Biomedical Engineering Reference
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(0.3 mm thick). From a solid-state laser source (wavelength of 532 nm and 250 mW), two
beams enter the oil immersion Nikon 1003 1.2 NA microscope objective (MO). The first
beam is directed along the optical axis while the second beam is slightly off-axis, resulting
in a 4 degree angle between the two beams. Another microscope objective (20 3 ) is used as
an imaging lens to obtain an image on the CCD plane. We consider the off-axis beam as
the reference beam in holographic setup. We used two CCD cameras (Figure 10.1A). The
first camera, CCD(I), is used to image the plane proximal to the beam waist (i.e., the
focal plane of the first MO) to visualize the optical trapping process. The second camera,
CCD(H), records the interferometric pattern (Figure 10.1B). The hologram is recorded and
numerically reconstructed by well-known algorithms [54] to obtain the whole complex
wave front from which the amplitude and phase map of the particle can be retrieved easily.
To clarify the setup geometry in Figure 10.3 , images recorded through the camera CCD(I)
and CCD(H) are displayed. Figure 10.3A shows the image from CCD(I) at the image plane
near to the MO focal plane. The two beams' waists are clearly separated and visible.
Figure 10.3B is an image of the same plane but a latex particle is captured in the object
beam. Finally, Figure 10.3C displays a plane well behind the focal plane of the 100 3 MO
and is recorded by the camera CCH(H) at the hologram plane H as shown in Figure 10.1B ,
and where two interfering beams produce good contrast fringe patterns.
In Figure 10.4 , several images illustrate the imaged volume comprised of the focal plane
(CCD(I)) and the hologram plane (CCD(H)).
(C)
F scat
(C)
(B)
F grad
(B)
(A)
(A)
Figure 10.2
Sketch of the forces acting on the particle. (AC) Three different positions of the particle while
guided in the funnel.
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