Aobo Li

High-Power Transistor-Based Tunable and Switchable Metasurface Absorber

High-power signals traveling along the surface of the shielding of transmitter systems may leak into the system through openings between connecting parts and cause damage to vulnerable electronic devices. This problem could be alleviated by implementing lossy coatings or recently developed passive power-dependent nonlinear surfaces. However, these solutions will either suppress the performance of the electromagnetic devices being shielded or be highly power-dependent. Applying transistors creates an active nonlinear metasurface that can allow the absorption of the surface to be directly controlled by the system, or tuned in response to the local power level using feedback control. This can provide a sharp absorption response with a wide range of controllable power threshold. Different absorption rates at the same power level can also be achieved by applying different biasing to the transistors. In this paper, the first transistor-based, thin, switchable, and tunable high-power surface wave absorber is proposed with full wave and circuit cosimulation analysis as well as waveguide and anechoic chamber measurement.

Metasurfaces and their applications

Abstract Metasurfaces are a topic of significant research and are used in various applications due to their unique ability to manipulate electromagnetic waves in microwave and optical frequencies. These artificial sheet materials, which are usually composed of metallic patches or dielectric etchings in planar or multi-layer configurations with subwavelength thickness, have the advantages of light weight, ease of fabrication, and ability to control wave propagation both on the surface and in the surrounding free space. Recent progress in the field has been classified by application and reviewed in this article. Starting with the development of frequency-selective surfaces and metamaterials, the unique capabilities of different kinds of metasurfaces have been highlighted. Surface impedance can be varied and manipulated by patterning the metasurface unit cells, which has broad applications in surface wave absorbers and surface waveguides. They also enable beam shaping in both transmission and reflection. Another important application is to radiate in a leaky wave mode as an antenna. Other applications of metasurfaces include cloaking, polarizers, and modulators. The controllable surface refractive index provided by metasurfaces can also be applied to lenses. When active and non-linear components are added to traditional metasurfaces, exceptional tunability and switching ability are enabled. Finally, metasurfaces allow applications in new forms of imaging.

Reconfigurable impedance ground plane for broadband antenna systems

We introduce a reconfigurable impedance ground plane for broadband antenna array systems. The reconfigurable antenna elements need an adjustment of the local reflection phase to operate over a broadband frequency range. The proposed ground plane is constructed based on high impedance surfaces with RF MOSFETs populated in the gaps between the patches. The transistors play the role of RF switches to manipulate the surface topology by turning on/off the switches. In this paper, we numerically demonstrate the novel tunable ground plane and simulation results to show the extreme tuning range from 2 to 17 GHz by using ideal transistor switches, while maintaining the artificial magnetic conductor properties of the high impedance surface.

Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

We demonstrate a series of nonlinear, active, and tunable metasurfaces for a variety of electromagnetic applications. The metasurfaces have achieved a range of exotic properties by populating nonlinear or active circuit components on a periodically patterned metallic surface. The circuit components such as diodes, varactors, transistors, and other devices can be controlled manually, actively, or self-adaptively. This allows nonlinear metasurfaces to have active tuning, power-dependent behavior, self-focusing, reconfigurable surface topology, or frequency self-tuning capabilities. The power-dependent metasurfaces can be applied to active RF absorbers that only absorb high-power surface waves to prevent malfunction or damages to sensitive devices. The rectifier-based waveform-dependent metasurface absorber can be specifically designed to absorb either high power pulsed waves or continuous waves. The transistor-based surface wave metasurface absorber provides another degree of freedom in that it can be manually switched to tune the absorber, or it can be tuned using computer controlled feedback. The self-focusing effect has been demonstrated for the first time at RF frequencies to automatically collimate high-power surface waves. The reconfigurable and self-tuning metamaterial surfaces can be implemented to support a broadband reconfigurable antenna system or to adapt to a wide range of incoming frequencies. In this paper, the concepts of nonlinear and active tunable metasurfaces are discussed, including results of full-wave simulation analysis, EM/circuit co-simulation, and experimental results in waveguides, using a near-field scanner, as well as far-field measurements in an anechoic chamber.

Study of the electric field enhancement of high-impedance surfaces

High-impedance electromagnetic surfaces (HIS) are known for many applications including low-profile antenna design. However, in high power antenna designs using HIS, the air-breakdown between neighboring elements is a limiting factor. We show that the electric field in the gap between neighboring elements is independent of the resonance quality factor of the HIS. Instead, the average electric field inside the HIS cavities (and therefore the stored energy) scales with the resonance quality factor (Q).

Theoretical analysis of nonlinear surface wave absorbing metasurfaces.

In this paper, we provide a theoretical analysis and discussion of the fundamental principles of nonlinear surface wave absorbers, in which ideal diodes are used to rectify surface currents to produce nonlinear harmonic terms including DC, and higher order modes (2f0, and 4f0, …). Interestingly, we find rectification converts most of the power to DC that can be completely absorbed by resistance in the surface, leading to advantages of nonlinear absorbers over conventional linear surface wave absorbers in both bandwidth and attenuation. We demonstrate the full-wave rectification case, and diode-rectifier-based nonlinear absorbing metasurfaces possess obvious advantages and can exceed the performance of linear absorbers, which relates the bandwidth and attenuation rate to the substrate thickness. For nonlinear metasurfaces, even with very thin substrates (for instance 0.35 mm thickness which is λ0/143 for center frequency 6 GHz), we can potentially achieve more than 60% relative bandwidth, three times of that in linear metasurfaces. To visualize the practical working mechanism, the distributed nonlinear network using ideal diode model is presented, and the full-wave simulations are demonstrated with nonlinear advantages. Differences between the theoretical case and practical case are addressed as well.

Periodic structures for scalable high-power microwave transmitters

High-power microwave (HPM) sources play an important role in applications including RF acceleration, radar, and telecommunications. However, these applications are often limited by weight and the cost. A novel highly scalable two-dimensional high-power microwave source is proposed in this paper. The idea is to first charge the periodic resonating structure with a DC source. Then, using fast high-power switches, the DC source is replaced with a short-circuit leading to an oscillating state in the structure. These oscillations coherently radiate into the far-field, converting the stored DC energy into the desired microwave frequency.

Experimental study of the interaction between DC discharge microplasmas and CW lasers

A high power (~ 1W) continuous wave (CW) laser was focused on argon microplasma generated in the microgap between two electrodes with submillimeter diameters. Dependence of breakdown (V(BD)) and quench (V(Q)) voltages of microplasma to the laser power, wavelength, and spot location were studied as the gap size and pressure varied. It was observed that the laser-plasma interaction can only occur thermally through the electrodes. Also, the thermal effect of the laser was noticeable at relatively higher pressures (> 10Torr), and in most cases led to a decrease in V(BD), proportional to the pressure.