We show for the first time that a planar metamaterial, an array of coupled metal split-ring resonators with a unit cell lacking mirror symmetry, exhibits asymmetric transmission of terahertz radiation (0.25-2.5 THz) propagating through it in opposite directions. This intriguing effect, that is compatible with Lorentz reciprocity and time-reversal, depends on a directional difference in conversion efficiency of the incident circularly polarized wave into one of opposite handedness, that is only possible in lossy low-symmetry planar chiral metamaterials. We show that asymmetric transmission is linked to excitation of enantiomerically sensitive plasmons, these are induced charge-field excitations that depend on the mutual handedness of incident wave and metamaterial pattern. Various bands of positive, negative and zero phase and group velocities have been identified indicating the opportunity to develop polarization sensitive negative index and slow light media based on such metamaterials.In contrast to three-dimensionally chiral structures (e.g. helices), planar chiral patterns (e.g. flat spirals) have the intriguing property that their sense of twist is reversed for observation from opposite sides. Not only human observers, but also circularly polarized waves incident on opposite sides of a planar chiral structure, see materials of opposite handedness. It has recently been discovered that planar chiral metamaterial patterns can show different levels of total transmission for circularly polarized waves of the same handedness propagating in opposite directions. The effect, which has been detected in microwave [1,2,3,4] and photonic [5,6] metamaterials and plasmonic nanostructures [7], is known as asymmetric transmission. Such asymmetric transmission phenomenon has not yet been observed for terahertz radiation. The terahertz spectral region has tremendous technological importance since many biological materials and substances have molecular vibration frequencies in this regime, making it highly attractive for sensing, material characterization, spectroscopy and biomedical imaging. In spite of intense research activity in this domain over the past decade terahertz radiation has proved to be extremely challenging to detect, measure, propagate and manipulate since electronic and magnetic responses of natural materials die out at these frequencies, thus earning the name of the socalled "terahertz gap". Recently, terahertz metamaterials [8,9,10,11,12,13,14,15,16,17,18] have shown potential for use in the terahertz gap with their fascinating novel properties but the region still suffers from a severe shortage of devices needed for fully exploiting the attractive potential applications of terahertz radiation.In this Letter we report the first experimental observation of asymmetric transmission in the terahertz do- main. We demonstrate a new type of polarization sensitive terahertz metamaterial device showing directionally asymmetric transmission of circularly polarized waves between 0.25 and 2.5 THz. The phenomenon resembl...
