Science 342, 1223–1226 (2013)

To maximize nonlinear effects, it is vital to match the phases of light beams that are involved in the interaction; this requirement is a consequence of the need to conserve the momentum of photons. Usually, this limits the wavelengths and beam directions that can be used in nonlinear processes, and necessitates the use of birefringent, periodic or carefully orientated materials to minimize any phase mismatch. The advent of artificially engineered metamaterials, however, has now provided researchers with a new opportunity to overcome these restrictions. Haim Suchowski and co-workers from the University of California at Berkeley and Lawrence Berkeley National Laboratory have now realized a metamaterial with a zero refractive index in which photons carry zero momentum. The result is momentum conservation is possible for photons travelling in both the backward and forward directions. The team has demonstrated equally efficient bidirectional four-wave mixing in a 'fishnet' zero-index metamaterial. The structure is composed of a stack of alternating thin metal (30-nm-thick gold) and dielectric (50-nm-thick magnesium fluoride) layers and features an array of cross-shaped holes. Such zero-index materials may prove useful for applications involving other nonlinear processes, such as sensing based on Raman spectroscopy and the generation of entangled photons by spontaneous parametric downconversion for quantum optics applications.