ture by using an objective L$_{1}$ ( × 50, numerical aperture=0.55). The transmitted light is collected and recollimated by using a second objective L$_{2}$ ( × 40, numerical aperture=0.6). The input and output states of polarization are respectively prepared and analyzed in the collimated part of the light path by using polarizers, half wave plates and quarter wave- plates. A sketch of the setup is provided below (see Fig. 4). The Mueller matrix is built by applying an experimental algorithm equivalent to the one described in [25]. More precisely, in order to write down the full Mueller matrix, we mea- sured here 6 × 6 intensity projections corresponding to the 6 unit vectors | x 〉 , | y 〉 , | + 45 $^{◦}$〉 , | − 45 $^{◦}$〉 , | L 〉 , and | R 〉 for the input and the output polarizations. Actually only 16 measures are needed to determine M [25]. Our actual procedure is thus more than sufficient to obtain M . The isotropy of the setup was first checked by measuring the Mueller matrix M glass with a glass substrate. Up to a normalization constant, we deduced that M glass is practically identical to the identity matrix I with individuals elements deviating by no more than 0.02. More precisely, the optical depolarization (i. e, the losses in polarization coherence) can be precisely quantified through the degree of purity of the Mueller matrix defined by [25] 9 FIG. 4: Principle of the polarization experiment. (a), Sketch of the optical set up described in the text. The images are recorded by using a CMOS camera. (b), A typical image of the transmitting nanohole showing the Airy spot associated with diffraction by the optical microscope. The scale bar is 2 µ m long. (c), Crosscut of the intensity profile along the yellow dotted line shown in (b).