Reconfigurable Microwave Circuits Research Articles

Electrical switching of magnetic order in an orbital Chern insulator.

Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favours ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields-a longstanding technological goal in spintronics and multiferroics1,2-can be achieved only indirectly. Here we experimentally demonstrate direct electric-field control of magnetic states in an orbital Chern insulator3-6, a magnetic system in which non-trivial band topology favours long-range order of orbital angular momentum but the spins are thought to remain disordered7-14. We use van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer to realize narrow and topologically non-trivial valley-projected moiré minibands15-17. At fillings of one and three electrons per moiré unit cell within these bands, we observe quantized anomalous Hall effects18 with transverse resistance approximately equal to h/2e2 (where h is Planck's constant and e is the charge on the electron), which is indicative of spontaneous polarization of the system into a single-valley-projected band with a Chern number equal to two. At a filling of three electrons per moiré unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate non-volatile electric-field-induced reversal of the magnetic state. A theoretical analysis19 indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favoured magnetic state. Voltage control of magnetic states can be used to electrically pattern non-volatile magnetic-domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultralow-power magnetic memories.

A quantized microwave quadrupole insulator with topologically protected corner states.

The theory of electric polarization in crystals defines the dipole moment of an insulator in terms of a Berry phase (geometric phase) associated with its electronic ground state. This concept not only solves the long-standing puzzle of how to calculate dipole moments in crystals, but also explains topological band structures in insulators and superconductors, including the quantum anomalous Hall insulator and the quantum spin Hall insulator, as well as quantized adiabatic pumping processes. A recent theoretical study has extended the Berry phase framework to also account for higher electric multipole moments, revealing the existence of higher-order topological phases that have not previously been observed. Here we demonstrate experimentally a member of this predicted class of materials-a quantized quadrupole topological insulator-produced using a gigahertz-frequency reconfigurable microwave circuit. We confirm the non-trivial topological phase using spectroscopic measurements and by identifying corner states that result from the bulk topology. In addition, we test the critical prediction that these corner states are protected by the topology of the bulk, and are not due to surface artefacts, by deforming the edges of the crystal lattice from the topological to the trivial regime. Our results provide conclusive evidence of a unique form of robustness against disorder and deformation, which is characteristic of higher-order topological insulators.

Design and characterization of a resonator-based metamaterial and its sensor application using microstrip technology

Design of a metamaterial based on an S-shaped resonator surrounded by a ground frame and excited by using a feeding transmission line on microstrip technology is presented. Since the resonator, ground frame, and its excitation mechanism are all realized on a microstrip, its characterization can be carried out using common laboratory equipment without needing any waveguide components or plane-wave-illumination techniques. The structure presented here may be realized on any microstrip and does not require special materials. The resonator and ground frame are both on the same side of the microstrip, thus the proposed topology may also be populated with active and passive microwave components, and hybrid active, passive, or reconfigurable microwave circuits may be realized. In metamaterial designs that require plane wave illumination, usually many numbers of periodic unit cells are needed; however, in our design, only one cell is capable of achieving metamaterial properties. The constitutive parameters of the metamaterial are retrieved and compared to demonstrate the agreement between simulations and measurements. The proposed topology is also demonstrated in a sensor application, where simulated and measured results agree well. Thus, it can be realized using standard microwave technology and used for numerous applications where metamaterial properties are needed.

A Tunable Metamaterial Resonator Using Varactor Diodes to Facilitate the Design of Reconfigurable Microwave Circuits

In this brief, we designed, constructed, and analyzed a frequency-tunable metamaterial resonator. The proposed design consists of an S-shaped resonator, a ground frame, and a feeding transmission line. First, the structure is designed as a nontunable resonator and then tunability is achieved by employing varactor diodes. In order to verify and demonstrate its tunable metamaterial properties, reflection and transmission parameters, group delay, electric and magnetic field distributions, and permittivity and permeability at each tuned frequency were analyzed. Simulation and measured results agree well and show that a high transmission and reflection peak occurs at each resonance frequency that may be tuned with the applied control voltage. Therefore, the proposed tunable metamaterial resonator may be used to realize reconfigurable microwave circuits such as reflection/transmission filters, antennas, and sensors, and for achieving flexibility in many other microwave circuits.

Circuit Agility

Over the last decade, mobile communication and its associated mass volume market has become one of the driving forces in the technology evolution of semiconductor and microwave circuits. For handheld communication devices, it is now mainstream to support increasing numbers of communication standards and localization services that occupy ever-expanding wide frequency ranges and bandwidths. At the same time, the physical dimensions of handheld user devices are shrinking, leading to even tighter specifications for the highly integrated front-end architectures of mobile radios. Todays radio front-end architectures use dedicated receive and transmit paths for each covered communication standard, thus the overall complexity and occupied area is increasing as well. The wide frequency allocation of the regulated communication bands along with the variety of standards, which have to be covered by these radios, calls for reconfigurable and frequency agile microwave subsystems. The scope of this article is to provide a comprehensive understanding of design principles for frequency agile and reconfigurable microwave circuits such as power dividers and couplers and how they can be used in radio-frequency (RF)-transceiver subsystems to pave the way toward reconfigurable radio front-end architectures. Introducing reconfigurability in microwave circuits is achieved by tunable passive components. Tunable passives can be implemented in a variety of technologies such as ferroelectric varactors, semiconductor diodes, and microelectromechanical systems (MEMS) components. Attractive applications for tunable circuits in the RF front end are tunable matching networks, filters, reconfigurable power amplifiers, tunable voltage controlled oscillator (VCO) circuits, and finally, couplers and dividers.

Low-voltage small-size double-arm MEMS actuator

The fabrication and characterisation of a double-arm cantilever-type metallic DC-contact MEMS actuator with low pull-down voltage are reported. Bi-layer TiW cantilevers with an internal stress gradient were fabricated using a microwave-compatible fabrication process. Owing to its small size, cantilever length (L=5–50 µm) and width (W=2–40 µm), i.e. ∼10–100 times smaller in lateral dimensions than a standard MEMS actuator, this actuator showed actuation voltages lower than 10 V. RF measurements of the 10 µm-wide actuators yielded an average insertion loss less than 1 dB and isolation higher than 40 dB up to 25 GHz. The developed actuator is well suited for integration in reconfigurable microwave circuits and systems such as reconfigurable antennas and arrays.