Assembling of Silicon Nanoflowers with Significantly Enhanced Second Harmonic Generation Using Silicon Nanospheres Fabricated by Femtosecond Laser Ablation

Silicon (Si) nanospheres (NSs) with diameters ranging from about 10 to 100 nm were fabricated by using femtosecond (fs) laser ablation of a silicon wafer immersed in deionized water. Si nanoflowers (NFs) looking like snowflakes were assembled by dropping and drying the colloid solution on a glass slide. Transmission electron microscope observation revealed that Si NFs were composed of self-assembled Si NSs with different sizes. The nonlinear optical responses of both single Si NSs and Si NFs were examined by using a focused fs laser at similar to 800 nm. While only second harmonic generation (SHG) with weak intensity was observed for single Si NSs, a significant enhancement in SHG was found for Si NFs. More interestingly, both the Stokes and anti-Stokes components of the Raman scattering of the SH were also revealed in the nonlinear response spectra of Si NFs, possibly due to the large enhancement in SHG. The electric field distributions were numerically simulated by using the finite-difference time-domain technique for single Si NSs and corresponding aggregates composed of seven closely packed NSs at the wavelengths of both the fundamental light and the SH. It was revealed that the significant enhancement in electric field achieved in the aggregates of Si NSs is responsible for the strong SHG observed in Si NFs.