  We present a gradient-index (GRIN) metamaterial based on an array of annular
slots. The structure allows a large variation of the effective refractive index
under normal-to-plane incidence and thus enables the construction of GRIN
devices consisting of only a small number of functional layers. Using full-wave
simulations, we demonstrate the annular slot concept by means of a 3-unit-cell
thin GRIN lens for the terahertz (THz) range. In the presented realizations, we
achieved an index contrast of Delta n = 1.5 resulting in a highly refractive
lens suitable for focusing THz radiation to a spot size smaller than the
wavelength.
