Strong third-order optical nonlinearities of Ag nanoparticles synthesized by laser ablation of bulk silver in water and air
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We demonstrate the correlation between strong nonlinear optical response of silver nanoparticles (Ag NPs) out of plasmon resonance and efficient third harmonic generation in the plasmas containing Ag NPs. The dynamics of nonlinear optical response of the 8 nm and 50 nm Ag NPs prepared by laser ablation of bulk silver in deionized water using 6 ns, 200 ps, and 60 fs laser pulses is systematically analyzed. Their optical limiting properties are studied at the wavelengths of 800 and 355 nm, using femtosecond and nanosecond laser pulses. Nonlinear absorption coefficient of 8 nm Ag NPs at the wavelength of 1064 nm was measured to be as high as 3 × 10−5 cm W−1. Nonlinear refraction shows the change of sign with variation of the wavelength and duration of probe laser pulses. The theoretical calculation of silver ablation shows the formation of a few nanometer sized particles. We also analyze third harmonic generation in the laser-produced plasmas containing Ag NPs and attribute the enhancement of this process to the influence of small silver clusters. The conversion efficiency of 800 nm towards the third harmonic was measured to be 4 × 10−3, which was a few times larger compared with similar process in air.
RAG thanks the financial support from Chinese Academy of Sciences President’s International Fellowship Initiative (Grant no. 2018VSA0001).
Natural Science Foundation of China (61774155, 61705227), National Key Research and Development Program of China (2017YFB1104700).
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