Giant Two-Photon Absorption in Bilayer Graphene

We present a quantum perturbation theory on two-photon absorption (2PA) in monolayer and bilayer graphene which is Bernal-stacked. The theory shows that 2PA is significantly greater in bilayer graphene than monolayer graphene in the visible and infrared spectrum (up to 3 mu m) with a resonant 2PA coefficient of up to similar to 0.2 cm/W located at half of the bandgap energy, gamma(1) = 0.4 eV. In the visible and terahertz region, 2PA exhibits a light frequency dependence of omega(-3) in bilayer graphene, while it is proportional to omega(-4) for monolayer graphene at all photon energies. Within the same order of magnitude, the 2PA theory is in agreement with our Z-scan measurements on high-quality epitaxial bilayer graphene deposited on SiC substrate at light wavelength of 780 and 1100 nm.