Spontaneous and stimulated Raman scattering from surface phonon modes in aggregated SiO2 nanoparticles

A spontaneous and stimulated Raman scattering from the surface phonon modes (SPMs) in aggregated amorphous silica (SiO2) nanoparticles is reported for the first time. The SPMs are located between Raman peaks corresponding to the amorphous silica bulk transverse-optical (TO) and longitudinal-optical CLO) bending phonon modes. An anomalously large TO-LO splitting (similar to 185 cm(-1)) of bending modes for bulk amorphous silica was observed. Six different SPA(ls corresponding to elliptical nanoparticles in two mediums of different effective dielectric constants were detected by the spontaneous Raman scattering spectra. One can obtain a good agreement with the theory far SPMs by assuming the anomalously large TO-LO splitting of bending modes in nanoparticles of the amorphous SiO2. The stimulated Raman scattering from SPMs was observed under second harmonic excitation of the YAG:Nd pulsed laser. The threshold value was estimated as similar to 60 MW cm(-2).