Versatile optical devices are designed from anisotropic and gradient materials based on transformation optics, which are usually hard to realize. Here, by utilizing conformal equivalence, we demonstrate that a flat gradient index optical thin-film waveguide is equivalent to a curved homogenous optical thin-film waveguide for light rays. Such a relationship provides us an alternative method to design optical devices with either an inhomogeneous medium or equivalently curved surfaces made from only a homogeneous material, leading to applications on on-chip optical devices. Moreover, we provide a prototype of the curved optical thin-film waveguide without rotational symmetry, which can serve as a concave lens. Our work gives a conformal landscape of a two-dimensional gradient refractive-index profile from the geometric optics perspective.

• Received 9 December 2019
• Revised 24 March 2020
• Accepted 15 April 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.054007