  In an attenuated total reflection (ATR) geometry, energy extracted from an
incident light beam by surface plasmon polariton (SPP) creation is implicitly
considered to dissipate as heat. However, we show that a sharpened tip
interacting with the SPP evanescent field can redirect some of the energy
before such dissipation occurs. This behavior is examined with simple computer
simulations and an analogy is drawn between it and a modified Mach-Zehnder
interferometer. We also discuss "interaction-free" measurements with our
set-up.
