maximum forward GH shift for the beam with $W = 320$ $\mu$m is about 495 times of the wavelength ($\sim 313$ $\mu$m), which is sufficiently large to be observable in figure 7.

We mention briefly on the GH shift of the transmitted beam, $\delta_t$ ($ = \Delta_t / \lambda$), as a function of the incident angle for several values of $W$. We find that $\delta_t$ is positive in all cases and depends very weakly on $W$. The maximum value of $\Delta_t$ is about seven times of the wavelength ($\sim 4.4$ $\mu$m). For the normalized half-width of the transmitted beam $W_t/ W$, we find that the value at the reflectance dip is slightly larger than 1. We also find that there is no splitting in the magnetic field intensity distribution of the transmitted beam. The spatial profiles of the transmitted beam look similar to those of the incident Gaussian beam. If we employ a substrate with its refractive index less than 1.5676, we obtain an evanescent transmitted beam. Then there will be another SPP excited at the metal-substrate interface, enhancing the GH shift even further.

There have been several experiments showing both positive and negative GH shifts. In particular, there exist 