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