A thin engineered surface has been developed that can protect sensitive electronic systems from strong signal interference, allowing them to communicate effectively with external antennas.
Conventional microwave absorbers have several uses, including in controlling unwanted signal reflections from antennas and radars, and in protecting sensitive communications electronics from strong interfering signals. Typically, these absorbers are made from a host material loaded with conducting iron- or carbon-based particles to absorb microwave energy and convert it into heat. A common feature of these absorbers is that the percentage of microwave power that is absorbed does not depend on the power of the incoming waves. Therefore, such devices cannot differentiate between strong signals, which can be harmful to communications electronics, and the weak signals that are needed for wireless communications with antennas. Writing in Physical Review Letters, Wakatsuchi et al.1 report an engineered surface (metasurface) that can absorb high-power microwave pulses but allow weaker signals to propagate.
What enables Wakatsuchi and colleagues' metasurface to distinguish low- from high-power signals is the use of semiconductor diodes in the structure. A diode is a basic electronic building block that conducts high-power signals but cannot respond to low-power ones. Therefore, a diode can be thought of as a switch that is 'on' for strong signals but remains 'off' for weak signals. Such behaviour is nonlinear because the output power is not proportional to the input power. This is what enabled the authors to make such a power-dependent microwave absorber. Specifically, the team used a periodic array of copper patches printed on a thin dielectric (insulating) substrate backed by a copper ground plate. The array's unit cell consists of a single patch and six electronic components — four diodes, a resistor and a capacitor — which are soldered between patches (Fig. 1a, b). The overall thickness of the metasurface absorber is just 1.52 millimetres, whereas the spatial period of the array is 18 mm.
Now, the reader may wonder why the authors used four diodes, and what the role is of the capacitor and resistor in each unit cell. Well, the answer is that the authors' metasurface absorber is more subtle and interesting than my description so far. A single diode has asymmetric conductivity even when it is switched on: it conducts well in one direction but poorly in the reverse. Ideally, for an absorber application — in which as much microwave energy should be converted into heat as possible — the absorber should conduct well in both directions when the power level is above a certain threshold. The use of four diodes does exactly this. It is a common topology used in electronics to make devices known as full-wave rectifiers. These have universal use; for example, they are common in power supplies that convert a.c. (alternating current) power to d.c. (direct constant current) to charge laptops and everyday consumer electronics. With signals being conducted in both directions, the metasurface absorber becomes more efficient and converts most of the incident energy into heat. And this last statement explains the presence of the resistor in the unit cell: the energy is eventually absorbed by the resistor, which naturally converts electrical energy into heat. But why involve a capacitor in the unit cell?
A capacitor is a simple device (two parallel conducting plates with a dielectric in between) that stores electrical energy. The rate at which this storage takes place is controlled by a quantity known as the RC time constant, which has units of time. This quantity endows the metasurface absorber with one of its most subtle and striking features: pulse-shape-dependent absorption. Energy is stored in the capacitor when a pulse impinges on the absorber. This energy is then dissipated in the resistor (the capacitor is discharged through the resistor) in the time between two successive pulses. In this way, the amount of absorption depends not only on the incoming power level of the pulse, but also on its shape. This is the reason that the title of Wakatsuchi and colleagues' paper is “Waveform-dependent absorbing metasurface”. Shorter pulses lead to high absorption, whereas longer pulses are not absorbed well (Fig. 1c).
To put the results in context, in the past three years there has been a renewed interest in the field of metasurfaces. However, most of the metasurfaces described so far have linear behaviour (the input and output signals are proportional to each other) and do not contain electronic devices such as diodes. For example, a metasurface has been designed2 to refract light by controlling the phase shift (delay) that the light undergoes as it propagates. Another example is a surface3 with a tailored absorption achieved using antennas made of metal nanoparticles, the absorptivity of which does not depend on the incident power. Moreover, a passive metasurface — one that consumes but does not produce energy — has been engineered4 to make thin cloaks for small dielectric cylinders, and a more general thin active cloak was reported last year5. However, Wakatsuchi and colleagues' metasurface is unique because its absorption performance is nonlinearly dependent on the shape and power level of the incoming wave.
The authors' waveform-dependent absorber has applications in several disciplines. For example, it could be applied to the skin of military vehicles or aircraft to protect sensitive electronics from strong electromagnetic-pulse threats, while allowing the electronics to communicate with external antennas. Moreover, one could imagine applying these absorbers to protect computer-network electronics such as those in data centres from strong interfering signals while allowing the electronics to operate properly. And one may foresee sensor or signalling applications based on the recognition of the width and power level of incoming pulses. In the authors' words, these metasurface absorbers could potentially create “new kinds of microwave technologies and applications”.
About this article
IEEE Antennas and Wireless Propagation Letters (2016)