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Nonlinear second harmonic optical activity of graphene covering a gold photonsieve was determined for different polarizations. The photon sieve consists ofa subwavelength gold nanohole array placed on glass. It combines the benefitsof efficient light trapping and surface plasmon propagation in order to unraveldifferent elements of graphene second-order susceptibility \chi(2). Thoseelements efficiently contribute to second harmonic generation. In fact, thegraphene- coated photon sieve produces a second harmonic intensity at least twoorders of magnitude higher compared with a bare, flat gold layer and an orderof magnitude coming from the plasmonic effect of the photon sieve; theremaining enhancement arises from the graphene layer itself. The measuredsecond harmonic generation yield, supplemented by semianalytical computations,provides an original method to constrain the graphene \chi(2) elements. Thevalues obtained are |d_{31} + d_{33}| < 8.1 x 10^3 pm^2/V and |d_{15}| < 1.4 x10^6 pm^2/V for a second harmonic signal at 780 nm. This original method can beapplied to any kind of 2D materials covering such a plasmonic structure.

Probing graphene \chi(2) using a gold photon sieve