How to make a light only emit speculars ?

Hello ! I'm trying to find a way to make so that my 3d mesh is lit only by specular light, and reflects no diffuse light at all. I know there is an option in Maya lights to separate diffuse and specular emmision, and to disable independently one or the other one. Is there a method to do the same in Unity ? Or maybe someone knows how to solve my problem in other way ?