1. OPTICAL
TWEEZERS
A bright shining tool
Floor van de Pavert

2. Outline Introduction and history How do optical tweezers work?
Applications
- Multipe traps
- Ring trap
Conclusion

3. Introduction and history
Particles (in size ranging from
nm to μm) can be trapped in
a tightly focused laser beam.
OT originally developped by
Arthur Ashkin (Bell Labs)
Ashkin in 1988
1970s: Slightly focused laser beam for 2D confinement (balanced by gravity or two counter-propagating beams)
1986: Single, highly focused laser beam for 3D confinement (single beam gradient trap)
Now used in many areas of research and commercially available as well

4. How do optical tweezers work?
Momentum transfer due to interaction with light
The optical forces can be decomposed in a gradient and a scattering force
Ray Optics (R >> λ)
(refraction of light rays)
Simple physical descriptions
when particle is much smaller
or much larger then wavelength
Rayleigh particles (R << λ )
(dipoles in an EM-field)
Picture taken from
Stiffness optical trap scales linearly with laser power

5. Applications
Widely used research tool in molecular biology, physical chemistry and soft condensed matter physics
Confinement and organization, tracking of moving particles, manufacturing structures, application and measurement of forces in the pN range.
10 μm

6. Applications
OT are especially useful for the study of physical properties of DNA and molecular motors
Bead is attached to
biomolecule as a ‘handle’
By moving the bead,
the DNA can be streched
books.nap.edu/books/ni000666/html/25.html

7. Application: multiple traps
Multiple optical traps can be created by:
Time sharing a single focused laser
(rapidly scanning from point)
 using acousto-optic deflectors
enables fast scanning and changing
patterns in time
Vossen et al.
Rev. Sci. Instrum. 75, 2960 (2004)
10 mm
A diffractive beam splitter
 using a holograph enables
creating arrays in 3D
2μm silica spheres trapped at the lattice cites of a diamond unit cell

8. Application: multiple traps
Some applications of arrays of OT :
Molecular sieves
Colloidal interaction measurements
Inducing phase transitions in colloidal suspensions
Mixture of tracer- and host
particles, host particles are
unaffected by the trap
When lattice spacing of
the template is small enough,
host particles start to
crystallize on it
10 µm
Thesis of D.L.J. Vossen, Utrecht University

9. Application: ring trap
Normally TEM00 mode is used
Mode can be modified with a helical phase profile
Focusing creates a ring shaped optical trap
Particles in the trap show circulation along the ring

10. Conclusion
Optical tweezers are a versatile research tool with a variety of interesting applications!

11. Questions?

12. Characteristics of optical traps
Forces are linearly related to the object displacement.
The slope of the force-displacement curve is called the stiffness of the optical trap (in N/m).
The stiffness dependence on the bead size and shape and the laser power.

13. z-confinement in line tweezers
from: McCann et al., Nature 1999