Engineering the harmonic generation in graphene

In the field of nonlinear optics, graphene has shown astounding nonlinear properties, including the generation of harmonics across a wide frequency range. Harmonic generation (HG) is a frequency conversion process that serves as the basis for a number of modern disciplines including materials science and nanotechnology. This field also holds great promise in laser physics, where it could be applied to the processing of optical signals in information technology and in the development of coherent light sources. Graphene HG experiments demonstrate that it exhibits the largest nonlinear optical susceptibilities in comparison with all other known materials to date, providing the basis for the development of broadband, ultrafast optical devices based on graphene. This review focuses on presenting the latest achievements in engineering graphene's HG. Different strategies to tune and enhance the HG in graphene are discussed including electrical gating, interlayer twisting angle, graphene nonlinear metamaterials or metasurfaces, and the effect of plasmonic or dielectric nanostructures. We envision that this article may serve as a concise overview of the recent progress of engineering the HG in graphene and provides a compact reference for the young researchers entering into the field. HG in graphene is expected to continue growing in the near future, thereby providing a practical basis for its application.