Hyper-Rayleigh and hyper-Raman scattering from silver nanoparticles trapped by a near-infrared laser beam

When near-infrared laser was focused in colloidal silver, not only optical trapping of silver nanoparticles, but also hyper-Rayleigh scattering were observed at the laser focus. By addition of NaCl and rhodamine 6G in the colloidal silver, hyper-Rayleigh scattering was enhanced and hyper-Raman scattering was also observed. These nonlinear optical responses showed temporal fluctuation in spite of the continuous laser irradiation. Experimental results suggested that aggregates which have a high activity for nonlinear optical responses were trapped and/or produced by a focusing near-infrared laser. Hyper-Raman signal, whose scattering cross section is much lower than normal Raman scattering, could be obtained easily by focusing a cw-YAG laser in colloidal silver including analytes. It was demonstrated that optical trapping of colloidal silver is a powerful technique to obtain the nonlinear optical responses.