Page 201 - Science at the nanoscale

RPS: PSP0007 - Science-at-Nanoscale
10:16
June 5, 2009
8.4. Optical Tweezers
CCD Camera
IIlumination
Light Source
TV Monitor
Sample Cell
S: Beam Splitter
M
L: Lens
S
M: Mirror
He-Ne Laser
Optical
Objective Lens
Microscope
IR Laser or
Beam Splitter
M
L
He-Ne Laser
Camera
L
Sample
TV
Sample Stage
Computer
Image Processing
Aqueous Suspension
of Colloidal Microspheres
VCR
Using Upright Optical Microscope
Using Inverted Optical Microscope
Figure 8.29.
Schematics of typical setups employed for optical
tweezing.
focused laser beam. In addition, the same objective lens can be
used to capture images of the microscopic particles trapped by
the optical tweezers. Figure 8.29 shows schematics of two typical
setups for optical tweezing. Both an upright or inverted optical
microscope can be utilised. The housing of these optical micro-
scopes consists of a side port that serves as the entrance point for
the parallel laser beam. Inside the optical microscope, the laser
beam is reflected by a beam splitter towards the objective lens
to achieve focusing. The sample chamber is made of transparent
housing that supports an aqueous colloidal suspension of micron-
sized objects. The sample chamber is placed on the sample stage
of the optical microscope. When the tightly focused laser beam is
present in the colloidal suspension, the micro-particles are readily 191 ch08
attracted towards the focused laser spot and become trapped.
When an intense focused laser beam is illuminated onto a
micro-particle, the force exerted by the laser beam onto the parti-
cle can be divided into two main components: the scattering force
and the gradient force. The scattering force arises from the change
in momentum when a photon scatters off from the particle. This
force tends to push the particle along the direction of beam prop-
agation. On the other hand, the gradient force provides the attrac-
tive force component that draws the particles towards the focal
point of the laser beam. At equilibrium the particle is held at a
location slightly beyond the focal point of the focused laser beam.