Nonlinear Propagation in Optical Zero-Index Materials
Forward phase mismatch (purple curve) is near zero for all wavelengths; backward (blue curve) has large phase mismatch except when index is near zero. Insets: (Bottom left) Scanning electron micrograph of zero-index metamaterial. (Top right) Measured four-wave mixing process in zero-index regime has almost the same yield in both directions.
The Subwavelength Metamaterials Program achieved the first experimental observation of nonlinear light generation from 3D metamaterials with zero refractive index: light gains strength in all directions while moving through the material.
Significance and Impact
The zero index elimates the need for phase matching and holds promise for quantum computing, networking, and future light sources.
A major problem in nonlinear optics is the inherent phase mismatch between interacting waves as light propagates through materials. Differences in phase velocities prevent constructive combination and lead to phase mismatch, or destructive interference, and thus very low efficiency. Phase matching is critical for coherent nonlinear optical processes such as frequency conversion and parametric amplification.
In conjunction with the goal of eliminating phase mismatch, Berkeley Lab researchers produced the first experimental observation of nonlinear generation from a 3D metamaterial, specifically a material with refractive index zero (zero-index material, ZIM). They used a unique optical ZIM to generate mismatch–free nonlinear light, meaning that the generated waves moved through the material gaining strength in all directions. The demonstration used four-wave mixing, in which three beams of light mix in a nonlinear medium to create a fourth. In the ZIM, equal amounts of nonlinearly generated waves were observed in both forward and backward propagation directions.
The removal of phase matching in nonlinear optical metamaterials may lead to applications such as efficient multidirectional light emissions for novel light sources and the generation of entangled photons for quantum networking. This phase mismatch-free quality holds promise for quantum computing and networking, and future light sources based on nonlinear optics.