In-plane focusing of terahertz surface waves on a gradient index metamaterial film

We 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.