Freeing Nonlinear Optics from Phase Matching

Optical metamaterials that have a refractive index of zero can boost the efficiency of nonlinear optical processes. The advent of lasers quickly led to the demonstration of nonlinear optical effects. One of the simplest examples of a nonlinear effect is doubling the input frequency of light with a nonlinear crystal. The output intensity of the frequency-doubled light then grows nonlinearly with the intensity of the laser beam. To maximize macroscopic nonlinear signals, the wavelets emitted by the elementary nonlinear sources need to add up in phase. On page 1223 of this issue, Suchowski et al. (1) report an interesting approach to phase matching with metamaterials—artificial nanostructures with unusual optical properties created by tailoring their structural features (2). The authors avoided the phase-matching problem by relying on a metamaterial that has a refractive index for the interacting wavelengths near zero (keeping in mind that the refractive index of the vacuum is not zero but unity). Furthermore, they achieve equal nonlinear emission in two opposite directions simultaneously, whereas conventional phase-matching techniques maximize the nonlinear signal in only one direction.