The development of artificially structured electromagnetic materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials 1 . This is especially important for the technologically relevant terahertz (1 THz 5 10 12 Hz) frequency regime; many materials inherently do not respond to THz radiation, and the tools that are necessary to construct devices operating within this range-sources, lenses, switches, modulators and detectors-largely do not exist. Considerable efforts are underway to fill this 'THz gap' in view of the useful potential applications of THz radiation 2-7 . Moderate progress has been made in THz generation and detection 8 ; THz quantum cascade lasers are a recent example 9 . However, techniques to control and manipulate THz waves are lagging behind.Here we demonstrate an active metamaterial device capable of efficient real-time control and manipulation of THz radiation. The device consists of an array of gold electric resonator elements (the metamaterial) fabricated on a semiconductor substrate. The metamaterial array and substrate together effectively form a Schottky diode, which enables modulation of THz transmission by 50 per cent, an order of magnitude improvement over existing devices 10 .A great deal of research into metamaterials has used microwave radiation; this is in part due to the ease of fabrication of sub-wavelength structures at these frequencies. Indeed, negative refractive index media 11,12 composed of negative permittivity 13 (e 1 , 0) and negative permeability 14 (m 1 , 0) metamaterial elements was first demonstrated at microwave frequencies. This has led to intense theoretical, computational and experimental studies of exotic phenomena, such as perfect lensing 15 and cloaking 16,17 . Recently, researchers have ventured to create functional metamaterials at near-infrared and visible frequencies [18][19][20] . Considerably less work has concentrated on THz frequencies 21,22 . However, the design flexibility associated with metamaterials provides a promising approach, from a device perspective, towards filling the THz gap.Metamaterials are geometrically scalable, which translates to operability over many decades of frequency. This engineering tunability is in fact a distinguishing and advantageous property of these materials. However, for many applications it is desirable to have real-time tunability. For instance, short-range wireless THz communication or ultrafast THz interconnects 23,24 require switches and modulators. Current state-of-the-art THz modulators based on semiconducting structures have the desirable property of being broadband, which is of relevance to THz interconnects, but are only able to modulate a few per cent 10 and usually require cryogenic temperatures 25 . Therefore, further improvement of the performance characteristics are required for practical applications. Here we present an efficient active metamaterial switch/modulator operating at THz frequencies. Although the modulation is based on a narrowband me...
