Gold films are widely used for different applications. We present the results of third- and high-order nonlinear optical studies of the thin films fabricated from Au nanoparticle solutions by spin-coating methods. These nanoparticles were synthesized by laser ablation of bulk gold in pure water using 200 ps, 800 nm pulses. The highest values of the nonlinear absorption coefficient (9 × 10⁻⁶ cm W⁻¹), nonlinear refractive index (3 × 10⁻¹¹ cm² W⁻¹), and saturation intensity (1.3 × 10¹⁰ W cm⁻²) were achieved using 35 fs, 400 nm pulses. We also determined the relaxation time constants for transient absorption (220 fs and 1.6 ps) at 400 nm. The high-order harmonic generation was studied during propagation of 35 fs, 800 nm pulses through the plasma during the ablation of gold nanoparticle film and bulk gold. The highest harmonic cutoff (29th order) was observed in the plasma containing gold nanoparticles.

In the past, common media for high-order harmonic generation (HHG) has been atoms and molecules. More recently, clusters, and nanoparticles have been introduced as HHG emitting media. Multi-particle media can enhance HHG yields but have more stringent requirements in determining the optimal parameters. Here, we demonstrate, for the first time, the effective application of 1-3 nm metal sulfide quantum dots (QDs) for harmonic generation in the 20 – 115 nm extreme ultraviolet range. We report on the syntheses, ablation of Ag₂S, CdS, and ZnS QDs, and HHG from laser-produced plasmas by using single- and two-color pumps. We compare HHG efficiency from the ablated QDs to that of bulk metal sulfides and show a seven-fold increase in harmonic yields. Further, the study also allows us to understand the effects of QD-contained plasma spreading dynamics on HHG yield.

As high-order harmonic emitters, quantum dots are produced through laser-induced plasmas. Subsequently, we generate high-order harmonics with 800-nm and 30-fs pulses from laser-produced plasmas containing quantum dots of different metal sulfides (Ag₂S, CdS, and Cd_0.5Zn_0.5S). The high-order harmonic generation is analyzed using different approaches, including two-color (800 nm + 400 nm) pump, application of alloyed quantum dots, and quasiphase matching of interacting waves. We discuss the self-phase modulation induced splitting of harmonics, the difference in the application of thick and thin crystals for second harmonic (400 nm) emission during two-color pumping of the quantum dot plasma, the spatial modulation of the quantum dot plasma for quasiphase matching, and the comparison of harmonic yields from monomer and quantum dot plasmas. This study allows us to determine the mechanisms of coherent extreme ultraviolet radiation generation using a few nanometer-sized emitters as well as optimal methods for further enhancing the high-order harmonic generation efficiency.