Subject: How to control optical density and hence the refractive index of a materia

How can one change the optical density of a material? I understand that light 
travels within a material by being absorbed by the atom/molecule and then re-
emitted and then travels through space to until it collides with the next 
particle. However, there should be a time difference in the rate of re-emission 
of light. What are the factors affecting it and how can we control the speed at 
which the light pass through the material? Thank you for your help. :)

Re: How to control optical density and hence the refractive index of a materia