Reseachers at QMUL have published work examining numerical coordinate transformations to produce a flat absorber equivalent to the pyramidal abosrbers in common use.
In the image above, the left hand shows the real part of the equivalent permittivity map for a single pyramid unit cell, whilst the right hand image shows the imaginary part of the equivalent permittivity map.
One of the novelties of this work was the examination of repeated transformations to achieve the objective, to understand if this provided greater control of the trade-off between material parameters and performance.