Optical Tweezers
Charlie Mueller
Jesse Fogleman
Qualitative Description
 In a nu tshell , optica l tweezers all ow us t he abili ty to re m ot ely c o ntrol m atte r w ith
the use of lasers.
 T he idea of m ov ing m att er w ith li ght is not n ew. Jo hann es K ep ler obser ved t hat
the t ail s o f com ets wer e a lwa ys pointing awa y from the S un. K ep ler k new t hat t h e
Su n had t o b e exer ting som e kind of rad iant pressure b ut c ould n ot verify t his.
 F our ce ntur ies later, the idea of us ing li ght to m ove m att er h ad bec om e a rea li ty
and q ua n tu m m ec hanics helps us unders tand how. L ight is able to m ove m att er
because p hot ons carr y m om entum . A photo n w ith any wa velength  carr ies a
m om entum p  h ( h is P lanck Õs c onstan t). T his m ea ns t hat w hen a n a to m emi ts

or absorbs a photo n itÕs m om entum c hanges, in acc orda nce w ith New to nÕs laws.
A lon g t he sa m e li nes, w hen a par ticle chan ges t he direc tion of a bea m of li ght as a
resu lt of re fl ec tion or refrac tion, it exper iences a force.
 T he firs t o ptica l traps were used i n 197 0 a t the A T & T Be ll labs in the U nited
S tates. T he ear ly traps Òlevitated Ó the par ticle by ba lancing t he fo rce of gra vity or
m ade use of tw o c ounte r-pr opaga tin g bea m s to trap the par ticles . In 198 6, it was
disc overed t hat grad ient forces alone w ould su ffi ce in t rapp ing a particle. N ow ,
only a tightly focused single bea m was requ ired to hold and m ove a par ticle in
three dim ensions.
Qualitative Description 2
 H ow do you focus a single laser bea m ? Pass i t throug h a mi cr osc ope Õs objec tive
lens! T he mi cr osc ope objec tive m us t have a high nu m er ica l aper ture (a m easure
of the angle at w hich the bea m appr oac hes t he focal point) . T his tigh tly focuses
the b ea m s o the g rad ient to t rap the par ticle is eff ectively s tron g enoug h. T he
sa m e lens m ay then be used to im age the trapped par ticle, usua lly routed to a
m onito r. A laser w ith only a milli wa tt of power is neces sar y to tra p a single,
tra nspare n t par ticle.
 Op tica l tweezers exploit a refrac tive inde x mi sm atch. T ra nspare nt par ticles have a
higher re frac tive inde x than t heir surr oundings and are att rac ted to area of
m ax im um inten sity w here the bea m is m ost tightly focused , k now n as t he bea m
wa ist.
 B y m oving t h e focus of the laser bea m ar ound a li ttle, dielec tric and biologica l
par ticles ca n be gra bb ed a nd m oved in three dim ens ions. H ow b ig of a pa rt icle
ca n the b ea m handle? A lth ough the o ptica l force of the t weezers is only on the
order of p iconew to ns, this is su itable enoug h to m ov e par ticles from tens o f
nan om eters a ll the wa y u p to tens of mi crons, bec ause at the mi cr o-level the
optica l forces are usua lly supre m e. DN A , itÕs prote ins and enzym es are m ost
com m only m anipu lated w ith optica l 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   1 0 0 0 n m
2 r 2   i   r   i 2
2
i
 2  1
 r  i2  i
r
i2
Dielectric
particle being
trapped
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  q2( E 1 1E 2  2p
Ed
E
EFq(E
B

(FE

E



) (
B )B )]
p1(E

E)E
FF(Eq[F
F)E


((d
[(p

)E
(
)E

EEE
BB)B))]E
2v
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
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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