Transformation optics with Fabry-P\'erot resonances

Transformation optics is a powerful tool to design various novel devices, such as invisibility cloak. Fantastic effects from this technique are usually accompanied with singular mappings, resulting in challenging implementations and narrow bands of working frequencies. Here in this article, we find that Fabry-P\'erot resonances can be used to design various transformation optical devices that are not only easy to realize but also can work well for a set of resonant frequencies (multiple frequencies). As an example, we fabricate a prototype for a cylindrical concentrator for microwaves.