OPTICAL TWEEZERS A bright shining tool Floor van de Pavert

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

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

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 www.st-andrews.ac.uk Stiffness optical trap scales linearly with laser power

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. www.uibk.ac.at/medphysik 10 μm

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

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 www.ieee.org 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

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

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 http://www.uibk.ac.at/medphysik/optical_traps/rotating_beads.mpg

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

Questions?

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.

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