Nature 497, 470–474 (2013)

Metamaterials offering negative refraction often consist of resonator arrays. In recent demonstrations at visible wavelengths, resonators with small features required to achieve the magnetic response needed for a negative index have approached the fabrication limits of lithography. Fortunately, gap plasmon waveguide modes formed in thin dielectric gaps in metals can also provide negative refraction and be easily made in much smaller sizes, thus allowing operation at shorter wavelengths. Using this approach, Ting Xu and colleagues from the USA and Canada have made a metamaterial that gives a left-handed response to ultraviolet light. They achieved a negative index over a broad range of incident angles of transverse magnetic light in a layered structure of stacked metal–dielectric layers. Their structure was only several layers thick, but their results indicate that its characteristics approached that of a bulk metamaterial. In principle, it is relatively easy to make thicker samples. Similar to other design approaches for achieving negative refraction, optical loss in the structures needs to be considered when determining the maximum size of practical samples.