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When collimated laser beams are focused, small particles with diameters ranging from submicrometers to tens of micrometers can be trapped. This action is explained by the conservation of momentum. The Figure shown at the right explains trapping along z-axis. When a beam enters a bead made of dielectric materials, the light changes direction, creating a change in the momentum. Since momentum is conserved for the whole system, this produces an equal but opposite change of momentum in the bead.
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| In the case of the left image where the bead locates (O is the center) upstream of orginal laser focus (F1), the light comes in and focuses to a new pistion (F2). It seems that the light is "held back" by the bead, as a result of conservation in momentum, the bead experiences a pulling force towards the new focus F2. In the case of the right image where the bead (O) locates behind the original focus, the change in the path of the light entering the bead results in a new focus (F2) behind the original focus (F1) where the bead is abscent. In this case, the beam is "pushed ahead" by the bead and the bead is held back towards F2. Similar arguments can explain the focusing effects at the x-y plance. |
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