what is meant by a noncentrosymmetric crystall?

Question

yes they exhibit nonlinear optical effects....how does it happen/

Answer 1

One must first know what a antisymmetric crystal is.

When the symmetry is not there, the materials response to electric fields (of photons) depends on orientation - hence you get things like birefringence.

Answer 2

This is a tough one to research, but here's an article that will get you up to speed:

Addendum: Bubsir has found a good link