Volk, M.F.M.F.VolkReinhard, B.B.ReinhardNeu, J.J.NeuBeigang, R.R.BeigangRahm, M.M.Rahm2022-03-042022-03-042013https://publica.fraunhofer.de/handle/publica/23288210.1364/OL.38.002156We designed and implemented a gradient index metasurface for in-plane focusing of confined terahertz (THz) surface waves. We measured the spatial propagation of the surface waves by two-dimensional mapping of the complex electric field using a THz near-field spectroscope. The surface waves were focused to a diameter of 500 m after a focal length of approximately 2 mm. In the focus, we measured a field amplitude enhancement of a factor of 3.en621535journal article