  We report on the observation of the reststrahl band assisted photocurrents in
epitaxial graphene on SiC excited by infrared radiation. The peculiar spectral
dependence for frequencies lying within the reststrahl band of the SiC
substrate provides a direct and noninvasive way to probe the electric field
magnitude at atomic distances from the material's surface. Furthermore our
results reveal that nonlinear optical and optoelectronic phenomena in 2D
crystals and other atomic scale structures can be giantly enhanced by a proper
combination of the spectral range and substrate material.
