(a) Phase histogram corresponding to data set 2. The histogram contains only phases within the rectangular area depicted in the inset. A fit of a sum of two Gaussians to the histogram is drawn in red. The phase of Ta (or no Ta) is estimated as an average over one 2 FWHM interval (dash-dotted black lines). The intervals correspond to the white and grey regions in the graph of the inset. (b) Estimations of corresponding to data set 1 for different values of γ using 2 FWHM intervals of the histograms. The reference as estimated from the inset shown in (a) is indicated as dash-dotted black line. The theoretical value for 25 µm W is indicated as a dashed black line. (c) Close-up of (b). Here it can be seen that γ = 2.496×103 is in good agreement with the reference.