Low-order nonlinearities and high-order harmonics generation in Zn and ZnSe nanoparticles synthesized during femtosecond laser ablation at a 50  kHz pulse repetition rate

We synthesize Zn and ZnSe nanoparticles during laser ablation of bulk material in ethanol and analyze them using the Z-scan technique and the generation of high harmonics using a 50 kHz, 40 fs, and 1030 nm laser. The low-order nonlinear absorption and nonlinear refraction coefficients in the colloidal suspensions of the formed nanoparticles were measured using 1030 nm and 515 nm probing laser pulses, respectively. The values of the nonlinear refractive index and nonlinear absorption coefficient of Zn NPs at 1030 nm wavelength were calculated to be γ = − 1.4 × 10 − 13 c m 2 W − 1 and β = 2.7 × 10 − 11 c m W − 1 , respectively. Similarly, in the case of ZnSe NPs, γ = − 4.9 × 10 − 14 c m 2 W − 1 and β = 0.5 × 10 − 11 c m W − 1 , respectively. In the case of 515 nm probing pulses, the corresponding nonlinear optical parameters of Zn NP suspension were calculated to be γ = − 1.7 × 10 − 14 c m 2 W − 1 and β = 1.0 × 10 − 11 c m W − 1 , and for ZnSe NP suspension, the value of β was calculated to be 2.1 × 10 − 11 c m W − 1 . The nonlinear refractive index of this suspension was γ = − 2.4 × 10 − 14 c m 2 W − 1 . We also report the studies of high-order harmonics generation in plasmas containing Zn and ZnSe NPs using single- (1030 nm) and two-color (1030 nm + 515 nm) laser fields. The maximal generated extreme ultraviolet (XUV) harmonics were 33rd (Zn NPs) and 27th (ZnSe NPs) orders. The decrease of the maximal generating harmonic orders was explained using the three-step model. The high brightness of the ninth and 10th harmonics generated in the two-color laser field was obtained due to the resonance properties of Zn NPs. The maximal generated XUV harmonics were the 33rd (Zn nanoparticles) and 27th (ZnSe nanoparticles) orders.