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 ∼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.

We investigate numerically the modification of the nonlinear optical properties of a nanoantenna in the trapping of nanoparticles (NPs) by using both the discrete dipole approximation method and the finite-difference time-domain technique. The nanoantenna, which is formed by two gold nanorods (GNRs) aligned end to end and separated by a small gap, can emit strong two-photon-induced luminescence (TPL) under the excitation of a femtosecond laser light which is resonant with its longitudinal surface plasmon resonance. In addition, the excited antenna can stably trap small NPs which in turn induce modifications in the emitted TPL. These two features make it a promising candidate for building highly sensitive detectors for NPs of different materials and sizes. It is demonstrated that sensors built with antennas possess higher sensitivities than those built with single GNRs and nanorodbased antennas are more sensitive than nanoprism-based antennas. In addition, it is found that the trapping probability for a second NP is significantly reduced for the antenna with a trapped NP, implying that trapping of NPs may occur sequentially. A relationship between the TPL of the system (antenna + NP) and the optical potential energy of the NP is established, enabling the extraction of the information on the optical potential energy and optical force by recording the TPL of the system. It is shown that the sequential trapping and releasing of NPs flowing in a microfluid channel can be realized by designing two different antennas arranged closely.