content_image/334635.jpg
F ( M ) = ( Tr [ M $^{†}$M ] −M 2 00 3 M 2 00 ) 1 / 2 ≤ 1. Here we measured F ( M $^{glass}$) = 0 . 9851. It implies that the light is not depolarized when going through the setup and that consequently we can rely on our measurement procedure for obtaining M . 

Two important points must be noted here: On the one hand we varied the incident illumina- tion spot size on the sample between 2 and 20 µ m without affecting the matrix, i. e., without introducing additional depolarisation. In the rest of the experiment on chiral structures we consider the case of a large gaussian spot with FWHM=20 µ m in order to illuminate the whole individual spiral. On the other hand, it can be observed that in our experiments the polarization in the Airy spot (see Fig. 4b) is homogeneous. This implies that we are actu- ally doing the polarization tomography of the central transmitting hole, i. e., we are dealing only with the SU(2) point symmetry of the Mueller Matrix. This situation clearly contrasts with previous SOP tomography measurements on metallic hole arrays in which the polar- ization degrees of freedom were mixed with spatial information responsible for SPP-induced depolarization [26]. 

## V. APPENDIX B: EXPERIMENTAL MUELLER MATRICES. 

The experimental Mueller matrices deduced from the polarization tomography are after normalization of M exp. 00 : 

<img src='content_image/334635.jpg'>

We have F ( M $^{$_{L}$exp.}$) ≃ 0 . 967 and F ( M $^{$_{R}$exp.}$) ≃ 0 . 939. 

We must also note that the normalization used here neglects a small additional coefficient of proportionality |M $_{L}$exp. ij / M $_{R}$exp. ij | ≃ 0 . 954 imputed to experimental errors and uncer- tainties. 

