Tunable Absorbers Based on Mushroom- Type Metasurfaces

So far our analysis has focused on demonstrating how to manipulate the bandgap of mushroom-type metasurfaces through nonuniform capacitive loadings. As clarified earlier in this chapter, periodic structures based on the mushroom metasurface, apart from EBGs, can also be employed to realize AMCs. However, it is important to emphasize once again that the underlying physics that govern the two functionalities are fundamentally different. This is simply because EBGs are responsible for controlling the surface wave propagating along the metasurface. In contrast, AMCs are responsible for controlling the properties of the electromagnetic field scattered off of them when they are illuminated by an impinging electromagnetic wave. In this section, we borrow elements from the EBG design methodology presented in the previous sections, and in particular the port reduction technique as summarized by Eq. (4.6), and we apply them toward the development of tunable AMCs and elementary tunable microwave absorbers. Note here that AMCs and absorbers are inherently related since once a loss mechanism is introduced in an AMC, it becomes an absorbing device.

Although research on microwave absorbers dates back to World War II, it still remains a very active research area due to its applicability in the development of stealth technology, and due to the specifications of modern telecommunication systems that require to isolate multifunctional and highly sensitive antenna platforms from either intentional or unintentional electromagnetic radiation/interference. The design of an electromagnetic absorber is essentially a matching problem where it is desired to match the input impedance of the absorbing device to that of free space, so that an incident wave can be completely or partially absorbed by a lossy medium [50]. An absorbing device usually comprises a multilayer structure where frequency-selective surfaces (FSS) are sandwiched in between its layers. The number, material constitution, and height of the layers as well as the filtering response of the FSSs are factors that determine the absorption performance of the device. In principle, it is desirable to devise an as electrically thin as possible absorber, which also exhibits wide absorption bandwidth. However, in reality, these two design goals contradict each other. For this reason, usually the designer needs to favor one of the two features and then attempt to improve the other by custom engineering the material constitution and the geometrical characteristics of the device.

A very popular absorber design is based on the periodic arrangement of the mushroom-type unit cell [51, 52]. It should be mentioned here again that the via of the mushroom unit cell is responsible for the propagation of backward guided waves across the metasurface, but this is irrelevant to the absorption mechanism. The absorption mechanism, as will be demonstrated later in this section, is solely dependent on the loss mechanism of the device and on the AMC properties of the shorted metasurface, where in this case the metasurface is simply a 2D periodic arrangement of metallic patches. These properties are essentially determined by the zero crossings of the reflection coefficient phase of the AMC. In principle, the only effect that the vias have on the performance of mushroom-type unit cell absorbers is to perturb the Q factor of the shorted spacer, and therefore they can be omitted.

The popularity of this type of absorber stems from their structural simplicity as well as from their ultra-thin profile. This type of absorber also has disadvantages. First, when a loss mechanism is introduced by either loading some resistors across their metallic patches or applying a uniform Ohmic sheet on their FSS, reasonable absorption can be achieved only for a very narrow frequency bandwidth, centered around the frequency where the phase of the corresponding AMC’s reflection coefficient phase goes to zero. Second, mushroom-based absorbers lack the ability to reconfigure their response since none of their structural components can be electrically tuned. Consequently, it becomes evident that if wider or tunable/reconfigurable absorption performance is required, then the absorber needs to be appropriately modified.

One way to introduce the aforementioned capabilities in a mushroom structure is by judiciously introducing lumped elements across the metasurface [53-55]. Note here that similar to the EBG configurations examined previously, a lumped element loaded absorber can also be described in terms of the 5-matrix of a multiport network, and therefore the optimization procedure and design methodology presented in the previous sections can be directly applied for the synthesis of absorbers as well. Given this strategy, in the following subsections, we present a tunable narrowband, a multiband, and a broadband absorber where all three designs are based on different flavors of a lumped element loaded metallic square patch metasurface.

 
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