Spatially graded index / graded material properties

Hi,

can anyone of you tell me if it is possible to define a graded index in comsol?
It seems as if sth. like this has been done in www.comsol.se/academic/papers/2871/
But how? Is it simply possible to use the coordinates as variables in the definition of global parameters for the dielectric properties?

This would mean, is just something like
parameter n
definition: factor * x
possible to achieve (simplified) a linear gradient of n over the geometric x dimension?
I would be happy about help.

Best regards

Arian