Third harmonic generation on silicon surface induced by femtosecond laser

The emission spectra of silicon (Si) surface have been studied by a Ti: sapphire femtosecond laser amplification system. The third optical harmonic and plasma emission were detected. The third harmonic generation (THG) is sensitive to the laser fluence and negatively correlated with the fluence. The nonlinear loss on the THG conversion efficiency at high laser fluence is mainly due to the two-photon absorption (TPA) effect. The micro-nanoparticles around the ablated area increase with decreasing laser fluence, which may be one of the reasons for the enhancement of the THG signal. Compared with the theoretical calculation of the THG (266.67 nm), the slight spectral blue-shift (<2 nm) can be attributed to the nonlinear optical processes of the self-phase modulation.