  Plasmonic catalysis holds significant promise for opening new reaction
pathways inaccessible thermally, or for improving the efficiency of chemical
processes. However, challenges persist in distinguishing photothermal effects
from effects due to energetic electrons and/or holes excited in the metal
comprising a plasmonic structure. Here we use a Au stripe waveguide along which
surface plasmon polaritons (SPPs) propagate, operating simultaneously as a
working electrode in a 3-electrode electrochemical cell. Cyclic voltammograms
were obtained under SPP excitation as a function of incident optical power and
wavelength ~ 1350 nm, enabling processes involving energetic holes to be
separated from processes involving energetic electrons by investigating
oxidation and reduction reactions separately. Redox current densities increase
by 10x under SPP excitation. The oxidation, reduction and equilibrium
potentials drop by as much as 2x and split beyond a clear threshold with SPP
power in correlation with the photon energy. The temperature of the working
electrode under SPP excitation is monitored in situ and independent control
experiments isolate thermal effects. Chronoamperometry measurements with on-off
modulated SPPs at 600 Hz, yield a diffusion-limited rapid current response
modulated at the same frequency, ruling out thermally-enhanced mass transport.
Our observations are attributed to the opening of optically controlled
non-equilibrium redox channels associated with energetic carrier transfer to
the redox species.
