An experimental setup for trapping and manipulating particles using upward-directed and downward-directed YAG laser beams at a wavelength of 1.06 mm having a TEM00 mode structure is shown in Fig. 1. The laser beam diameter is increased from 0.7 mm to 8.2 mm by a beam expander to fill the entire aperture of the objective uniformly. A quarter-wave plate is placed to generate a circularly polarized beam. The intensity of the laser beam is varied using a ND filter. The beam is divided by a beam splitter to enter two objective lenses for focusing the downward-directed and upward-directed laser beams. A dichroic filter is used to separate the transmitted image from the trapping beam. The total transmittance efficiency from YAG laser to the objective is about 35%. The transmittance effi
ciency from YAG laser to the immersion
oil objective lens (NA = 1.25) is 21% at a wavelength
of 1.06 mm.
Fig. 1. Experimental setup for trapping and manipulating
microobjects using upward-directed and downward-directed YAG laser beams with
wavelength of 1.06 mm
Figure 2 shows photographs of the experimental setup. Microobjects are
suspended in an aqueous medium in a chamber. The chamber is made similar
to a hole of a spacer (50 mm to 150 mm in depth)
inserted between a coverslip (150 mm in depth) and a glass slide. The particle is
trapped so as to be pulled to the focus of a strongly converging laser beam
transmitted through the coverslip. When the objective lens moves, the particle
follows the objective motion. The trapping behavior of the microobjects is
monitored using a CCD camera. We have measured the minimum laser power for both
axially trapped and transversely trapped particles 
by balancing the gravitational force and the viscous drag force,
respectively.
Fig. 2 Photograph of optical trapping apparatus.
Upper and lower objective lenses are seen around the microscope stage lower
left in the figure (a). Two objective lenses used in trapping particles in
water with downward-directed and upward-directed laser beams (b)