Page 204 - Science at the nanoscale

10:16
RPS: PSP0007 - Science-at-Nanoscale
June 5, 2009
Nanotools and Nanofabrication
194
the optical tweezers. Accordingly,
1
2
k B T =
2
2
where x is the displacement of the particle from its trapped equi-
librium position, α is the stiffness, k B is the Boltzmann constant
and T is the temperature of the system.
Since the advent of the optical tweezers, different methods have
been employed to shape the laser beam into different configu-
rations for the purpose of optical trapping of assemblies of col-
loidal particles. These optical traps provide an important tool in
the mesoscopic environment to trap and manipulate microscopic
objects. Patterned laser beams have also been developed for many
purposes such as the assembly of polymerised colloidal structure
into micro-fluidic devices, driving forces for micro-devices and for
cell-manipulation. One commonly adopted technique to create an
array of laser spots is to use a diffractive optical element. Other
techniques include the use of galvanometer scanning mirrors or
a piezoelectric scanning mirror, computer-generated holograms
and acoustically modulated light beams. Examples of optical trap-
ping of an array of microspheres are shown in Fig. 8.32. Besides
microspheres, other micron-sized objects such as biological cells
and CuS micro-stars can be readily manipulated.
(a)
(b)
(c) 1 αhx i (8.6) ch08
Figure 8.32. (a) Optical trapping of a 5 × 5 array of microspheres.
Optical manipulation of the (b) CuS microstars and (c) fish blood cells
into “NUS” letters formation (from author’s lab).