Optical absorption in graphene. (a) Optical absorption of approximately 2.3% for pristine graphene from ref. . (Inset) Graphene crystallites were placed over a metal with several apertures. (b) For vertical incident light, an optical absorption of approximately 2.3% can be achieved by single-layer graphene. (c) By integrating graphene along with a waveguide (i.e., light transmits horizontally through graphene), large light-graphene interaction and higher absorption can be achieved. Reproduced from ref. .