Nonlinear emission from a single metal nanoantenna excited by 8-fs laser pulses

While the linear optical properties of metal nanoantennas have been studied in detail over the past few years, the origin of incoherent and coherent nonlinear emission is still controversial. Up to now, laser pulses ranging from picoseconds down to 100 fs have been employed for excitation. A 8-fs laser pulses was used to excite a single gold bowtie nanoantenna. The paper studies THG emission depending on antenna size and gap distance on arrays of gold nanoantennas fabricated by electron-beam lithography. A clear correlation of fractional antenna resonances with THG emission intensity is observed. Raster scanning a single nanoantenna under the fixed exciting laser focus allows us to record THG and SHG emission maps. Setting the exciting laser polarization parallel or perpendicular to the long antenna axis leads to the excitation of different plasmon resonances in the bowtie nanoantenna, which are identified in the THG maps. Our 8-fs light pulse allows us to measure the plasmon dephasing inside a single nanoantenna directly in the time domain. The strong THG signal of the bowtie antenna combined with frequency-resolved optical gating (FROG) indicates how the plasmon oscillator dephases. This technique opens up the door for future tailoring of single metal nanoantennas for ultrafast and nonlinear applications.