Abstract:In this work we demonstrate optical trapping and manipulation of microparticles suspended in water due to laser-induced convection currents. Convection currents are generated due to laser light absorption in an hydrogenated amorphous silicon (a:Si-H) thin film. The particles are dragged towards the beam's center by the convection currents (Stokes drag force) allowing trapping with powers as low as 0.8 mW. However, for powers >3 mW trapped particles form a ring around the beam due to two competing forces: Stokes drag and thermo-photophoretic forces. Additionally, we show that dynamic beam shaping can be used to trap and manipulate multiple particles by photophotophoresis without the need of lithographically created resistive heaters.

We report the dynamics of dissipative solitons in a ring cavity passively mode-locked fiber laser with a strict control of the polarization state. We study the relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. We have found that at pulse ellipticities between 5° and 15°, the laser generates one bunch of pulses in the cavity, while at higher ellipticities the laser generates multiple bunches. At constant ellipticity we rotated the polarization azimuth and observed a regime transition from the generation of noise-like pulses (NLP) to that of soliton crystal. The NLP regime was found when the azimuth was rotated towards smaller low-power transmission through the polarizer. The number of solitons in the soliton crystal also depended on the azimuth in a straightforward way: the higher the initial transmission, the bigger the number of solitons.

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An experimental and theoretical study about selective photodeposition of metallic zinc nanoparticles onto an optical fiber end is presented. It is well known that metallic nanoparticles possess a high absorption coefficient and therefore trapping and manipulation is more challenging than dielectric particles. Here, we demonstrate a novel trapping mechanism that involves laser-induced convection flow (due to heat transfer from the zinc particles) that partially compensates both absorption and scattering forces in the vicinity of the fiber end. The gradient force is too small and plays no role on the deposition process. The interplay of these forces produces selective deposition of particles whose size is directly controlled by the laser power. In addition, a novel trapping mechanism termed convective-optical trapping is demonstrated.