Aimad Saib

An unbiased integrated microstrip circulator based on magnetic nanowired substrate

A very compact planar fully integrated circulator operating at millimeter wavelength has been designed using a magnetic substrate combining a polymer membrane with an array of ferromagnetic nanowires. The original feature of this substrate, called magnetic nanowired substrate (MNWS), relies on the fact that the circulation effect is obtained without requiring any biasing dc magnetic field. This leads to a significant reduction of device dimensions since no magnetic field source is needed, and a realistic ability for integration with monolithic microwave integrated circuits. The circulator design is performed by an efficient analytical model including a self design of the impedance matching network. This model also allows a physical understanding of the circulation mechanism through the access to the electromagnetic field patterns inside the circulator substrate. Based on the excellent agreement between the theoretical and experimental results, the model is used to predict the improvement of circulator performances resulting from a reduction of dielectric and conductor losses. Insertion losses lower than 2 dB with an isolation higher than 45 dB are expected for MNWS circulators with a low-loss substrate and thick metallic layers.

Magnetic photonic band-gap material at microwave frequencies based on ferromagnetic nanowires

We present an experimental investigation of a class of microwave photonic band-gap (PBG) materials, in which the magnetic permeability μ varies periodically within the material. This material is fabricated using a periodic arrangement of arrays of magnetic nanowires. As for dielectric or metallic PBG, the band-gap behavior varies with the geometrical parameters fixing the spatial periodicity of the magnetic structure. The magnetic photonic band gap is induced by the presence of a ferromagnetic resonance effect in the vicinity of the band gap.

Unbiased microwave circulator based on ferromagnetic nanowires arrays of tunable magnetization state

A planar microwave circulator operating without biasing static magnetic field is presented, using bistable ferromagnetic nanowires embedded in a dielectric medium. We show that its circulation mechanism is strongly dependent on the magnetization state at zero external magnetic field. A theoretical model, yielding the permeability components of the magnetic nanowired composite at remanent or partially magnetized state and the transmission characteristics of the circulator, is proposed. It explains the essential role played by the magnetization state on the circulation performances. Composites with very high remanence are necessary for an optimal circulation effect. It is also shown that the insertion losses can be significantly reduced by improving the matching between the access lines and the circulator.

Carbon nanotube composites for broadband microwave absorbing materials

In this paper, we present a new shielding and absorbing composite based on carbon nanotubes (CNTs) dispersed inside a polymer dielectric material. The extremely high aspect ratio of CNTs and their remarkable conductive properties lead to good absorbing properties with very low concentrations. A broadband characterization technique is used to measure the microwave electrical properties of CNT composites. It is shown that a conduction level of 1 S/m is reached for only 0.35 wt % of a CNT, while, for a classical absorbing composite based on carbon black, 20% concentration is mandatory. The conductive properties are explained by a phenomenological electrical model and successfully correlated with rheological data aiming at monitoring the dispersion of conductive inclusions in polymer matrices

Periodic Metamaterials Combining Ferromagnetic Nanowires and Dielectric Structures for Planar Circuits Applications

This paper presents RF applications of metamaterials combining ferromagnetic and dielectric structures for planar devices. Materials under scope are periodic dielectric substrates including ferromagnetic inclusions or not with planar metallic patterns, forming microstrip photonic band-gap (PBG) structures. These topologies are modeled, fabricated, and measured in order to design planar sensors and filters up to millimeter wave frequencies. In addition, defect modes that make the spectral properties of the PBG devices more attractive for narrow bandpass filters and high Q resonators are also investigated. A theoretical explanation of defect modes operation in a magnetic PBG structure is proposed.

Transmission lines on periodic bandgap metamaterials: from microwaves to optics applications

The paper presents a transmission line (TL) approach developed in the field of metamaterials for planar devices and circuits. Materials under consideration are dielectric substrates, containing or not nanoscaled ferromagnetic inclusions, that can be arranged in a periodic way with specific impedances and phase velocities. They form photonic or periodic bandgap (PBG) substrates, on which planar guiding structures are deposited. These PBG substrates show some similarities with integrated Bragg reflector topologies previously investigated in the optical range. Using the TL concept, defect modes in PBG planar devices are studied. A theoretical explanation of defect mode operation, which is validated by measurement on a dielectric PBG structure, is proposed. It is then transposed to a magnetic PBG structure based on arrays of nanoscaled ferromagnetic nanowires. The effect on its frequency response of varying the defect length is discussed, in order to design planar sensors and filters. Extension of the magnetic PBG structure for applications in the optical range is finally proposed.

Design of a stopband filter based on a magnetic photonic bandgap material

In this paper, we present a stopband filter synthesis using a new class of photonic bandgap (PBG) materials, in which the magnetic permeability varies periodically. This magnetic PBG (MPBG) is fabricated using arrays of ferromagnetic nanowires. The MPBG design procedure, which will be described, is divided into two stages. First, we perform an analytical prediction of the filter properties for given MPBG parameters. Then, the frequency response of the MPBG is simulated using a variational-chain matrix model, which is validated by measurements.