QUEST-related research led by QMUL has been published in the open-access journal Scientific Reports, part of the Nature Publishing Group. In this paper, one of the fundamental issues limiting current practical implementations of designs based on transformation optics is addressed: limited bandwidth.
This bandwidth limitation arises because most designs require extreme material property values, often ones not easily found in natural materials. Hence, these values are achieved through the use of metamaterials. Metamaterials are artificial materials with properties that depend on the geometry of the materials, as well as the properties of the constituent materials. Such materials are usually realised with arrays of specific metallic shapes, which produces the required material properties around specific resonant frequencies, with narrow bandwidths of operation.
An alternative approach is explored in the research performed at QMUL. It uses an “all-dielectric” approach to avoid the use of metamaterials and achieve broad operating bandwidths, as well as simplifying fabrication. The method uses a discrete co-ordinate transformation (DCT) numerical method to determine the new permittivity values in the transformed region, with the DCT method optimised to achieve quasi-conformal transformations. Conformal and quasi-conformal transformations result in non-magnetic materials (i.e., a permeability of 1), again helping to achieve greater bandwidths and simplifying fabrication.
In the Scientific Reports paper, the DCT method is used to design an all-dielectric planar hyperbolic lens that retains the same focusing properties of the original curved lens. The redesigned lens demonstrates operation with high directivity and low side lobe levels for an ultra-wide band of frequencies, spanning over three octaves.