The accumulation of Gouy phase is a well-known phenomenon associated with propagation of Gaussian beams passing through a focal waist in free space. The Gouy phase can be understood from a purely heuristic perspective as the effect of change in optical mode volume [1], and as such does not have to be restricted to Gaussian beams. Here I demonstrate that an evanescent field of surface waves, which undergoes adiabatic tapering along its propagation also accumulates a (transverse) Gouy phase in addition to the conventional dynamic and Gouy phase due to its propagation along the surface. The theory of this effect, built on the calculation of the average value of the distribution of wave-vector components [2], is developed for the case of the evanescent field of a surface plasmon guided by a thin conductive layer with spatially changing surface conductivity. A non-uniform electric bias of a single layer graphene, which gives rise to the spatially non-uniform Fermi level and surface conductivity, is proposed as an approach for experimental testing of this effect. The accumulation of the Gouy phase of surface plasmon guided by such nonuniformly biased graphene layer is confirmed by numerical simulations. The discovered effect promises a new approach to actively control phase in plasmonic devices and can be useful for a variety of practical applications, from interferometry to sensing.
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