the beam occurs near the critical thickness of the metal layer, but not at the exactly same thickness. We observe that $W_r/W$ is larger and the overall curve is broader when the GH shift is smaller and when the incident beam is narrower.

![\[fig5\] (a) Forward normalized GH shift of the reflected beam versus incident angle for several different values of $W$ when $x_m=0.08657$. (b) Normalized half-width of the reflected beam versus incident angle for the same values of $x_m$ and $W$ as in (a).](fig5){width="8.5cm"}

In figure 5(a), we plot $\delta_r$ as a function of the incident angle for several values of $W$ when $x_d$ is 0.5055 and $x_m$ is 0.08657. We find that the maximum value of the [*forward*]{} lateral shift, which is about 140 times of the wavelength when $W$ is 320 $\mu$m and about 42 times of the wavelength when $W$ is 160 $\mu$m, is an order of magnitude smaller than the value in the plane-wave incidence, but is still very large. This value decreases as $W$ decreases. Since each Fourier component in the incident beam experiences a different phase shift from each other, the reflected beam, which is the sum of the reflected Fourier components, gets 