Dear Md Shamiul Fahad, 

In measuring the amount of light that was reflected back, recent researches found it there is a reflection of a particular pulse. More specifically, the material's optical gain property occurs because as the pump laser pulse strikes the graphene, its electrons become excited with more charge carriers winding up in the Dirac cone than in the lower cone.

Thank you Dr. Liangxu. Should there be graphene on Cu foil after CVD, (which means graphene layer on top of Cu), on that case, can graphene still reflect light?

Hi Shamiul,

In fact monolayer graphene is not truly transparent, it has a very broadband absorption of about 2.3% per pass. But like semiconductors, this absorption can be saturated by a strong pulse. Graphene have been used on top of Distributed Bragg Reflector to make a fast saturable absorber. See for example the reference :

https://www.osapublishing.org/oe/abstract.cfm?URI=oe-21-25-31548

I hope this helps.

Hi Laurain, thanks for the comment. That was really helpful.