Combining Digital Holographic Microscopy with Microfluidics: A New Tool in Biotechnology - Biomedical Optical Phase Microscopy and Nanoscopy - page 191

A latex particle floating inside the chamber is captured by the highly collimated laser beam.
From Figure 10.4A to Figure 10.4D , it can be seen that the particle is moving in the light
beam up to the trapping site. From Figure 10.4E to Figure 10.4I , images of the planes
between the trapping site and the hologram plane are displayed.

The on-axis beam of the optical tweezers ( Figure 10.2 ) is used to trap or direct micro-
objects along preferred directions [ 42,55 ].

In Figure 10.2 A-C show images of a particle at three different depths as it is positioned by
the scattering force of the light beam.

The object beam has the role of capturing the particle and driving it along the optical funnel
to reach the “holographic position” [55] . As a result, the setup parameters are fixed in such a
way that the gradient force holds the particle on the beam axis, while the scattering force
tends to move it in the direction of the optical beam. Therefore, it can be considered a 2D
confinement and all the particles that fall into this “weak trap” are pushed along the desired
trajectory. Another way to drive particles in the preferred corridor is to focus the object beam
under the sample cell. Indeed, if the beam waist is not fully into the chamber, then the
particle cannot be trapped, as illustrated in Figure 10.2 . The simple sketch of the sample
allows us to explain the optical forces acting on the particle. A Gaussian beam from the solid-
state laser emitting at 532 nm is mildly focused by means of the 100 3 microscope objective.
The focal plane is placed inside the glass slide, that is, outside the sample chamber.

10.2.3 Experimental Results

In our “drive and analyze” holographic setup, the on-axis beam acts as object beam
while the off-axis one has the role of holographic reference beam. As illustrated in

(A) (B) (C)

Reference beam Reference beam

Object beam

Object beam

Captured
particle

Figure 10.3
(A) Image of the two separated beams. (B) A particle is trapped by the object on-axis beam.
(C) Frame acquired in a plane well behind the focal plane of the 100 MO, the hologram fringe

3

pattern is clearly visible.