Biophotonics_Concepts_to_Applications

In addition to the gradient force, the scattering of incident photons from the
sphere will exert an axial force on the sphere. This force points downstream in the
direction of travel of the light beam and is known as thescattering forceFscat.
For an optical tweezers or optical trap implementation, typically a microscope
objective is used to focus a collimated input light beam into an exit beam with a
tightly focused axially symmetric Gaussian intensity profile, as is shown in
Fig.11.2. A very strong electricfield gradient exists within the narrowest portion of

Low intensity

ray 1

High intensity

ray 2

Photon momentum

change of ray 1

Photon momentum

change of ray 2

Intensity

Position

Incident

photons

Force F 2

from ray 2

Force F 1

from ray 1

Intensity

distribution

Fig. 11.1 Basic Scattering force Fscat
geometric-optics principle of
optical tweezersaA
transverse intensity variation
gives a gradient force pointing
towards the region of highest
intensity;bScattering of
photons yields a forward
scattering force

x Particle displacement

Beam waist

Microscope

objective

Trap center

Gaussian input beam

Lateral force

Low

intensity

Low

intensity

High

intensity

Intensity profile

across beam face

Fig. 11.2 Exit beam from a
microscope objective with a
tightly focused axially
symmetric Gaussian intensity
profile

326 11 Biophotonics Technology Applications