{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][3][4][5][6][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...
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