Third harmonic generation in graphene

We report the measurement of optical third harmonic generation from single-layer graphene and few-layer graphite flakes produced by exfoliation. In the measurements, femtosecond near-infrared laser pulses were used to irradiate the samples. The emission observed scales with the cube of the intensity of the incident near-infrared pulse and with one third of the incident wavelength - both are clear evidences of third harmonic generation. We deduced an effective third-order susceptibility for single layer graphene to be on the order of 10$^{-16}$ m$^2$/V$^2$. By measuring a set of flakes with different numbers of atomic layers, we found that for a few layers, the emission scales with the square of the number of atomic layers.