Optical Tweezers - The Nanobiotechnology - page 442

19.2.3 Optical Trapping for intermediate Size Particles
For.particles.whose.size.is.comparable.with.the.laser.wavelength.(i.e.,. r .∼.λ),.quantitative.
description. of. the. optical. forces. is. dificult. due. to. the. lack. of. available. electromagnetic.
theories.in.this.intermediate.size.range..However,.most.valuable.objects.used.for.optical.
trapping.study,.including.microspheres,.bacteria,.organelles.of.cell,.and.various.types.of.
cells,.are.all.in.this.regime..A.computation.toolbox.has.recently.been.developed.to.calcu-
late.the.optical.forces.and.torque.on.dielectric.particles..It.covers.a.particle.size.range.of.
0.1-10.λ.and.is.suitable.for.both.spherical.and.nonspherical.shape.particles.(Nieminen.et.al..
2007)..The.trapping.forces.on.a.dielectric.particle.that.have.a.size.within.0.1-10.λ.in.both.
a.Gaussian.and.non-Gaussian.beam.are.also.provided.in.Figure.19.2..MATLAB ® .codes.of.
this.toolbox.are.made.free.to.the.public,.providing.researchers.support.and.guidance.for.
their.optical.trapping.practice.(Optical.Tweezers.Toolbox.1.2.Website).
19.3 Optical Trap Instrument and Design
19.3.1 general Optical Trap Setup
A.common.holographic.optical.tweezers.setup.is.schematically.illustrated.in.Figure.19.3..The.
optical.trapping.module.is.mounted.on.an.inverted.microscope.for.convenient.visualization.
of.the.trapping.and.manipulation.process..A.powerful.laser.source,.usually.with.a.Gaussian.
TEM 00 .mode.proile,.is.used,.whose.wavelength.is.well-separated.from.another.beam.used.
for.imaging..The.trapping.laser.beam.is.irst.expanded.by.a.telescope.lens.pair.(M1:M2).and.
if. multiple. traps. are. needed,. the. beam. is. further. directed. into. a. relective. liquid-crystal.
spatial.light.modulator.(SLM),.a.relay.optics.unit.with.holographic.optical.trapping.(HOT),.
and.a.beam.block..The.laser.beam.is.then.guided.into.an.objective.by.a.dichroic.mirror..The.
objective.often.requires.a.high.numerical.aperture.(NA).with.excellent.transmissions,.such.
as.60.×./100.×.oil/water.immersion.objective.with.NA.=.∼1.2.to.1.4.from.Zeiss.Inc.or.Nikon.
Inc..When.multiple.separated.trapping.beams.are.involved,.an.SLM.with.diffraction.grating.
is.usually.computer-controlled.to.steer.these.beams.to.form.a.trapping.lattice.
The.trapping.position.and.force.measurement.is.done.by.a.3D.back-focal.plane.interferometer.
with.a.second.laser.(Gittes.and.Schmidt.1998;.Rohrbach.and.Stelzer.2002;.Rohrbach.et.al..2003;.
Dreyer.et.al..2004)..The.second.laser.beam.is.also.expanded,.split,.and.guided.into.the.objective..
The.generated.interference.pattern.changes.are.recorded.by.two.quadrant.photodiode.(QPD).
detectors..The.QPD.signals.are.further.digitalized.with.an.analog.to.digital.(A/D).converter.
and.analyzed.by.software.like.LabVIEW.to.provide.information.on.the.planar.position.( XY .
plane,. or. the. specimen. plane). and. the. displacement. of. the. trapped. object. to. the. trapping.
equilibrium.( z .direction.or.the.light.propagation.direction)..For.the.imaging.module,.a.bright.
ield.light.is.guided.from.the.top.of.the.inverted.microscope.and.illuminates.the.sampling.
zone. through. a. condenser. lens.. Part. of. the. light. is. directed. to. a. charged. coupled. devices.
(CCD).camera.for.real-time.monitoring.and.imaging.of.the.trapping.process.
19.3.2 Major Components of Optical Trap and Their Current emerging Techniques
19.3.2.1 Trapping Laser
The.power.of.the.trapping.laser.varies.from.a.few.milliwatts.to.several.watts,.depending.
on.the.desirable.trapping.force/torque.and.the.optical.transmittance.of.the.actual.optical.