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 rather than their chemistry or band structure. 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.
Major Research Areas
The spectral response of metamaterials are very sensitive to the local environment due to the concentration of the incident electromagnetic fields into a reduced volume. A greatly enhanced electric field will exhibit pronounced sensitivity to changes in the surrounding dielectric properties, including refractive index, conductivity, and coupling strength. As a result, the dielectric change of local materials is equivalently enhanced by metamaterial composites represented by the change of effective parameters e(w) or m(w). We have extensively utilized this sensitivity to make tunable metamaterials wherein the electromagnetic response is altered by changing the local environment by, for example, changing the carrier concentration of the material in the gaps using light or an applied voltage. This has enabled us to create numerous metamaterial devices. We use metamaterials hybridized with a variety of technologies and natural materials for our tunable devices including:
- Liquid crystals
The operating principle of the perfect absorber lies in the ability of metamaterials to yield an exact and designed electric e and magnetic m response. A common configuration for such a metamaterial consists of two distinct metallic elements: a patterned layer providing an electrical resonance and a ground plane. The magnetic response is created through a coupling between both the electric resonator and the ground plane, generating anti-parallel currents resulting in resonant m(w) response. The magnetic resonance can therefore be tuned independently of the electric resonator by changing the geometry of the cut wire and the distance between elements. By tuning each of the resonances it is possible to match the impedance Z(w) to free space, i.e. e~m à Z=Z0 and minimize the reflectance at a specific frequency. The ground plane is fabricated to be significantly thicker than the penetration depth of the incident electromagnetic waves at the operational frequency, thus resulting in zero transmission. It is useful to highlight several key advantages of metamaterial perfect absorbers, in the context of detectors and imaging systems.