The formation and relaxation of plasmon-induced hot electrons in Au nanodisks (AuNDs) with relatively large particle size was studied by femtosecond pump-probe spectroscopy. We temporally resolved the plasmon-induced formation of nonthermal electrons through intraband transitions and subsequent redistribution of nonthermal electrons to thermal electrons. The transient extinction spectra were found to exhibit clear dichroism for the pump polarization on a time scale of a few hundred femtoseconds. The electric field distribution was calculated using the Finite-difference time-domain (FDTD) method to be inhomogeneous inside the AuNDs under irradiation by linearly polarized light. The formation and relaxation of hot electrons occur locally inside the AuNDs, which results in the observed ultrafast dichroic response.
Metasurfaces of gold (Au) nanoparticles on a SiO2-Si substrate were fabricated for the enhancement of second harmonic generation (SHG) using electron beam lithography and lift-off. Triangular Au nanoprisms which are non-centro-symmetric and support the second- order non-linearity were examined for SHG. The thickness of the SiO2 spacer is shown to be an efficient parameter to spectrally tune to maximise SHG. Electrical field enhancement at the fundamental wavelength was shown to define the intensity of the second harmonics. Numerical modeling of light enhancement was verified by experimental measurements of SHG and reflectivity spectra at the normal incidence. At the plasmonic resonance, SHG is enhanced up to ∼3.5×103 times for the optimised conditions.
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