Active terahertz metamaterials

Terahertz radiation—electromagnetic waves in the 0.1–10THz frequency range—occupies a middle ground between microwaves and infrared light. The unique properties of terahertz radiation make it very attractive for numerous applications in molecular spectroscopy, biomedical imaging, short-range ultrahigh-bandwidth wireless communications, non-destructive inspection, security screening, and even quantum computing in silicon devices. For example, in a similar way to how infrared radiation can probe the vibrations corresponding to the motion of atoms bound together in an individual molecule, terahertz spectroscopy can probe the much weaker forces governing the interaction between molecules. Its use could revolutionize our understanding of how complex biomolecules interact. Terahertz waves are also well suited to non-destructive testing and security screening applications because, like microwaves and millimeter waves, they are non-ionizing, making them much safer than x-rays, and they can travel through many materials including plastics and cloth. However, the shorter wavelength of terahertz waves compared to microwaves and millimeter waves enables significantly higher image resolution. Historically, the terahertz range has been largely unexplored due to the difficulty of generating, detecting, and manipulating terahertz radiation. Electronic devices that have driven the widespread use of microwaves are generally limited to much lower frequencies. In addition, a paucity of suitable materials means that photonic technologies that have been wildly successful in the infrared, visible, and ultraviolet regimes run into severe limitations in the terahertz range. One powerful way to circumvent the limitations of existing materials is to create artificial ‘metamaterials.’ These consist of arrays of conducting elements that are small enough to appear as an effectively continuous material to the electromagnetic waves (much as we can often ignore that natural materials are really composed of tiny atoms instead of being continuous). Metamaterials can also be engineered to exhibit exotic electromagnetic phenomena not observed in natural materials, including negative refraction and anomalous Figure 1. Microscope image of our superconducting metamaterial. YBCO: Yttrium barium copper oxide.