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Portable Optical Tweezers Kit Chapter 8: Teaching Tips
Page 80 MTN012639-D02
The net momentum change of the entire beam is depicted in Figure 66.
Focused Laser Beam Change in Momentum
The laser beam exhibits a momentum change downwards. Hence, the bead’s momentum
is changed upwards. T
he bead moves against the beam’s main propagation direction and
into the laser focus. Analogously, one can show that the bead is pushed downwards when
the laser focus is beneath it.
In essence, a single laser beam can create an ideal three-dimensional optical trap. One
thing is crucial for functioning optical tweezers: in order to trap an object in a three-
dimensional trap, a highly focused laser beam with a suitable intensity profile is required.
The object is always pushed in direction towards the point with highest intensity:

Radially into the middle of the laser beam due to the (Gaussian) intensity profile

Vertically into the direction of the focus
As soon as a particle moves away from the focus, a restoring force acts on it to push it
back into the focus.