We demonstrate free-space focusing of terahertz (THz) radiation by scattering plasmonic surface-waves into the air. We use a grating of shallow holes which contains non-equidistant defects which act as scattering centers. The scattering occurs with defined phase delays such that the waves emitted in free-space interfere constructively to form a focus above the waveguide surface. In contrast to conventional lenses, this structure does not require any free-space on its backside and has great potential for integrated THz optics.
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