Study of Surface Plasmon Induced Hot Electron Relaxation Process and Third-Order Optical Non linearity in Gold Nanostructures

The relaxation dynamics of hot electrons and the third-order optical nonlinearity in gold nanorods (GNRs) with different aspect ratios were investigated with the help of femtosecond optical Kerr (OKE) technique. Relaxation process on the time scale of picosecond (ps) was obtained at the longitudinal surface plasmon resonant (SPR) wavelength. As the aspect ratio (AR) of the GNRs varied from 4.2 +/- 0.3 to 3.3 +/- 0.3, the SPR wavelength changed accordingly from 808 to 750 nm, but the corresponding relaxation time and the third-order optical susceptibility did not change much. However, as the pump power increased from 150 mW to 400 mW, the relaxation time of the nanorod with AR = 4.2 +/- 0.3 increased by 50% from 1.20 +/- 0.01 ps to 1.84 +/- 0.04 ps, indicating that the pump power is an important factor that affects the response time of the nanostructures. Comparative studies between Au nanorods, Au nanobipyramids, and Au triangular prisms measured under the same setup demonstrated that excitation conditions (excitation wavelength or pump power) and structure shapes were key factors to modulate the relaxation time of hot electrons. These results are important for the control and modulation of the response time in metallic nanostructures, which is essential for their applications in photovoltaic, photocatalytic, and ultrafast optic devices.