GH shift, when the metal layer is relatively thick. Shen [*et al.*]{} have shown that the forward shift changes to the backward shift when the metal layer thickness changes by only a few nanometers. We notice that the sign of the GH shift changes from negative to positive as $d_m$ increases in our case of the Otto configuration, whereas it changes from positive to negative in Shen [*et al.*]{}’s case of the Kretschmann configuration.

![\[fig3\] (a) Forward normalized GH shift of the reflected wave, $\delta_r$, calculated from Artmann’s formula for plane waves when $x_m=0.08657$ and (b) the corresponding phase of the reflected wave, $\Phi_r$, versus incident angle $\theta_i$. (c) Backward normalized GH shift of the reflected wave calculated from Artmann’s formula when $x_m=0.08638$ and (d) the corresponding phase of the reflected wave versus incident angle.](fig3){width="9cm"}

In figures 3(a) and 3(b), we show the forward GH shift for the reflected plane wave and the corresponding phase of the reflected wave as a function of the incident angle, when $x_m=0.08657$. We find that $\Phi_r$ changes very rapidly and the GH shift becomes extremely large near $\theta_i=62.33^\circ$. The maximum $\delta_r$ is about 7450 at $\theta_i=62.332^\circ$. Similarly, in figures 3(c) and 3(d), we show 