Biomedical Engineering Reference
In-Depth Information
19.3.2.2 Single Beam and Dual Beam
Single.beam.trapping.relies.on.the.large.gradient.forces.on.objects.for.its.use.of.a.powerful.
and.highly.focused.laser.beam..To.attain.suficient.trap.force,.photodamage.is.inevitable,.
but. can. be. reduced. by. choosing. an. appropriate. combination. of. laser. source,. immersion.
medium,.and.trap.mode..In.terms.of.various.trap.modes,.two.counter-propagating.dual.
beam.traps.cause.much.less.damage..This.comes.from.their.different.trapping.mechanisms.
other.than.the.one.illustrated.in.Figure.19.1..Instead.of.the.attributions.to.the.gradient.force,.
counter-propagating.dual.beam.trapping.relies.on.the.use.of.the.scattering.force.for.object.
capture.. The. scattering. forces. push. objects. away. from. the. illuminating. light. from. both.
sides,.which.reach.a.balance.at.a.speciic.position.and.create.a.trap.(Figure.19.5a)..In.many.
cases,. such. dual-beam. traps. occur. at. the. center. of. the. gap. between. the. two. beams,. but.
drifting.may.occur.if.the.scattering.forces.from.the.two.sides.are.uneven..If.two.divergent.
Beam 1
Beam 2
Gradient
force
Scattering
force
Light
path
Net
force
(a)
Trapped cell
Stretched cell
Divergent lasers
0.1-0.3 mm
Optical
fiber
Optical
fiber
(b)
Beam profile
Beam1
Beam2
(c)
Figure 19.5
(See companion CD for color igure.)
. Other. optical. tweezers. conigurations.. (a). Two. counter-propagating.
beams.for.trap..(b).Two.diverging.beams.for.optical.stretching..(c).Dual.beam.interferometer..(Zhang,.H..and.
Liu,.K.,.Optical.tweezers.for.single.cells,.
J. R. Soc. Interface
,.5,.671,.2008..Reproduced.by.permission.of.The.Royal.
Society.of.Chemistry.)
