Nonlinear metamaterials: a new degree of freedom

Abstract

Novel types of microstructured materials exhibit electromagnetic properties not found in natural matter. In particular, composite materials created by arrays of conducting wires and splitring resonators can exhibit negative refraction.1 These so-called ‘left-handed metamaterials’ were first mentioned as a theoretical curiosity 40 years ago, and although actual applications have not yet been realised, nonlinear metamaterials may enable manipulation of electromagnetic waves in previously unimaginable ways. They also have vast potential for future optical applications such as more efficient frequency converters, power limiters, and parametric amplifiers. It is well known that a material’s response to electromagnetic radiation can be characterized by its magnetic permeability and electric permittivity. The product of these two physical characteristics defines the refractive index that measures how fast the material transmits light and how light is bent on entering thematerial: the higher the refractive index, the slower the propagation and the stronger the deflection. Metamaterials allow us to access even negative values of the refractive index. Importantly, they can also be designed with a spatially varying index of refraction. This is prerequisite for creating so-called electromagnetic ‘invisibility cloaks,’ the first of which—for microwaves—has already been designed.2 We have added a degree of freedom for metamaterial design by showing how the properties of a section of the material can be externally tuned by either applying an external constant magnetic or electric field,3, 4 or application of DC biasing to embedded electronic components.5 We go further and suggest the concept of nonlinear metamaterials, whose properties are changed by electromagnetic waves. As an example, the material can be transparent for weak electromagnetic waves yet become opaque once the input power is increased. Nonlinear effects are uniquely useful in optics and telecommunications. This suggests that nonlinear metamaterials can offer very unusual functionalities. Figure 1. Structure of nonlinear tunable metamaterial. Inset shows a close-up view of a split-ring resonator with variable-capacity diode. Each resonator measures about 1cm.