SP wavelength λ$_{SPP}$ ≃ 760 nm (for an excitation at λ ≃ 780 nm). We recorded optical transmission spectra at normal incidence with unpolarized light for both isolated structures (Fig. 1). As it can be seen, both enantiomers behave like resonant antennas with quasi identical transmission properties. This resonant behaviour is a direct indication of the SP excitation by the grating similarly to what is observed for circular antennas [24]. To observe and fully characterize the optical signature of planar chirality we perform a full polarization tomography [25, 26] with the aim of determining the 4 × 4 Mueller matrix M associated with each enantiomer. Experimental results M $_{L}$exp. , and M $_{R}$exp. respec- tively obtained for left and right handed spirals are given in appendixes A and B. Here the important point is that the degree of purity F of the Mueller matrices [25] is near unity with F ( M $^{$_{L}$exp.}$) ≃ 0 . 967 and F ( M $^{$_{R}$exp.}$) ≃ 0 . 939. This shows that the coherence in polar- ization is not degraded by the structure and that we can therefore restrict our discussion to Jones matrices [3, 25]. In the convenient left | L 〉 and right | R 〉 circular polarization basis, these Jones matrices tie the excitation [ E in L , E in R ] to the transmitted [ E out L , E out R ] electric fields. In the case of planar chiral structures displaying 2D chiral activity, they have the following form [5, 22]: where A , B and C are complex valued numbers such that | B | ̸ = | C | . This inequality account for chirality. Being non diagonal, these matrices correspond to polarization con- verter elements with no rotational invariance around the z axis (Fig. 1). They are thus fundamentally different from Jones matrices associated with optical activity, e.g., gamma- dions. Importantly the conditions | B | ̸ = | C | implies the non unitarity of J th. L , R which means that reversing the light path through the chiral structures is not equivalent to reversing the time. From equation (1) we deduce the associated theoretical forms for the Mueller matrices M $_{L}$th. , M $_{R}$th. (see appendix C) which are used to fit J L and J R from experimental results. After normalization by A we deduce J $_{L}$th. =    A B C A    , J $_{R}$th. =    A C B A    , (1) J $_{L}$fit =    1 . 000 0 . 166 + i 0 . 221 − 0 . 131 + i 0 . 099 1 . 000    , J $_{R}$fit =    1 . 000 − 0 . 129 + i 0 . 098 0 . 170 + i 0 . 230 1 . 000    . (2) 5