  Planar metamaterials with tailorable electromagnetic properties in the
terahertz domain offer customized optics solutions that are needed for the
development of imaging and spectroscopy systems. In particular, metamaterials
carry the potential to substitute conventional bulky optics or can be basic
building blocks for completely novel devices. In this respect it is
advantageous that metamaterials can be devised to be impedance-matched to
another material regarding their electromagnetic properties. Here, we design,
fabricate and investigate a metamaterial with tailorable refractive index and
impedance-matching to free space. The unit cell is comprised of two different
pairs of cut-wires that provide almost independent control over the electric
and magnetic response to an electromagnetic wave. For the example of a
metamaterial with a uniform effective refractive index of 1.18, we
experimentally demonstrate an amplitude transmission of more than 90% and a
reflectivity of about 5% at a working frequency of 0.444THz. As a more
functional device, we fabricate a terahertz gradient index beam steerer with a
linear refractive index gradient from 1.14 to 2.66. We numerically simulate the
electromagnetic wave propagation through the beam steerer and compare the
electric field amplitude of the deflected wave with the terahertz field
distribution that is measured behind the beam steerer in an imaging terahertz
time-domain spectroscope. The numerically simulated deflection angle of
6.1{\deg} agrees well with the measured deflection angle of 5.95{\deg}. The
measured peak amplitude transmission of the beam steerer amounts to almost 90%,
which also agrees well with a simulated value of approximately 88%.
