarized waves is presented. This capability is useful for the implementation of ...dual-polarized beam-reconfigurable reflectarrays, as well as for new reflector functionalities such as linear/circular polarization-flexibility and polarization-twisting with simultaneous phase control. The concept is demonstrated on a unit cell operating at 8 GHz, using surface-mounted diodes. In addition to its polarization capability, the cell implements a new concept to overcome the usual tradeoff between phase range and loss in monolayer reflective cells, using a combination of PIN and varactor diodes. For each linearly-polarized component, a dynamic phase range over 360° is achieved at 8 GHz under normal incidence, with 4 dB and 2 dB of maximum and average loss, respectively. The modeling of the cell in a general periodic environment, along with measurements in a waveguide simulator, show that similar performance is preserved within a 5% bandwidth and under TE and TM oblique incidences up to 45°. The cell size is only 0.42¿ at 8 GHz.
In this letter, we present a reconfigurable reflectarray cell operating at 12 GHz and fabricated in a monolithic MEMS process. A 5-bit digital control allows reconfiguration of the reflection phase ...over the full 360deg range, while alleviating the impact of MEMS and bias voltage tolerances on the device performances. The designed reflectarray cell exhibits low frequency phase error (large bandwidth) and excellent measured reflection loss (-0.3 dB) with regard to state-of-the-art. Close agreement between measurements and full-wave simulations is observed.
This paper proposes the concept, analysis and design of a sinusoidally modulated graphene leaky-wave antenna with beam scanning capabilities at a fixed frequency. The antenna operates at terahertz ...frequencies and is composed of a graphene sheet transferred onto a back-metallized substrate and a set of polysilicon DC gating pads located beneath it. In order to create a leaky-mode, the graphene surface reactance is sinusoidally modulated via graphene's field effect by applying adequate DC bias voltages to the different gating pads. The pointing angle and leakage rate can be dynamically controlled by adjusting the applied voltages, providing versatile beamscanning capabilities. The proposed concept and achieved performance, computed using realistic material parameters, are extremely promising for beamscanning at THz frequencies, and could pave the way to graphene-based reconfigurable transceivers and sensors.
Isolators, or optical diodes, are devices enabling unidirectional light propagation by using non-reciprocal optical materials, namely materials able to break Lorentz reciprocity. The realization of ...isolators at terahertz frequencies is a very important open challenge made difficult by the intrinsically lossy propagation of terahertz radiation in current non-reciprocal materials. Here we report the design, fabrication and measurement of a terahertz non-reciprocal isolator for circularly polarized waves based on magnetostatically biased monolayer graphene, operating in reflection. The device exploits the non-reciprocal optical conductivity of graphene and, in spite of its simple design, it exhibits almost 20 dB of isolation and only 7.5 dB of insertion loss at 2.9 THz. Operation with linearly polarized light can be achieved using quarter-wave plates as polarization converters. These results demonstrate the superiority of graphene with respect to currently used terahertz non-reciprocal materials and pave the way to a novel class of optimal non-reciprocal devices.
Age-related changes in the niche have long been postulated to impair the function of somatic stem cells. Here we demonstrate that the aged stem cell niche in skeletal muscle contains substantially ...reduced levels of fibronectin (FN), leading to detrimental consequences for the function and maintenance of muscle stem cells (MuSCs). Deletion of the gene encoding FN from young regenerating muscles replicates the aging phenotype and leads to a loss of MuSC numbers. By using an extracellular matrix (ECM) library screen and pathway profiling, we characterize FN as a preferred adhesion substrate for MuSCs and demonstrate that integrin-mediated signaling through focal adhesion kinase and the p38 mitogen-activated protein kinase pathway is strongly de-regulated in MuSCs from aged mice because of insufficient attachment to the niche. Reconstitution of FN levels in the aged niche remobilizes stem cells and restores youth-like muscle regeneration. Taken together, we identify the loss of stem cell adhesion to FN in the niche ECM as a previously unknown aging mechanism.
The propagation of surface waves along spatially dispersive graphene-based 2-D waveguides is investigated in detail. Graphene is characterized using a full-k ρ conductivity model under the ...relaxation-time approximation, which allows to obtain analytical and closed-formed expressions for the wavenumber of plasmons supported by sheets and parallel plate waveguides, respectively. Per unit length equivalent circuits are introduced to accurately characterize the propagation in different waveguides, and analytical relations between the effective TM-mode circuit lumped elements and graphene conductivity are derived. The proposed circuits allow identifying the different mechanisms involved in spatially dispersive plasmon propagation, explaining their connection with the intrinsic properties of graphene. Results demonstrate that spatial dispersion, which significantly decreases the confinement and the losses of slow surface plasmons, must be accurately assessed in the design of graphene-based plasmonic components at millimeter-waves and low terahertz frequencies.
Recently, a technique called beam-space MIMO has been demonstrated as an effective approach for transmitting multiple signals while using a single RF-chain. In this work, we present novel design ...considerations and a compact antenna solution to stimulate the deployment of beam-space MIMO in future wireless applications. Targeting integration in small wireless devices, the novel antenna is made of a single integrated radiator rather than an array of physically-separated dipoles. It also drastically simplifies the implementation of variable loads and DC bias circuits for BPSK modulated signals, and does not require any external reconfigurable matching circuit. Finally, we show that this antenna system could be reconfigured by dynamic adjustment of terminating loads to preserve its beam-space multiplexing capabilities over a 1:2 tuning range, thereby promoting the convergence of MIMO and dynamic spectrum allocation via reduced-complexity hardware. A prototype achieving single-RF-chain multiplexing at a fixed frequency is designed and measured, showing excellent agreement between simulations and measurements.
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