What are Metamaterials?
Materials and Fields - In perhaps the broadest definition, metamaterials are structures that allow us to engineer the interaction of fields with matter. For those neither well-acquainted with fields nor metamaterials, an analogy may help to introduce the concepts. Imagine standing in a meadow filled with tall grass on a breezy day. As the wind blows, the tall grass sways back and forth, responding to the strength and direction of the wind. A physicist or mathematician might describe the wind as a pressure field—a continuous distribution of pressure that exists everywhere throughout space. The wind is invisible—a very dilute gas of molecules—but you can sense its presence as it pushes against you or other objects, moving them around. Likewise, each blade of grass in the meadow feels a force caused by the wind, and moves accordingly. As you watch the collection of tall grass oscillating in the meadow, driven by the wind, you are witnessing a field-matter interaction; the meadow can be thought of as a material; the wind, a field.
The picture of grass swaying in a breeze can help us visualize other field-matter interactions, such as a beam of light passing through a shard of glass. Light is an electromagnetic field, consisting of electric and magnetic fields that travel throughout space and can be likened to the breeze blowing across the grassy meadow. Electrons within the glass—or any other material—are analogous to the grass, driven by the electric and magnetic fields rather than the pressure from the wind. Were we able to see electrons, we’d see them oscillating and jiggling, their motion driven by the fields swirling around them.
Once we understand the field-matter interaction, we can do useful things with it. For example, a windmill extracts energy from the wind by converting it to mechanical motion and eventually to other forms of energy that can be stored and later accessed as needed. Many other types of fields can also be controlled or harnessed by engineering their interaction with materials. Materials are the means by which we can take hold of and manipulate what is otherwise invisible. Metamaterials take things a step further, allowing us to create the exact kind of material response we want for some particular application, rather than relying randomly on the properties of conventional materials and compounds. Just as we fashion windmills to produce power from wind rather than trying to use grass, we can structure metamaterials to control fields in ways just not possible with conventional materials.
Our group and other groups in our Department study all aspects of metamaterials and their impact in many areas of science and engineering. Taking the broadest view of the field-matter interaction, we explore metamaterials relevant to light, microwaves, terahertz radiation, acoustics, fluid flow, and many other phenomena. On this site you will find information about metamaterials—what they are, their history, why they’re interesting, and how they are becoming increasingly useful.
Metamaterials: A Definition - Metamaterials are artificially structured materials used to control and manipulate light, sound, and many other physical phenomena. The properties of metamaterials are derived both from the inherent properties of their constituent materials, as well as from the geometrical arrangement of those materials. Though there are many structures that qualify as metamaterials, the most common is that of an arrangement of elements whose size and spacing is much smaller relative to the scale of spatial variation of the exciting field. In this limit, the responses of the individual elements, as well as their interactions, can often be incorporated (or homogenized) into continuous, effective material parameters; the collection of discrete elements is thus replaced conceptually by a hypothetical continuous material.
The advantage of this homogenization procedure is that sophisticated and complex materials can be engineered—sometimes with properties beyond what Nature provides—in a simple and intuitive manner. The metamaterial elements can be thought of as the molecules of the artificial material, and can be designed using a variety of numerical methods that have been developed over the years. Metamaterials provide a path to multiscale design, in that the properties of the metamaterial elements can be first determined, with an equivalent, hypothetical continuous material used for subsequent system design.
It is difficult to arrive at a strict and unambiguous definition of a metamaterial that wouldn’t exclude many types of structures that rightfully should be considered part of the metamaterials field. For this reason, the definition above is purposefully somewhat vague. What is important, though, is that a metamaterial is not a tangible thing; rather, a metamaterial results from a design approach that satisfies the spirit of the above definition. The metamaterial concept has influenced the way we think about materials and device design, and in many cases has allowed us to identify solutions to problems in efficient and novel ways.
Origins - The expansive metamaterial concept was developed in 1999 by Rodger Walser, then at the University of Texas in Austin, who suggested that carefully constructed composite materials could achieve physical properties well beyond their constituent components. The idea was not without precedent: Fiber composites—mixtures of carbon or glass fiber and polymers—had existed for decades prior, and were widely used for creating strong yet lightweight structures. Still, Walser and many others believed there was much more to the concept that had not been fully exploited, especially in areas outside of structural engineering. Indeed, as the metamaterials community grew and started avidly exploring the concept, a wealth of new physical paradigms and engineering approaches were quickly discovered. So profound were these discoveries that some of our most basic physical concepts were challenged. In fact, the advent of metamaterials prompted a re-examination of underlying concepts in the field of optics as well as in many other fields. To this day, the metamaterials concept continues to inspire and influence thought across many disciplines of science and engineering.