Download Optical Tweezers

Optical Tweezers
Charlie Mueller
Jesse Fogleman
Qualitative Description
 In a nutshell , optical tweezers all ow us the abili ty to remotely control matter with
the use of lasers.
 The idea of moving matter with li ght is not new. Johannes Kepler observed that
the tail s of comets were always pointing away from the Sun. Kepler knew that the
Sun had to be exerting some kind of radiant pressure but could not verify this.
 Four centuries later, the idea of using li ght to move matter had become a reali ty
and quantum mechanics helps us understand how. Light is able to move matter
because photons carry momentum. A photon with any wavelength  carries a
momentum p  h  ( h is PlanckΥs constant). This means that when an atom emi ts
or absorbs a photon itΥs momentum changes, in accordance with NewtonΥs laws.
Along the same li nes, when a particle changes the direction of a beam of li ght as a
result of reflection or refraction, it experiences a force.
 The first optical traps were used in 1970 at the AT&T Bell labs in the United
States. The early traps levitatedΣ the particle by balancing the force of gravity or
made use of two counter-propagating beams to trap the particles. In 1986, it was
discovered that gradient forces alone would suffice in trapping a particle. Now,
only a tightly focused single beam was required to hold and move a particle in
three dim ensions.
Qualitative Description 2
 How do you focus a single laser beam? Pass it through a mi croscopeΥs objective
lens! The mi croscope objective must have a high numerical aperture (a measure
of the angle at which the beam approaches the focal point). This tightly focuses
the beam so the gradient to trap the particle is effectively strong enough. The
same lens may then be used to image the trapped particle, usua lly routed to a
monitor. A laser with only a milli watt of power is necessary to trap a single,
transparent particle.
 Optical tweezers exploit a refractive index mi smatch. Transparent particles have a
higher refractive index than their surr oundings and are attracted to area of
maximum intensity where the beam is most tightly focused, known as the beam
waist.
 By moving the focus of the laser beam around a li ttle, dielectric and biological
particles can be grabbed and moved in three dimensions. How big of a particle
can the beam handle? Although the optical force of the tweezers is only on the
order of piconewtons, this is suitable enough to move particles from tens of
nanometers all the way up to tens of mi crons, because at the mi cro-level the
optical forces are usua lly supreme. DNA, itΥs proteins and enzymes are most
commonly manipulated with optical tweezers.
Ray Tracing Analysis of Force Trap
1 sin(i )  2 sin( r )
2 sin(i 2 )  1 sin( r 2 )
180  ( r   i 2 )
1
h
Incoming light
p

ray   1000nm
2 r 2   i   r   i 2
2
i
2  1
 r  i 2  i
r
Dielectric
particle being
trapped
i 2
Exiting light ray
p
h
'
Scattering force vector
Force vector exerted by light on
dielectric particle as a result of
momentum conservation
Electric Dipole Approximation
• The force on charge particle in an electric field
is given by the Lorentz Force Equation:
d
1
1
dE
1
p
E
2
2
2
FFE

q(E

E


B)

q(E

v

B)
)E

F

(d
(



)E
E
)


E
E


(
 E)
B)B)]
p


E
FF(E
q(E
[F
F



(
[(E
p

E


)E
(



E)


B)
B)]
1
2
1
1
tt tt
22
2
t
1
B
B E
2
 [ E EE( ) 
 B)]
2
t
t
t
1

2
 [ E  (E  B)]
2
t
QuickTime™ and a
decompressor
are needed to see this picture.
Sources Used:
•
•
•
A. Ashkin, J.M. Dziedzic, J.E. Bjorkholm and S. Chu. 1986. "Observation of a
Single-Beam Gradient Force Optical Trap for Dielectric Particles." Opt. Lett. 11
(5) 288-290.
Block S. M. 1992. "Making light work with optical tweezers." Nature
360(6403):493-5.
Dholakia, Kishan, Gabriel Spaulding, and Michael Macdonald. "Optical
Tweezers: the next Generation." Physics World Oct. 2002. Physicsweb.org. Oct.
2002. Web. 13 Nov. 2010.
http://physicsworldarchive.iop.org/index.cfm?action=summary&doc=15%2F1
0%2Fphwv15i10a37%40pwa-xml&qt
•
Pressurized Viruses, Gelbart, et al.Science 27 March 2009: 16821683.DOI:10.1126/science.1170645
http://www.absoluteastronomy.com/topics/Optical_tweezers