This paper reports the first experimental investigation of the nonlinear optical properties of graphene flakes. The study shows that a single-layer graphene exhibits a strong third-order nonlinear optical response at near-infrared frequencies, with a nonlinear susceptibility of |χ³| ~ 10⁻⁷ esu, which is comparable to that of other strongly nonlinear materials like carbon nanotubes. This response is essentially dispersionless over the wavelength range of 760-840 nm, making it useful for high-contrast optical imaging of graphene flakes. The nonlinear response is exploited to achieve image contrasts several orders of magnitude higher than those obtained using linear microscopy. The nonlinear optical response of graphene was measured using four-wave mixing, a technique that involves the interaction of two pump laser beams with a sample to generate a third coherent beam. The study shows that the nonlinear emission is collected in the backward direction and detected using a photomultiplier tube or a spectrometer. The results indicate that the nonlinear response of graphene is independent of the wavelength of the incident light, which is attributed to the dispersion relation of graphene, where all photon energies in the four-wave mixing process can match electronic transitions. The study also compares the nonlinear optical response of graphene with that of other materials, such as gold films, and finds that the nonlinear susceptibility of graphene is comparable to that of carbon nanotubes. The results suggest that the frequency independence of the nonlinear response could be useful in graphene applications where a constant response is important, such as in nonlinear devices like bistable mirrors for mode-locking, optical limiters, and switchable elements. In conclusion, the study demonstrates that graphene exhibits a wavelength-independent, strongly nonlinear optical response in the near-infrared spectral region, described by a third-order susceptibility of |χ³| ~ 10⁻⁷ esu. Nonlinear measurements are shown to be a useful tool for visualizing graphene flakes on dielectric substrates. The study highlights the potential of graphene for optical experiments and suggests that further studies on a broad range of frequencies, including the low-energy (THz) range, could be beneficial.This paper reports the first experimental investigation of the nonlinear optical properties of graphene flakes. The study shows that a single-layer graphene exhibits a strong third-order nonlinear optical response at near-infrared frequencies, with a nonlinear susceptibility of |χ³| ~ 10⁻⁷ esu, which is comparable to that of other strongly nonlinear materials like carbon nanotubes. This response is essentially dispersionless over the wavelength range of 760-840 nm, making it useful for high-contrast optical imaging of graphene flakes. The nonlinear response is exploited to achieve image contrasts several orders of magnitude higher than those obtained using linear microscopy. The nonlinear optical response of graphene was measured using four-wave mixing, a technique that involves the interaction of two pump laser beams with a sample to generate a third coherent beam. The study shows that the nonlinear emission is collected in the backward direction and detected using a photomultiplier tube or a spectrometer. The results indicate that the nonlinear response of graphene is independent of the wavelength of the incident light, which is attributed to the dispersion relation of graphene, where all photon energies in the four-wave mixing process can match electronic transitions. The study also compares the nonlinear optical response of graphene with that of other materials, such as gold films, and finds that the nonlinear susceptibility of graphene is comparable to that of carbon nanotubes. The results suggest that the frequency independence of the nonlinear response could be useful in graphene applications where a constant response is important, such as in nonlinear devices like bistable mirrors for mode-locking, optical limiters, and switchable elements. In conclusion, the study demonstrates that graphene exhibits a wavelength-independent, strongly nonlinear optical response in the near-infrared spectral region, described by a third-order susceptibility of |χ³| ~ 10⁻⁷ esu. Nonlinear measurements are shown to be a useful tool for visualizing graphene flakes on dielectric substrates. The study highlights the potential of graphene for optical experiments and suggests that further studies on a broad range of frequencies, including the low-energy (THz) range, could be beneficial.