@article{4254,
  keywords = {nanophotonics, surface plasmons},
  author = {Vasily Kravtsov and Sultan AlMutairi and Ronald Ulbricht and Ryan Kutayiah and Alexey Belyanin and Markus Raschke},
  title = {Enhanced Third-Order Optical Nonlinearity Driven by Surface-Plasmon Field Gradients},
  abstract = {<p>Efficient nonlinear optical frequency mixing in small volumes is key for future on-chip photonic devices. However, the generally low conversion efficiency severely limits miniaturization to nanoscale dimensions. Here we demonstrate that gradient-field effects can provide for an efficient, conventionally dipoleforbidden nonlinear response. We show that a longitudinal nonlinear source current can dominate the thirdorder 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 2ω<sub>1</sub> - ω<sub>2</sub> four-wave mixing response as a function of detuning ω<sub>1</sub> - ω<sub>2</sub>, we find up to 10<sup>-5</sup> conversion efficiency with a gradient-field contribution to χ<sup>(3)</sup><sub>Au</sub> of up to 10<sup>-19</sup> m<sup>2</sup>/V<sup>2</sup>. The results are in good agreement with the theory based on plasma hydrodynamics and underlying electron dynamics. The associated increase in the nonlinear conversion efficiency with a decreasing sample size, which can even overcompensate the volume decrease, offers a new approach for enhanced nonlinear nano-optics. This will enable more efficient nonlinear optical devices and the extension of coherent multidimensional spectroscopies to the nanoscale.</p>},
  year = {2018},
  journal = {Physical Review Letters},
  volume = {120},
  month = {2018-05},
  issn = {0031-9007},
  url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.203903},
  doi = {10.1103/PhysRevLett.120.203903},
}