Radial extent of the evanescent zone (Δ r) normalised by the radius of the midpoint of the evanescent zone (r_evanescent) vs. the natural logarithm of the coupling factor () for each frequency in the frequency range ν_max ± 3.5Δν computed using Eqs. (9)–(11). The colour-coding is the same as in Fig. 6. The grey line indicates the fit as indicated in the legend (see text for more details). The values of q_computed are shown in the top axis.