A new path

TL;DR: This work has demonstrated that metamaterials consisting of thin wires can behave like low-loss dielectrics at wavelengths much longer than the wire thickness, but barely longer than twice the unit cell, which has some interesting implications for controlling light propagation.

Abstract: n many photonic structures composed of two or more dielectrics, the absolute value of the refractive index contrast is critical to performance. Increasing the index contrast would therefore be extremely useful. Recent work on ultralow-refractive-index metamaterials (ULIMs) suggests a path toward this goal. Metamaterials, an extension of the concept of artificial dielectrics, typically consist of periodic structures of a guest material embedded in a host material. While homogeneous dielectrics derive their optical properties from the subnanometer-scale structure of their atoms, metamaterials derive their properties from the subwavelength-scale structure of their component materials. When the wavelength of the field interacting with the structure is much longer than the feature sizes and unit cell, the metamaterial can be treated as a homogeneous dielectric with macroscopic parameters like effective refractive index n eff. The proper choice of component materials and geometries can yield metamaterials with novel optical properties, which allow the metamaterials to control light in unconventional ways with potential applications in photonic integration. Traditional methods of deriving an effective refractive index apply in the long-wavelength limit. 1 We have demonstrated that metamaterials consisting of thin wires can behave like low-loss dielectrics at wavelengths much longer than the wire thickness, but barely longer than twice the unit cell (see figure 1). 2 Photonic crystals also have been assigned an effective refractive index based on their band structure at wavelengths only slightly longer than the features and unit cells. 3–4 Research on ultralow-refractive-index metamaterials focus-es on achieving an effective refractive index of less than unity at optical frequencies. For example, a metamaterial composed of a 2-D square array of thin silver wires that is embedded in an air host medium behaves on refraction, reflection, and transmission as a low-loss dielectric with the real part of the effective index below unity (0 < Re(n eff) < 1). This property has been verified at visible and near-IR (NIR) wavelengths. Such a material has some interesting implications. A beam incident on a planar interface between air (n = 1) and a ULIM would be refracted away from the normal, instead of refracted toward the normal as with most optical materials. If one were to build a plano-concave lens with a ULIM, the Ultralow-index metamaterials present new possibilities for controlling light propagation.