|Title||Enhanced Third-Order Optical Nonlinearity Driven by Surface-Plasmon Field Gradients|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Kravtsov, V, AlMutairi, S, Ulbricht, R, A. Kutayiah, R, Belyanin, A, Raschke, MB|
|Journal||Physical Review Letters|
|Keywords||nanophotonics, surface plasmons|
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 2ω1 − ω2 four-wave mixing response as a function of detuning ω1 − ω2, we find up to 10−5 conversion efficiency with a gradient-field contribution to χ(3)Au of up to 10−19 m2/V2. 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.