Near - zero refractive index photonics

The control and manipulation of light on the nanoscale — the primary aim of nanophotonics — is of fundamental scientific interest and plays a key role in telecommunication technologies and energy management. Yet, because light–matter interactions are usually weak and hard to confine, they often need to be assisted by the use of suitably designed macroscopic media. For instance, the use of carefully engineered metamaterial structures1–3 empowers a finer control of light including bending, focusing, filtering, or even its trapping and storage, as well as the realization of all-optical information processing tasks. Near-zero refractive index photonics — the study of light–matter interactions in the presence of structures with near-zero parameters, that is, continuous media or artificial electromagnetic materials in which one or more of the constitutive parameters are near-zero1,4–6 (for example, relative permittivity or relative permeability) — exhibits a number of unique features that differentiate it from other materialinspired approaches. In turn, it enables not only unprecedented light– matter interactions, but also the exploration of qualitatively different wave dynamics. According to their predominant electromagnetic response, structures with near-zero parameters at a given frequency can be classified as epsilon-near-zero (ENZ), ε ≈ 0, mu-near-zero (MNZ), μ ≈ 0, and epsilon-and-mu-near-zero (EMNZ), μ ≈ 0 and ε ≈ 0 media. All aforementioned classes exhibit a near-zero index of refraction п = √ με — ≈ 0 at the frequency of interest and can be jointly addressed as zero-index media. A representative sample of different physical realizations of structures exhibiting near-zero parameters is shown in Box. 1. By simple inspection of time-harmonic source-free Maxwell curl equations, ∇ × E = iωμH and ∇ × H = −iωεE, for the electric, E, and magnetic, H, fields at radian frequency ω, it is clear that near-zero constitutive parameters (ε ≈ 0 and/or μ ≈ 0) result in a decoupling of electricity and magnetism, even at a non-zero frequency1,4–6. This effect is also accompanied by an enlargement — that is, ‘stretching’— of the wavelength, schematically depicted in Fig. 1a, and thus a decoupling of spatial (wavelength) and temporal (frequency) field variations. Consequently, the phase distribution of electric and magnetic fields is necessarily nearly constant in a medium with near-zero permittivity and/or permeability. We emphasize that, far from being a theoretical peculiarity, the connection between wavelength and frequency has a deep technological impact restricting, for example, the size of a device operating at a given frequency and/or the maximal resolution of an imaging device. Thus, ‘loosening’ the connection between frequency and wavelength in structures with near-zero parameters gives us access to field dynamics relevant from both a Near-zero refractive index photonics