▎ 摘　　要

Using an optical parametric oscillation laser as the excitation source, the dependence of the saturable absorption of multiple-layer graphene upon photon energy is investigated and, in all cases, the saturation intensity is lower for lower excitation photon energy. This result experimentally proves the hourglass shape of the energy band of graphene, which is a well-known theoretical deduction from first principles. The modulation depth increases from 1.5% to 5.1% when the layer number increases from a monolayer to 5-7 layers and, at the same time, the saturation intensity decreases with increasing number of layers. The results demonstrate that, as a saturable absorber of a pulsed laser, graphene can more easily achieve optical modulation in the low energy region, i.e., the infrared waveband. (C) 2015 AIP Publishing LLC.