  A theoretical analysis of the lateral displacement (Goos-H\"anchen shift) of
spatially limited beams reflected from Attenuated Total Reflection (ATR)
devices in the Otto configuration is presented when backward surface plasmon
polaritons are excited at the interface between a positive refractive index
slab and a semiinfinite metamaterial with negative refractive index. First, the
stationary phase approximation and a phenomenological model based on the
properties of the complex poles and zeroes of the reflection coefficient are
used to demonstrate that: i) the excitation of backward surface waves can lead
to both negative and positive (and not exclusively negative) Goos-H\"anchen
shifts, and ii) the sign of the shift depends on whether the value of the
coupling layer thickness is higher or lower than a critical value
characteristic of the ATR structure. Second, these findings are verified
through rigorous calculations of the spatial structure of the reflected beam.
For incident beams with a Gaussian profile, the lateral shift calculated as the
first moment of the field distribution of the reflected beam agrees quite well
with the predictions of approximate analysis. Near the resonant excitation of
the backward surface plasmon polariton, large (negative or positive)
Goos-H\"anchen shifts are obtained, along with a splitting of the reflected
beam.
