Strong third-order optical nonlinearities of Ag nanoparticles synthesized by laser ablation of bulk silver in water and air

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 8nm and 50nm Ag NPs prepared by laser ablation of bulk silver in deionized water using 6ns, 200ps, and 60fs laser pulses is systematically analyzed. Their optical limiting properties are studied at the wavelengths of 800 and 355nm, using femtosecond and nanosecond laser pulses. Nonlinear absorption coefficient of 8nm Ag NPs at the wavelength of 1064nm was measured to be as high as 3x10(-5)cmW(-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 800nm towards the third harmonic was measured to be 4x10(-3), which was a few times larger compared with similar process in air.