Biomedical Engineering Reference
In-Depth Information
counter-propagating.beams.are.employed,.the.additional.surface.forces.on.the.object.can.
simultaneously.stretch.the.trapped.objects.if.they.are.deformable.(e.g.,.macromolecules.and.
cells,.Guck.et.al..2000),.as.shown.in.Figure.19.5b..Another.type.of.dual.beam.trap.occurs.
when.the.parallel.propagating.beams.are.used.to.create.an.optical.lattice.through.beam.
interference.(Figure.19.5c)..By.slightly.adjusting.the.dual-beam.path.or.displacement,.a.3D.
optical. lattice. can. be. created,. providing. no. fringe,. high. intensive. trapping. regions,. and.
complete.coninement..In.this.interfering.trapping.mode,.more.beams.can.get.involved.to.
tailor.the.phase.and.intensity.of.the.interference.pattern.on.particles.of.various.refraction.
indexes.or.trap/sort.particles.in.low..More.advanced.interfering.trapping.modes.involve.
the. use. of. SLM. equipped. with. galvanometer. scanning. mirrors,. acousto-optic. delectors.
(AOD),.electro-optic.delectors.(EOD),.or.HOT.beam.delectors.
19.3.2.3 Time Sharing Multiple Trapping
Multiple-point. trapping. is. more. favorable. in. recent. optical. tweezers. applications,. cell.
trapping.in.particular..It.can.be.realized.with.several.strategies,.including.(1).splitting.the.
incident.beam.in.an.optical.circuit.(Flynn.et.al..2002;.Ozkan.et.al..2003a,b);.(2).scanning.one.
beam. with. an. AOD. delector. (Visscher. et. al.. 1993;. Vossen. et. al.. 2004);. and. (3). computer-
generating.dynamic.holograms.through.SLM.(Curtis.et.al..2002,.2003;.Curtis.and.Spatz.2004;.
Martin-Badosa.et.al..2007)..As.splitting.physically.reduces.the.power.of.each.individual.beam,.
scanning. between. the. optical. trap. sites. at. a. rate. much. faster. than. the. Brownian. motions.
of.the.targeted.trapping.object.is.widely.adopted.in.multiple.position.trapping.nowadays..
Several.multiple.trapping.technologies.used.in.optical.tweezers.are.shown.in.Table.19.1.
Galvanometer. scanning. mirrors. are. widely. used. in. early. beam. steering.. They. have. a.
general.operating.speed.of.1-2.kHz.with.a.step.response.of.100.μs.or.more.and.a.delection.
repeatability.of.8.μrad..The.optical.diffraction.grating.in.AOD.delectors.is.produced.in.a.
transparent.crystal.whose.density.changes.with.an.acoustic.traveling.wave..The.grating.
period. is. determined. by. the. wavelength. of. the. ultrasound. signal. in. the. crystal. and. the.
diffraction.eficiency.relies.on.the.amplitude.of.the.acoustic.wave..A.2D.trapping.can.be.
accomplished. with. two. independent. AOD. delectors. conigured. in. an. orthogonal. way..
Current.commercial.AOD.delectors.operate.at.238.kHz.with.a.response.time.of.4.2.μs..To.
generate. multiple. traps. in. all. three. dimensions,. dynamic. HOTs. are. necessary.. General.
dynamic.holographic.SLMs.modulated.optical.tweezers.are.illustrated.in.Figure.19.3..In.
these.HOT.optical.tweezers,.through.a.liquid.crystal.SLM.and.other.diffractive.optics,.a.
single. beam. can. split. and. steer. between. multiple. traps. to. create. phase-only. holograms..
Compared.to.SLM/AOD,.the.real-time.hologram.algorithm.allows.the.accurate.creation.of.a.
large.array.of.optical.trap.patterns..Moreover,.these.HOT.optical.tweezers.can.conveniently.
manipulate.objects.for.various.complicated.motions.other.than.trapping.with.appropriate.
algorithms..As.not.all.ports.of.an.incident.beam.is.diffracted.by.the.SLM,.the.undiffracted.
portion. can. be. focused. into. another. plane. other. than. that. of. the. optical. traps. to. avoid.
TABLe 19.1
Comparison.of.Various.Beam.Steering.Approaches
Beam Steering Method
Operating Speed
Switching Time
Delection Repeatability
Galvanometer.scanning.mirrors
1-2.kHz
100.μs
8.μrad
Acousto-optic.delectors
238.kHz
4.2.μs
High
Dynamic.holographic.SLMs
10-2.kHz
<1.s
N/A
Electro-optic.delectors
10.MHz
100.ns
High
