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

TL;DR

This paper demonstrates that gradient-field effects can significantly enhance third-order optical nonlinearity in gold nanostructures, enabling more efficient on-chip nonlinear optical devices at nanoscale dimensions.

Contribution

The study introduces a novel approach using surface-plasmon field gradients to boost nonlinear response, surpassing traditional dipolar mechanisms in nanostructures.

Findings

Achieved up to 10^{-5} conversion efficiency in four-wave mixing.

Gradient field contribution to χ^{(3)}_{Au} up to 10^{-19} m^2/V^2.

Nonlinear efficiency increases as sample size decreases, counteracting volume effects.

Abstract

Achieving efficient nonlinear optical frequency conversion in small volumes is key for future on-chip photonic devices that would provide a higher-speed alternative to modern electronics. However, the already intrinsically low conversion 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 for enhanced nonlinear optics in nanostructures. We show that a {\em longitudinal} nonlinear source current can dominate the third-order optical nonlinearity of the free electron response in gold in the technologically important near-IR frequency range where the nonlinearities due to other mechanisms are particularly small. Using adiabatic nanofocusing to spatially confine the excitation fields, from measurements of the $2\omega_1 - \omega_2$ four-wave mixing 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 $10^{-19}~\mathrm{m}^2 / \mathrm{V}^2$. The results are in good agreement with theory based on plasma hydrodynamics. Our results demonstrate an increase in nonlinear conversion efficiency with decreasing sample size that can offset and even overcompensate the volume decrease of conventional dipolar pathways. This will enable more efficient nonlinear optical devices and frequency converters and facilitate the extension of coherent multidimensional spectroscopies to the nanoscale.