Introduction to Optical Trapping Science and Technology

November 7, 2017 | Author: Felix Lu | Category: Scattering, Radiation, Physics & Mathematics, Physics, Materials Science
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Introduction to the physics of optical trapping, overview of optical tweezer vendors, tradeoffs of different approaches...


Introduction to biological applications of optical trapping science and technology Felix Lu Duke University March 29, 2007

Outline 1. What is Optical Trapping (OT)? 2. History of optical trapping 3. Use of OT in Bio applications 4. Optical Trapping Physics 5. Optics for a moveable trap 6. The technology of optical trapping 7. Commercial Optical trapping systems

Typical size of a non-commercial OT setup

8. Opportunities for improvement 9. Summary


What is Optical Trapping (OT)? Mesoscopic scale – • 10’s of nm – 100’s of µm, • forces from femtonewtons - nanonewtons, • time scales ranging from a microsecond and up. 0Introduction.htm

Trapped dielectric spheres in optical vortices p2b/

Trapped : dielectric spheres, viruses, bacteria, living cells, organelles, small metal particles, and even strands of DNA.

Bacteria et.htm


History of optical trapping Timeline 1958 Invention of Laser, 1970 Arthur Ashkin levitates micron sized particles, Bell Labs, 1986 Demo of Optical tweezers 1987 Ashkin manipulates live bacteria and viruses 1989 OT used to manipulate live sperm Block&Berg measure stiffness of bacterial flagella 1990 Steven Chu reports manipulation of DNA using OT

Undated photo – Arthur Ashkin

1991 Karl Greulich and colleagues use optical tweezers to isolate individual chromosomes 1993 J. J. Krol develops multiple trap system 1993…a whole bunch of biology stuff I don’t know how to appreciate…2004 2004 Block builds first combination optical trap/fluorescence instrument Adapted from article by Don Monroe, “The Optical Trap”, The Scientist 2005, 19(16):48

4 A. Ashkin, Phys. Rev. Lett. 24, 156 (1970)

Uses of Optical Trapping in biological applications In Biology: Intra- and inter cellular processes responsible or respiration, reproduction and signaling. • confinement and organization (e.g. for cell sorting), • tracking of movement (e.g. of bacteria), • application and measurement of small forces, • altering of larger structures (such as cell membranes).

Cell sorting

Lab on a chip Driving of micromotors Microsurgery Localized heating

Membrane deformation

Mesoscopic force measurements search/hubmayr/strain_laser.cfm


Optical Trapping Physics The intuitive view • • •

Scattering force (in direction of light propagation) Gradient force (in direction of light intensity gradient) Trapping stable when gradient force > scattering force. (Fallman 97) If particle has larger index than medium

Gaussian light beam Maximize Gradient Force

Dielectric particle

Gradient force Scattering force


focus beam using a high NA objective lens. Scattering force

After Dholakia (2002)

The single most important element of the OT is the objective lens used to focus the trapping laser. 6

Optical Trapping Physics Limiting cases for force calculations Object >> λ : ray optics calculations Radiation pressure from refracted rays trap particle.

Object Schek, III, and Alan J. Hunt, “Advanced Optical Tweezers for the study of cellular and molecular biomechanics”, IEEE Trans. On Biomed Eng, vol 50 no 1, Jau 2003, p 121

* Trading off diffraction efficiency


Aberrations and tolerance Small particles (~1 µm) are more strongly affected by beam aberrations. Trap performance defined as mean squared displacement of a corrected trap vs. uncorrected trap. Beam aberrations corrected by holographic SLMs



0.8 µm

2 µm

5 µm

Improvement for small particles is greater Red ellipses indicate the rms displacement Images after Wulff et al. (2006)


Commercial Optical trapping systems -- Cell Robotics -

OT work stations Add-on modules for existing microscopes. Laser scissors ® system : Cell Robotics Premier workstation

shorts bursts of UV light from a nitrogen laser to ablate specimen parts with submicron precision.


Commercial Optical trapping systems -- Arryx -

Sample layout of OT system using a holographic SLM

Up to 200 beams can be independently manipulated using the holographic SLM.

BioRyx® 200 Optical Trapping System

Reference on Holographic optical tweezers:


Commercial Optical trapping systems -- P.A.L.M. Microlaser Technologies –

Laser Microdissection and Pressure Catapulting (LMPC) technology How does P.A.L.M.'s LMPC-technology function? An UV beam is focused and used to cut out specimen. The isolated specimens are ejected out of the object plane and catapulted directly into a sample holder with the help of a single defocused laser pulse and can be "beamed" several millimeters away, even against gravity.


Opportunities for improvement Beam steering system • AOD’s – Very fast but have relatively small addressable area – Power fluctuation with trap movement increases instability • Holographic SLMs are very flexible

but have a slow update (5-10 Hz). • Current* systems do not have simultaneous Multi-beam, multi-color capability. •Gary J. Brouhard, Henry T. Schek, III, and Alan J. Hunt, “Advanced Optical Tweezers for the study of cellular and molecular biomechanics”, IEEE Trans. On Biomed Eng, vol 50 no 1, Jau 2003, p 121

* As far as I can tell from the literature, March 29, 2007


Summary • Optical trapping is a tool that can be used to manipulate microscopic objects. • The objective lens with a high NA is the most critical element in the system. • Beam types and beam steering elements add flexibility to the system. • Drawbacks to current implementations of OT systems include tradeoffs among optical power, speed, aberrations, and addressable area. • Using a tilting mirror based system is acknowledged to be an efficient approach to improving system characteristics. [Ref 4] 17

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Keir C. Neuman and Steven M. Block, “Optical Trapping”, Review of Scientific Instruments, Vol. 75, no. 9, Sept 2004 David McGloin, “Optical Tweezers: 20 years on”, Philosophical Transactions of the Royal Society A (2006) 364, 3521-3537 Erik Fallman and Ove Axner, “Design for fully steerable dual-trap optical tweezers”, Applied Optics, Vol 36, No. 10, April 1997 Erik Fallman, Magnus Andersson, and Ove Axner, “Techniques for moveable traps: the influence of aberration in optical tweezers”, Keynote paper, Proc. of SPIE vol. 6088 David G. Grier, “A Revolution in Optical Manipulation” David McGloin, Veneranda Garces-Chavez, and Kishan Dholakia, “touchless tweezing”, spie’s OE magazine, January 2003 Justin E. Molloy and Miles J. Padgett, “Lights, Action: Optical tweezers”, Kishan Dholakia, Gabriel Spalding, and Michael Macdonald, “Optical Tweezers: the next generation”, Physics World, October 2002 Bradley A. Brown and Phyllis R. Brown, “Optical tweezers: Theory and current applications”, American Laboratory, November 2001, page 13 Kurt D. Wulff, Daniel G. Cole, Robert L. Clark, Roberto DiLeonardo, Jonathan Leach, Jon Cooper, Graham Gibson and Miles J. Padgett, “Aberration correction in holographic optical tweezers”, Optics Express 1 May 2006 vol. 14, no. 9 p 4169


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