A simple method to trap and manipulate metallic micro/nano-particles on the surface of photorefractive crystals is proposed. After inducing inhomogeneous charge density and space-charge fields in photorefractive crystals by non-uniform illumination, both uncharged and charged metallic particles can be trapped on the illuminated surface due to dielectrophoretic force and electrophoretic force, respectively. A transition from dielectrophoresis to electrophoresis is observed when manipulating nano-silver particles with high surface space-charge field. Our results show that this method is simple and effective to form surface microstructures of metallic particles.
We experimentally show that the generation and erasure of femtosecond laser-induced periodic surface structures on nanoparticle-covered silicon inducted by irradiation with a single laser pulse (800 nm, 120 fs, linear polarization) depend on the pulse fluence. We propose that this is due to competition between periodic surface structuring originating from the interference of incident light with surface plasmon polaritons and surface smoothing associated with surface melting. Experimental results are supported by theoretical analysis of transient surface modifications based on combining the two-temperature model and the Drude model.
Generation and evolution of plasma during femtosecond laser ablation of silicon are studied by steady-state and time-resolved spectroscopy in air, N2, SF6, and under vacuum. The plasma is generated faster than 200 ps (time resolution of our experiment) after excitation and mainly contains atoms and monovalent ions of silicon. Time-resolved spectra prove that silicon ions are faster than the silicon atoms which may be attributed to Coulomb repulsion and a local electric field when they are ejected from the silicon surface. During plasma evolution, ambient gas causes a confinement effect that enhances the dissociation of ambient gas molecules and the re-deposition of the removed material and leads to higher intensity and longer lifetime of the emission spectra. In SF6, a chemical reaction increases the plasma density and weakens the re-deposition effect. The different processes during plasma evolution strongly influence microstructure formation.
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