The time delay is a measure of the photon transit time and is defined as τ d = ∂φ/∂ω, where φ is the phase difference between the input and output port of the vector network analyzer and ω is the operational angular frequency. Since the 3D periodicity of the woodpile PhC structure prevents to take a continuous spatial measurement along the longitudinal z- direction, we injected the receiver antenna inside the PhC structure with a tilting angle of 16° with respect to the y- axis until the center of the structural height at specific points. The difference of the time delays measured at these points have been divided to the corresponding point-to-point spatial distance to obtain the average group velocity, which is then divided to the speed of light in free space to estimate the average group index \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.