Nonlinear solution

Nature Mater. http://dx.doi.org/10.1038/nmat3148 (2011)

Phase conjugation implemented via a thin nonlinear film has been proposed as an alternative to the use of lossy, narrowband and complex metamaterials for realizing the phenomenon of negative refraction, albeit at microwave frequencies. Stefano Palomba and colleagues from the USA, the UK and Israel have now demonstrated a simpler solution that operates at optical frequencies by exploiting degenerate four-wave mixing (FWM) in a 20-nm-thick nanostructured gold film. They experimentally demonstrated that a beam generated during the FWM process can emerge at a negative angle relative to the exciting wave. Mixing two plane waves at frequency ω₁ and incidence angle θ₁ with a third plane wave at frequency ω₂ and incidence angle θ₂ gives rise to two beams at a new frequency of ω₃ = 2ω₁ − ω₂ and incidence angle θ₃ in the forwards and backwards directions. The intriguing phenomenon occurs for θ₁ = 0, where the forwards nonlinear FWM beam exhibits negative refraction relative to the probe beam. The ratio between the sines of the incidence and refracted angles is a constant that depends only on the two wavelengths, which implies that the refractive index can be modified simply by tuning the wavelengths of the interacting waves. The team have experimentally shown that FWM for Au/SiO₂/Au nanodisk metamaterials is around ten times stronger than that for smooth gold film, owing to the strong field confinement at localized surface plasmon resonances. They also obtained FWM efficiencies of 9 × 10⁻⁸ for a smooth gold film and 10⁻⁶ for nanodisk metamaterials. The findings are potentially useful for high-resolution and background-free imaging at the nanoscale.