  Reversible exchange of photons between a material and an optical cavity can
lead to the formation of hybrid light--matter states where material properties
such as the work function\cite{Hutchison_AM2013a}, chemical
reactivity\cite{Hutchison_ACIE2012a}, ultra--fast energy relaxation
\cite{Salomon_ACIE2009a,Gomez_TJOPCB2012a} and electrical
conductivity\cite{Orgiu_NM2015a} of matter differ significantly to those of the
same material in the absence of strong interactions with the electromagnetic
fields. Here we show that strong light--matter coupling between confined
photons on a semiconductor waveguide and localised plasmon resonances on metal
nanowires modifies the efficiency of the photo--induced charge--transfer rate
of plasmonic derived (hot) electrons into accepting states in the semiconductor
material. Ultra--fast spectroscopy measurements reveal a strong correlation
between the amplitude of the transient signals, attributed to electrons
residing in the semiconductor, and the hybridization of waveguide and plasmon
excitations.
