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Portable Optical Tweezers Kit Chapter 8: Teaching Tips
Page 78 MTN012639-D02
Unfocused Off-Axis Laser Beam Total Change in Momentum
Figure 62 demonstrates that the laser beam experiences a momentum change to the left
and upwards. This results in a momentum change of the bead to the right and downwards.
Therefore, the bead is drawn to the position with maximum intensity, forming a two-
dimensional trap. The bead, however, is still pushed downwards which means that it is not
trapped in once place.
Focused Laser Beam, Particle on the Beam’s Main Axis Below the Laser Focus:
We will next consider a focused laser beam. The particle is assumed to be in the center of
the beam but below the focus of the laser. The laser is assumed to be highly focused which
implies that the incident partial beams are no longer parallel.
Figure 63 depicts two partial beams of the focused laser. The center of the bead is
assumed to be below the focus.
Focused Laser Beam Incident on a Polystyrene Bead