▎ 摘　　要

We present an experimental observation of nonlinear up- and down-converted optical luminescence of graphene and thin graphite subject to picosecond infrared laser pulses. We show that the excitation yields to a high-density electron-hole plasma in graphene. It is further shown that the excited charge carriers can efficiently exchange energy due to scattering in momentum space. The recombination of the resulting nonequilibrium electron-hole pairs yields to the observed white-light luminescence. Due to the scattering mechanism, the power dependence of the luminescence is quadratic until it saturates for higher laser power. Studying the luminescence intensity as a function of layer thickness gives further insight into its nature and provides a new tool for substrate independent thickness determination of multilayer flakes.