and $d_d$ were chosen so as to maximize the largest value of the GH shift.

We observe in figure 4(a) that the GH shift of the reflected beam changes its sign from negative to positive as $x_m$ increases through a critical value $x_{m,\rm cr}$. This value and the corresponding critical thickness of the metal layer, $d_{m,\rm cr}$, are found to vary very little as $W$ is decreased from 320 $\mu$m $( \approx 505.53\lambda)$ to 40 $\mu$m $( \approx 63.19\lambda)$. As $W$ is decreased further, $d_{m,\rm cr}$ decreases noticeably. $x_{m,\rm cr}$ is about 0.0862 and $d_{m,\rm cr}$ is about 54.56 nm when $W$ is 40 $\mu$m, while $x_{m,\rm cr}$ is about 0.0825 and $d_{m,\rm cr}$ is about 52.22 nm when $W$ is 10 $\mu$m $(\approx  15.8\lambda)$. The maximal forward and backward lateral shifts are close to the half-width of the incident beam. For example, the beam with $W = 505.53\lambda$ is shifted by $\Delta_r = 494.72\lambda$ in the forward direction and by $\Delta_r =
-527.2\lambda$ in the backward direction. For $\lambda =
633$ nm, these correspond to 313.2 $\mu$m and $-333.7$ $\mu$m respectively. Our numerical results are summarized in Table 1. We notice that for $W/\lambda \approx 15.8$, the forward GH 