Nonlinear optical properties of metallic nanoantennas investigated by angle resolved confocal microscopy with femtosecond laser pulses
Résumé
Metallic nanostructures exhibit fascinating linear and nonlinear optical properties once they are excited by appropriate incident light. Localized Surface Plasmons (LSPs) generated by a collective oscillation of electrons in the conduction band offer the possibility of enhancing and concentrating the electrical field in a subwavelength volume enabling the nanostructures to act similarly to antennas in the microwave or radiowave regime. The influences of LSP resonances on the linear optical properties of plasmonic nanoantennas, such as the scattering, the absorption, and the one-photon photoluminescence have been rigorously and systematically investigated. However, nonlinear optical processes in plasmonic nanoantennas, such as two-photon photoluminescence (TPL) and second harmonic generation (SHG), are still not fully understood. A comprehensive study of TPL and SHG from separated and connected gold nanodimers fabricated using an electron-beam-lithographic technique was conducted here. In particular, the influence of the gap size and nanodisc diameter on their nonlinear optical response is addressed in detail. Analyzing the nonlinear optical spectra and performing polarization resolved measurements by rotating the excitation field, using an experimental setup combining a femtosecond laser source with a parabolic mirror, we show that SHG and TPL exhibit a different behavior despite the fact that these two nonlinear optical processes have the same fundamental intensity dependence. The underlying physical mechanisms explaining the differences are revealed using a surface integral equation method for nonlinear computations. The different evolutions of SHG and TPL with the nanodimer geometry are due to their distinct physical natures, resulting in different coherence properties and specific rules for plasmon enhancement.