$x_m = d_m / \lambda$, when $\theta_i$ is fixed to $62.33^\circ$. As $x_m$ is increased through the critical value $x_{m,\rm cr}\approx 0.08648$, $\delta_r$ changes rapidly from large negative values to large positive values. The value of the largest backward normalized GH shift is about $-7957$ at $x_m\approx 0.08642$, while that of the largest forward normalized GH shift is about 4214 at $x_m\approx 0.08654$. We note that the change of $x_m$ from $0.08642$ to $0.08654$ corresponds to the thickness change of just 0.076 nm. From many numerical calculations, we have found that the maximum GH shifts are obtained when $\theta_i\approx 62.33^\circ$. Since the values of the GH shift for plane waves are much larger than those obtained for Gaussian beams and shown in figure 4(a) below, we show them in separate figures.

Shen [*et al.*]{} have presented the results of the calculation and the measurement of the phase shift of the reflected wave in the Kretschmann configuration as a function of the incident angle [@Shen]. It has been shown that the phase shift decreases monotonically with $\theta_i$, which corresponds to the forward GH shift, when the metal (Ag) layer is relatively thin, while it increases monotonically, which corresponds to the backward 