Optical TweezersOptical Tweezers

Sarah NicholsSarah Nichols

March 17, 2004March 17, 2004

OutlineOutline

•Motivations•Operation•Layout•Successes and Difficulties•Future Directions

Why use optical Why use optical tweezers?tweezers? Can manipulate living cells and organellesCan manipulate living cells and organelles Can measure stretching of large molecules Can measure stretching of large molecules

such as DNAsuch as DNA Measure and manipulate mechanoenzymes Measure and manipulate mechanoenzymes

and other physically acting moleculesand other physically acting molecules

Relatively cheap and easy to build a simple Relatively cheap and easy to build a simple setupsetup

Useful tool for learning various optics Useful tool for learning various optics procedures and termsprocedures and terms

How it worksHow it works

ddobjectobject>>d>>dbeambeam -> ray optics -> ray optics approximation is validapproximation is valid

Reflection and refraction Reflection and refraction create opposing forces on create opposing forces on a dielectric objecta dielectric object

If the beam is strong If the beam is strong enough, the gradient enough, the gradient force can overcome the force can overcome the scattering and scattering and gravitational forcesgravitational forces

Rays from edge of lens Rays from edge of lens are deflected most, so are deflected most, so they have the biggest they have the biggest trapping impacttrapping impact

Amendola, 2001

ComponentsComponents 20 mW diode laser20 mW diode laser 2 cylindrical lenses for 2 cylindrical lenses for

shapingshaping Mirrors for redirectionMirrors for redirection 2 spherical lenses for 2 spherical lenses for

resizingresizing Periscope to raise the Periscope to raise the

beambeam Dichroic mirror to direct Dichroic mirror to direct

beam into microscopebeam into microscope Microscope with 100x oil Microscope with 100x oil

immersion objective, immersion objective, eyepiece removedeyepiece removed

Camera connected to TV Camera connected to TV screen for viewingscreen for viewing

Laser Cylindrical lenses

M1

Spherical lenses

M2

Periscope

Microscope

Infinity-correcting lens

LayoutLayout

LayoutLayout

Camera

Filter

Dichroic mirror

Beam block

Microscope

OptimizationOptimization

Beam must be aligned to travel exactly Beam must be aligned to travel exactly vertically through microscopevertically through microscope

For best trapping, beam should be sphericalFor best trapping, beam should be spherical Optimal beam size overfills the objective Optimal beam size overfills the objective

slightly FWHM 6 mm for our setup (Amendola, slightly FWHM 6 mm for our setup (Amendola, 2001)2001)

Microscope objective does not collect light Microscope objective does not collect light from infinity, so a lens should focus light 160 from infinity, so a lens should focus light 160 mm behind the objectivemm behind the objective

Trapping most easily achieved with moving Trapping most easily achieved with moving objects; surfactants used in bead solutionsobjects; surfactants used in bead solutions

ResultsResults

Despite numerous alignments, beam was Despite numerous alignments, beam was weak until very recentlyweak until very recently

Objective replacement improved beam Objective replacement improved beam strengthstrength

Weak trapping demonstrated for yeast cells (~ Weak trapping demonstrated for yeast cells (~ 5 um) as well as 5 and 10 um polystyrene 5 um) as well as 5 and 10 um polystyrene spheresspheres

With small yeast cells, can drag horizontally With small yeast cells, can drag horizontally and verticallyand vertically

ResultsResults

Untrapped 5 um spheres Trapped sphere

QuestionsQuestions

Why is it easier to trap with yeast cells for a Why is it easier to trap with yeast cells for a

given size? (they exhibit more Brownian given size? (they exhibit more Brownian

motion, but why?)motion, but why?)

Why does overfilling rule not hold? “Optimal” Why does overfilling rule not hold? “Optimal”

FWHM size: 6 mm. Measured size: 6-6.5 mm. FWHM size: 6 mm. Measured size: 6-6.5 mm.

Trapping size: 4 mm.Trapping size: 4 mm.

Why does non-infinity correction make Why does non-infinity correction make

trapping harder?trapping harder?

Suggested Suggested ImprovementsImprovements Clean lenses, replace mirrorsClean lenses, replace mirrors Replace diode laser for a less elliptical beamReplace diode laser for a less elliptical beam Determine reason for cell/sphere distinctionsDetermine reason for cell/sphere distinctions Mount a lens on a translation stage for beam Mount a lens on a translation stage for beam

steering with less intensity losssteering with less intensity loss Determine reason for failure to agree with Determine reason for failure to agree with

predictions - pure intensity loss or other predictions - pure intensity loss or other reasons?reasons?

Upgrade camera for digitized still and motion Upgrade camera for digitized still and motion picturespictures

AcknowledgementsAcknowledgements

Thanks to Dr. John Noé and Yiyi Deng for their Thanks to Dr. John Noé and Yiyi Deng for their help in adjusting to the vagaries of the system. help in adjusting to the vagaries of the system. Thanks also to Duke Scientific for their donation Thanks also to Duke Scientific for their donation of microspheres to the Laser Teaching Center for of microspheres to the Laser Teaching Center for testing.testing.

ReferencesReferencesP. Amendola. “Design and Construction of an Optimized Optical P. Amendola. “Design and Construction of an Optimized Optical Tweezers.” Intel Science Talent Search Report (2001).Tweezers.” Intel Science Talent Search Report (2001).

Y. Deng, “Optical Tweezers”, http://laser.physics.sunysb.edu/~yiyi (2003).Y. Deng, “Optical Tweezers”, http://laser.physics.sunysb.edu/~yiyi (2003).

S. Smith et al., S. Smith et al., Am. J. PhysAm. J. Phys. . 6767, 26 (1999)., 26 (1999).

Z. Ulanowski and I.K. Ludlow, Z. Ulanowski and I.K. Ludlow, Meas. Sci. TechnolMeas. Sci. Technol. . 1111, 1778 (2000)., 1778 (2000).

Slide SetupSlide Setup

Deng (2003)