Active Control of Multiple, Simultaneous Nonlinear Optical Processes in Plasmonic Nanogap Cavities

Plasmonic structures are promising to enhance and control nonlinear optical processes as the subwavelength-scale elements not only increase the local electric field intensities, but also result in relaxed phase matching conditions. This opens the possibility to observe and further manipulate multiple nonlinear optical processes simultaneously, which would be forbidden in bulk crystals due to incompatible phase matching requirements. Here we enhance and control the relative strength between third harmonic generation (THG), sum frequency generation (SFG), and four wave mixing (FWM) arising from 1 to 7 nm Al₂O₃ layers sandwiched between a gold film and silver nanorectangles. We demonstrate that the relative strength of the three, simultaneous nonlinear optical processes can be precisely controlled by either the ratio between the powers of the two excitations or the thickness of the Al₂O₃ layer. Furthermore, enhancements up to 10⁶-fold for THG and FWM are observed along with 10⁴-fold enhancements for SFG response when the resonance of the transverse and longitudinal mode of the cavity are matched to the two pump excitations. The ability to obtain and control multiple, nonlinear optical processes simultaneously open new capabilities for advanced on-chip manipulation and processing of optical signals on the deep nanoscale.