  A high spatial resolution surface plasmon near field scanning tunneling
microscope (STM) has been used to study the properties of localized surface
plasmons (SPO) in so-called hot spots on a gold surface, where the local
electromagnetic field is extremely high. A CW semiconductor laser and a
femtosecond Ti:Sa laser were used to excite the plasmons and the SPO excited
tunnel current was used as the detector. When scanning the STM from negative to
positive bias and reversed, hysteresis in the tunnel signal was found,
excluding (or rather minimizing) the role of the presence of a Casimir effect
in the process. It was found, however, that a multiple image charge induced
double well potential may explain our experimental findings. The stepwise
behaviour of the area of the observed hysteresis loops is a new, additional
indication of the non-classical properties of the SPOs.
