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Philipp Rauch, Torsten Jähnke, Optical Tweezers for Quantitative Force Measurements and Live Cell Experiments, Microscopy Today, Volume 22, Issue 4, 1 July 2014, Pages 24–31, https://doi.org/10.1017/S1551929514000741
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Introduction
Optical tweezers employ light to manipulate objects. This manipulation takes place on the microscopic scale, allowing interrogation of small objects such as individual cells, cell compartments, single nanoparticles and (bio)molecules. Optical tweezers have a successful history starting from the first attempts to stably trap microparticles with dual counter-propagating beam setups in the early 1970s by Arthur Ashkin and colleagues [1]. Instrumental developments since that time [2–6] have led to the sophisticated turnkey systems of today. Up to now, optical tweezers have been mostly used by physicists able to design and construct an instrument from scratch. JPK has developed the NanoTrackerTM 2, a compact, table-top instrument [7] that brings the technology to a broader range of scientists.
What are optical tweezers?
The forces generated in an optical trap stem from the change of direction that tightly focused laser light undergoes when it is refracted at the interface of two optically different materials. The basic geometry is shown in Figure 1 where a spherical particle (nparticle>nmedium) is placed in a focused beam that establishes a gradient of light intensity with cylindrical symmetry. Photons hit the particle surface at an angle and are deflected toward the vertical optic axis. Their momentum changes are transferred to the particle in the opposite direction. When the particle is displaced from the trap center, it is no longer in a stable position within the symmetric intensity gradient potential, and more momentum is transferred on the side facing the trap center. Thus, so-called gradient forces arise that drive the particle back to the trap center. Backscattering of photons gives rise to a second force pointing in the direction of light propagation that superimposes on the gradient force. As long as the intensity gradient is strong enough, for example, by the use of high numerical apertures, the gradient forces exceed the scattering force and the particle is stably trapped.
