Tunable metamaterials: the key step to practical application

Over the last decade, metamaterials—artificial materials deliberately designed to have properties not found in nature—have constituted a research frontier in electrodynamics, solid-state physics, and materials science that is drawing wide interest in applied fields ranging from microwave engineering to optics. Representing a next level up from conventional materials in how they are structured, metamaterials offer an unbeatable range of unusual properties, including negative refraction (basically, bending light the wrong way) and all its consequences.¹ Their chief attraction, however, is that not only are we free to design a material with desired features. We can also assemble it so the response can be tuned during operation.

We initially proposed this idea² in 2004 in the context of nonlinear metamaterials.^3,4 We suggested it might be possible to remotely control how an incident electromagnetic wave interacts with a metamaterial, i.e., whether it is transmitted, reflected, or absorbed. This control was subsequently demonstrated experimentally.^5,6 An alternative is to exploit the reconfigurability of liquid crystals within a metamaterial structure.^7,8 All these approaches, however, require complicated engineering at the level of individual atoms, or elements, of metamaterial and become increasingly challenging at higher frequencies because the structural details must be smaller. Here, we describe a new, straightforward way around these difficulties: structural tunability.⁹

This concept has a clear analogy in solid-state physics and crystallography. Researchers in these disciplines know very well that the overall properties of a material not only are determined by the nature of the constituent atoms but also depend dramatically on the lattice structure. Consider how distinct graphite, diamond, and graphene are. Yet each is composed of carbon atoms, simply differently arranged. This effect is governed by mutual interaction of the atoms in a lattice, which is defined by the lattice geometry. In natural materials the lattice normally is fixed. In a very few substances, it can be slightly tweaked to enable phenomena such as piezoelectricity. With metamaterials, however, we can design the structure so the lattice is easily adjustable ‘on the fly,’ making it possible to reconfigure a metamaterial device during operation.

Figure 1. Schematic illustration of a lateral lattice shift (δa) in a metamaterial with lattice constraints a and b. The inset shows an array of five single-split resonators. Stacking such boards together produces a tunable metamaterial.

A few years ago, we showed¹⁰ that mutual interaction between the elements of these materials is essential, and must be taken into account when calculating macroscopic characteristics (such as permittivity or permeability) from the microstructure. Furthermore, the ability to densely pack the meta-atoms results in very strong interactions. These advantages underpin the concept and implementation of direct structural tunability.⁹

For a practical demonstration, we chose an anisotropic metamaterial composed of uniaxial single-split resonators (see Figure 1, inset), which provides an artificial magnetic response through a resonant circular current in each element. In a lattice, such elements are magnetically coupled to each other, and we can calculate their interaction in terms of mutual inductance. This strong coupling changes the overall response remarkably. As a consequence, the resonance of the effective permeability can be finely controlled by adjusting the lattice geometry, opening the way to tunable transmission.

Figure 2. Change in the relative resonance frequency (ω/ω_o) achieved through the lateral lattice shift. (Comparison of theoretical results and experimental data.)

Figure 3. Experimental transmission (S₂₁) in a waveguide with a tuned metamaterial slab. The curves from left to right correspond to the increasing lattice shift, δa, as indicated.

We found that the optimal tuning mechanism consists in gradually shifting half of the layers of the resonators in the lateral direction (see Figure 1). Overall mutual inductance in the lattice decreases with shift, leading to a steady increase in the resonant frequency (see Figure 2, solid curve). In a system of finite size, the actual effect is somewhat stronger (see Figure 2, squares) than that calculated for an ‘infinite’ continuous medium because the edge elements are expelled from the structure. Finally, our experiments at the Nonlinear Physics Centre in Canberra, Australia, showed excellent results (see Figure 3), with a stronger effect than predicted (see Figure 2, stars). The explanation is the additional capacitive coupling between the adjacent elements, which positively contributes to the overall tuning process.

In summary, we have developed a novel, efficient approach to directly tuning metamaterial properties through continuous adjustment of the lattice structure.⁹ Our approach makes a tremendous resonance frequency shift (up to 30%) easily realizable, suggesting a number of important applications in areas such as sensing, switching, and filtering. At the same time, the implementation is simple and cost-efficient, and the design is readily scalable in a wide range of frequencies. We are currently looking into further possibilities offered by this approach and will report results in the near future.