Shynkarenko Y. Transient and nonlinear optical processes in media with gold nanoparticles under femtosecond laser excitation.

The work presents the results of experimental study of changes of optical properties of gold nanoparticles under femtosecond laser excitation. Fast optical response of metal nanoparticles in comparison with semiconductor devices is associated with a small lifetime of localized plasmons. This makes such systems promising for ultrafast (subpicosecond) optical elements. There were investigated spherical gold nanoparticles in fused silica; aqueous colloidal solutions of gold nanoparticles of spherical shape; aqueous colloidal solutions of gold nanorods. In order to measure changes of optical properties in subpicosecond time domain time-resolved techniques of transient absorption "Pump-probe" and secondary emission - "optical Kerr gate" were improved. Detailed experimental and theoretical investigations of accompanying effects during these measurements were done and temporal resolution and efficiency of "optical Kerr gate" are presented. The experimental study of transient optical properties of gold nanoparticles in different media affected by the femtosecond laser pulses with energy from 1 uJ to 120 uJ is presented. It is shown that after the interaction of femtosecond laser pulses with gold nanoparticle the intensity of the plasmon absorption band decreases and the spectral width increases. Numerical simulations of heating of the electron gas and the lattice of a nanoparticle under the influence of the femtosecond laser pulses and corresponding changes of gold dielectric function taking into account interband transitions are presented. It is shown both experimentally and theoretically that the rise of the relaxation time of the optical properties from 2 ps to 10 ps with increasing the excitation pulse power density from 0.2 mJ/cm2 to 1.2 mJ/cm2 is caused by the change of the heat capacity of the electron gas of the gold nanoparticle. Under high-intensity laser irradiation gold nanorods have also irreversible changes of the absorption spectrum and hence the colour of the solution. TEM images show partial transformation of nanoparticles from cylindrical to spherical shape.