To experimentally evidence that the group velocity of the propagating wave converges to zero towards the localization regions, we performed a series of time delay measurements45. Figure 7(g–i) show the measured average group indices \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bar{n}}_{g}$$\end{document}n¯g for the frequencies equal to 7.52 GHz, 7.80 GHz and 7.91 GHz, respectively. We find that in all three cases \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bar{n}}_{g}$$\end{document}n¯g gradually increases towards the localization region. At these regions, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bar{n}}_{g}$$\end{document}n¯g varies between 90 and 110, which verifies the slow light originating localizations20. We should note that in theory one expects to obtain zero group velocity at the localization regions, however there are two major factors that prevent obtaining such a measurement. The first one is the lateral losses occurring at the localization region, which impose an additional decay rate to the system and cause deviation of the group velocity from zero. The second one, which is rather a practical issue, is the difficulty to obtain the time delay measurements continuously along the longitudinal direction. As was said above, due to the 3D periodicity of the structure, the time delay measurements had to be taken at discrete positions which transforms the τ d = ∂φ/∂ω differential relation into a finite difference approximation. To further reveal that the localized modes approach the band edge regions, the bandstructure in the propagation Γ-X′ direction for the instantaneous az periods at the localization regions were numerically calculated (see upper left insets of Fig. 7(g–i)). One can infer from these figures that the localized modes are indeed close to the band edge region and, thus, are expected to be slowed down and be localized.