Biomedical Engineering Reference
In-Depth Information
F
a
b
a
F
scat
b
a
F
grad
F
b
(a)
F
scat
Laser
F
grad
F
grad
F
scat
(b)
Objective
Figure 19.1
(See companion CD for color igure.)
.Schematic.of.optical.forces.on.a.dielectric.particle.and.optical.trap.mecha-
nism..(a).The.scattering.(
F
scat
).and.gradient.(
F
grad
).forces.from.two.rays.of.light.of.a.Gaussian.beam,.
a
.and.
b
.(light.
intensity.increases.from.
a
.to.
b
),.on.a.dielectric.particle..(Hormeno,.S..and.Ricardo.Arias-Gonzalez,.J.:.Exploring.
mechanochemical.processes.in.the.cell.with.optical.tweezers..
Biol. Cell
..2006..98..679-695..Copyright.Wiley-VCH.
Verlag.GmbH.&.Co..KGaA..Reproduced.with.permission.).(b).Single.beam.optical.trap.principles..The.gradient.
force.pulls.the.particle.toward.the.focal.region,.if.it.is.out.of.the.trap.center.
other. forces. on. the. particle. (e.g.,. Brownian. motions),. resulting. in. the. capture. of. particle.
(i.e.,. optical. trap).. As. illustrated. in. Figure. 19.1,. when. staying. at. the. trapping. center,. the.
particle.refracts.light.symmetrically..The.gradient.force.cancels.the.scattering.force.(from.
the.light.relection).so.that.there.is.zero.net.lateral.force.on.the.particle.and.it.stays..If.the.
particle.is.somehow.shifted.out.of.the.focus.center,.the.optical.forces.become.unbalanced.
and.the.resultant.net.force.pulls.the.particle.back.to.the.trapping.position.
19.2.2 Optical Trapping in raleigh Scattering regime
If. the. particle. size. is. substantially. smaller. than. the. laser. wavelength. (
r
=.λ),. the. Raleigh.
scattering. conditions. are. satisied.. The. particle. can. be. approximated. as. a. point. dipole.
and.the.corresponding.gradient.force.and.scattering.force.from.a.Gaussian.beam.can.be.
expressed.as
2
5 6
2
128
3
π
λ
r m
m
−
+
1
2
n
c
m
F
I
.
(19.2)
scat
=
4
2
.
