Enhanced third-order optical nonlinearity driven by surface-plasmon field gradients

摘要

Achieving efficient nonlinear optical frequency conversion in small volumesis key for future on-chip photonic devices that would provide a higher-speedalternative to modern electronics. However, the already intrinsically lowconversion efficiency severely limits miniaturization to nanoscale dimensions.Here we demonstrate that gradient-field effects can provide for an efficient,conventionally dipole-forbidden nonlinear response, offering a new approach forenhanced nonlinear optics in nanostructures. We show that a {\em longitudinal}nonlinear source current can dominate the third-order optical nonlinearity ofthe free electron response in gold in the technologically important near-IRfrequency range where the nonlinearities due to other mechanisms areparticularly small. Using adiabatic nanofocusing to spatially confine theexcitation fields, from measurements of the $2\omega_1 - \omega_2$ four-wavemixing response as a function of detuning $\omega_1 - \omega_2$, we find up to$10^{-5}$ conversion efficiency with a gradient field contribution to$\chi^{(3)}_{\mathrm{Au}}$ of up to $10^{-19}~\mathrm{m}^2 / \mathrm{V}^2$. Theresults are in good agreement with theory based on plasma hydrodynamics. Ourresults demonstrate an increase in nonlinear conversion efficiency withdecreasing sample size that can offset and even overcompensate the volumedecrease of conventional dipolar pathways. This will enable more efficientnonlinear optical devices and frequency converters and facilitate the extensionof coherent multidimensional spectroscopies to the nanoscale.