Graphene optical absorption not what it was thought to be

Graphene has immense potential for improving photonic and optoelectronic devices such as lasers, solar cells, LEDs, and photodetectors, which are cornerstones of the optoelectronic industry that includes telecommunications. Advantages of graphene over traditional materials include its transparency, flexibility, mechanical strength and high thermal and electrical conduction. When constructing optoelectronic devices, graphene is integrated with silicon, III-V semiconductors, or other industry materials. The optical performance of such multilayer devices depends on the optical properties of graphene, such as its transparency, reflectivity, or optical absorption. However, to date, the absorption of a single layer of graphene has always been taken as 2.3%, a value that has been calculated and measured for a freestanding graphene layer with air on both sides. Now researchers have found that this value can significantly differ from its nominal value if the graphene lies on top of semiconducting materials. The finding will affect all research and applications of optics of graphene.

Researchers from Graphenea joined efforts with partners from Instituto de Energia Solar of the Polytechnical University in Madrid and the Adolphe Merkle Institute of the University of Fribourg in Switzerland, having published the results in ACS Photonics earlier this month. Graphenea’s CVD graphene was transferred onto a III-V semiconductor structure, in this case the widely used AlInP, which was made atop a transparent substrate. To check the optical absorption of graphene, samples with one, two and three layers of graphene were made, as well as those without any graphene at all. Optical reflectance and transmittance were carefully measured, and since the expected differences between the samples were very small, the results were compared to numerical simulations.

In consistency with earlier results on freestanding graphene sheets, it was found that the absorption increases with the number of graphene layers. However, the exact percentage of light absorbed does not match the freestanding case if there is a substrate under the graphene. Also, the absorption differs across the visible and near infrared part of the optical spectrum, and, strikingly, the absorption of graphene varies with the thickness of the underlying AlInP layer. These findings are of extreme importance for the design and implementation of optoelectronic and photonics devices that include graphene.