2D and 3D structures periodic only in one direction

Dear MPB Users, Dear Steven

Is it possible to use MPB in order to study a 2D or 3D structure whose 
refractive index n(x,y,z) is periodic only in one direction 
n(x,y,z)=n(x,y,z+L) ?

Reading the manual, I noticed the parameter ensure-periodicity[boolean], 
but it seems to affect the periodicity in all the directions and I 
couldn't find any example of its use.

Thank very much,
Best regards
Paolo Bardella