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Portable Optical Tweezers Kit Chapter 7: Teaching Tips
Page 74
MTN024417-D02
Chapter 7 Teaching Tips
To engage students, it is particularly helpful to point out that they have a Nobel winning
experiment in front of them! 48 years after his paper entitled “Acceleration and Trapping
of Particles by Radiation Pressure” (Physical Review Letters, 24, No. 4, Jan. 26, 1970 (pp.
156–159)), Arthur Ashkin was awarded the Nobel prize “for the optical tweezers and their
application to biological systems”. This emphasizes the profound meaning this technology
has in fundamental research. Students may also enjoy the Nobel lecture on optical
tweezers, found via:
https://www.nobelprize.org/prizes/physics/2018/ashkin/lecture/
In this chapter we present a simplified approach to help students in understanding the
basic physical principles of optical tweezers. In the following, we discuss how a particle is
trapped in a focused laser beam and how a three-dimensional optical trap works.
For keeping an object in place, a restoring force has to act as soon as the object leaves
its equilibrium position. Since the object is trapped only by the laser, the restoring force
has to be applied by the laser itself.
An essential precondition for trapping an object is that it is transparent to the laser light.
This means that at least part of the light has to be transmitted. Part of the laser light is
reflected off the object’s surface, while the other part is refracted and transmitted. Thereby,
the laser faces a change in direction which corresponds to a change in the momentum.
This in turn means that there has to be a force between the laser light and the object.
Intuitively, one would expect the particle to be deflected. However, in an optical trap the
object is drawn into the laser focus.
The laser beam exhibits a certain intensity profile. In order to create an optical trap, this
profile needs to possess a point with maximal intensity, such as in a Gaussian profile.