Probing Graphene χ⁽²⁾ Using a Gold Photon Sieve

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 to unravel different elements of graphene second-order susceptibility χ⁽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 χ⁽²⁾ elements. The values obtained are |d₃₁ + d₃₃| ≤ 8.1 × 10³ pm²/V and |d₁₅| ≤ 1.4 × 10⁶ pm²/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.