Nonlinear optics in nanoantennas

Nonlinear optical processes are inherently weak in nanoantennas, but this can be significantly enhanced through the use of plasmonic resonances and field enhancements. This thesis presents an experimental investigation of two different designs for plasmonic nanoantennas to demonstrate different mechanisms via nonlinear optical measurements. In the first, the existence of a critical coupling condition that maximises field enhancement is explored in coupled waveguide-dimer nanoantennas. The prediction formed by fitting experimental data to the coupled oscillator model is confirmed via measurements of second harmonic generation and third harmonic generation that vary with a geometric parameter within the nanoantenna design. A second plasmonic nanoantenna is used to demonstrate the stimulated emission tomography technique, which allows for characterisation of a spontaneous parametric process by studying its stimulated analogue. Four-wave mixing is observed in individual nanoantennas, and stimulated emission tomography is performed to reconstruct the joint spectral density maps for spontaenous four-wave mixing. Furthermore, a key derivation in the technique is used to assess the spontaneous four-wave mixing generation rate in the nanoantennas, which is found to be approximately 100 pairs generated per second per mW of pump power and per nanoantenna. Further optimisation of the nanoantenna design is expected to increase this value significantly.