“Nanophotonics and Optical
Control of Single Nanoparticles”
Keonwoo Nam
Supervisor: Professor A. A. Fedyanin
Lomonosov Moscow State University, Faculty of Physics
Moscow 2012
Introduction to Optical Tweezers
OUTLINE
 Brief History of Optical Tweezers
 Principles of Optical Tweezers
 Optical Trapping Forces
 The Optical Tweezers Set up &
The Experimental Method
 Research Areas and Possible uses
What is…
Optical Tweezers - one of the techniques,
which use a highly focused beam to control
and hold microscopic particles.
Light transfers momentum to matter
History

Johannes Kepler
(1571-1630):
noticed comet's tail always points
away from the sun, because of the sun's radiation pressure.

James Maxwell
(1831-1879):
existence of the light pressure was
demonstrated

P. N. Lebedev

Albert Einstein
(1866-1912):
measured the light pressure
(1879-1955):
confirmed that photons possess its
own momentum

Arthur Compton
(1892-1962:)
showed the existence of the light
momentum on his experimental work
History
Arthur Ashkin

In 1970 A. Ashkin proved that light can grab and release
nanometer particles by its momentum, using the light
Quantum Theory.

In 1986, A. Ashkin proved that he could trap 10nm diameter
dielectric particles only using gradient force

In 1987, A. Ashkin showed the damage-free manipulation on
cells using an infrared laser
Types of OT
 Single Beam Optical Tweezers
 Dual beam Optical Tweezers
 Holographic Optical Tweezers
Principles of OT
(a) If the particle is to the left, say, of the center of the beam,
it will refract more light from the right to the left, rather than
vice versa.
The net effect is to transfer momentum to the beam in this direction,
so, by Newton’s third law, the particle will experience an equal and
opposite force – back towards the center of the beam. In this
example the particle is a dielectric sphere
(b) Similarly, if the beam is tightly focused it is possible for the
particle to experience a force that pushes back towards the laser beam.
How optical tweezers work
Optical tweezers: the next generation, Kishan Dholakia,
2002, physics world

(c) We can also consider an energetic argument: when a polarizable
particle is placed in an electric field, the net field is reduced. The
energy of the system will be a minimum when the particle moves to
wherever the field is highest – which is at the focus. Therefore,
potential wells are created by local maxima in the fields.
Exert a laser beam to the very small particle, the light will be
reflected or refracted from the surface of the particle. The
momentum of photon, refracted to the particle, will be
changed and by the law of the conservation of the
momentum, the force of the variation of momentum will be
exerted to the small particle.
Conditions of OT
The Electromagnetic Regime of Optical Tweezers
The Ray Optical Regime of Optical Tweezers
Wonhoe Koo, Seoul, 2005
Two Forces
The Electromagnetic Regime of Optical Tweezers
The Gradient Force
Two Forces
The Ray Optical Regime of
Optical Tweezers Wonhoe
Koo, Seoul, 2005
The Gradient Force
The Basic OT Set up
A generic optical tweezers diagram with only the
most basic components
The Optical Tweezers, Wikipedia
Research areas
 Study and manipulate particles such as atoms,
molecules and small dielectric spheres
(in range from mm to nm).
 Force measurements of biological objects in
piconewton range.
 Biological investigations involving cells
 Cutting and ablating biological objects
(Cell fusion and DNA cutting)
 Force measurements of cell structures and DNA coiling
 Elasticity measurements of DNA
Thank you ^-^
Keonwoo Nam