  Perfectly transparent metamaterial structures of arbitrary shapes,
constructed from coordinate stretching and contractions, are presented.
Coordinate stretching has been used for 2 decades in perfectly matched layers
(PMLs) to electromagnetically simulate infinite domains in numerical
techniques, but this concept is applied here for the first time to realize a
physical transmission medium. The transparent medium does not scatter
electromagnetic waves, i.e. it is reflection-less for all incidence angles and
all excitation frequencies. It may be implemented in the form of metasurfaces
and will clearly find a myriad of applications if it can be efficiently
manufactured.
