Gap-dependent chiral coupling in T-shaped gold nanodimers

The mutual non-orthogonal orientations of its horizontal and vertical bars make T-shaped gold nanodimers chiral. Because of the broken symmetry second-harmonic generation from the structure has different efficiencies for left- and right-hand circularly-polarized fundamental light. The chiral signature arises from the coupling between the bars. One would therefore assume that the chiral signature is largest when the gap size is very small, because then the coupling is presumably the strongest. Counter-intuitively, the measurement results show a very small chiral signature for the smallest gap. To explain the results, one needs to consider the distribution of the local field in the unit cell of the structure.