We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 10 −16 m 2 /V 2 , which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths. By measuring a set of flakes with different numbers of atomic layers, we find that the emission scales with the square of the number of atomic layers, which suggests that the susceptibility of graphene is independent of layer number, at least for a few layers.Graphene is a monolayer of carbon atoms arranged in a hexagonal two-dimensional lattice. It has a linear dispersion relationship between energy, E, and wavenumber, k, E(k) = ±hv F k, where the Fermi velocity v F ≈ 10 6 m/s [see Fig. 1 -3]. Since the material became readily available less than a decade ago [4], it has been the subject of extensive investigations [e.g., see Refs. 5 and 6 and references therein]. Graphene exhibits a number of unusual and remarkable transport properties that make it attractive for nano-electronic applications [7][8][9], including high mobilities [4,5,9, 10] and nearly-ballistic transport at room temperature [5,6]; however, it is the optical properties of graphene that are of primary interest here.The linear absorbance of graphene is flat and approximately 2.3% across the entire visible spectrum [11,12]. Thus, graphene can be considered to be both highly absorbing and/or highly transmitting, depending upon one's point of view or application. Since a negligible fraction (< 0.1%) is reflected [12], 97.7% of the incident light is transmitted. In this sense the sample is certainly transparent. On the other hand, if one were to assign an effective absorption coefficient, α, to a monolayer of thickness 0.33 nm (even though it may be questionable to use macroscopic parameters, such as α, for such thin samples), it would be very large (α ≈ 7 × 10 5 /cm). As suggested by this large effective absorption coefficient, graphene interacts strongly with light. The strong broadband nature of the interaction of light with graphene is consistent with its linear bandstructure, where interband transitions of roughly equal strength are available throughout the visible. The combination of broadband transparency and the high mobilities mentioned earlier makes graphene a promising candidate for use as a transparent conductor [13][14][15][16] in photovoltaic devices [17,18] and touch screens [14]. The high broadband absorption (i.e., optical conductivity) and high carrier mobility suggest that graphene may find applications in a range of optically-controlled transport devices, such as broadband and ultrafast photodetectors [19][20][21][22][23][24][25] The s...