THORLABS OTM211, User Manual

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OTM211 Chapter 3: Description
Rev A, October 7, 2015
Page 5
Chapter 3 Description
3.1. Introduction
The Thorlabs OTM211 is an optical tweezers system designed for inverted microscopes. It provides two
independent trapping beams that allow the manipulation of nano- and micro particles. Each path can be used to
create several additional traps. The system is completely computer controlled and offers a graphical user interface
which provides the user with a set of tools for the most common trapping experiments. The detector module included
in the OTM211 allows users to perform quantitative force measurements and particle tracking. The included
Software Development Kit enables users to easily create applications optimized for their specific needs and allows
integration into more complex setups.
The OTM211 system integrates with existing imaging modalities, such as fluorescence, Raman, confocal, phase
contrast. Further on the system is compatible with all major microscope platforms.
3.2. Theory
Optical Tweezers, or traps as they are often called, are created by using a high numerical aperture objective to
tightly focus a laser beam thereby creating a spot where a dielectric particle with dimensions of the order of 0.3µm
to a few microns will experience a force due to transfer of momentum from the scattering of photons.
Arthur Ashkin in the early 1970s originally demonstrated that optical forces can manipulate micron-sized dielectric
particles in water (
Ashkin et al, Optics Letters, Vol 11, No. 5, may 1986
). This technique has become an
important tool in a wide range of fields such as bioengineering, material science, and physics due to their ability to
hold and manipulate micron-sized particles and to measure forces in the piconewton and femtonewton ranges.
Figure 1 Theory of Optical Trapping
The capability of optical tweezers to exert measurable forces on dielectric particles offers a unique and valuable
tool for studying cell components such as biological proteins and molecular motors. In many investigations, optical
tweezers apply force to functionalized microspheres that have been attached to molecules of interest. Optical
tweezers apply a force toward the focus of the trapping laser beam with a magnitude proportional to the distance
of the particle from the focus, for small displacements from the center of the trap. This allows the optical tweezers
to be modeled by Hooke’s law,
F = -kd
, where
d
is the displacement from the center of the trap and
k
is the force
constant. As the laser beam passes through a trapped particle, it will be deflected by an amount that depends on
the position of the particle. Accurate force measurements depend on precise calibration of the spring constant,
k
,
and the particle position which varies with laser power and particle properties.
Three methods for ascertaining the force constant,
k
are: Equipartition, PSD Rolloff and Stokes’ drag. Since each
method relies on a different physical principle, the combined results provide a convenient way to verify the
calibration.