Page 24 - Optofluidics Fundamentals, Devices, and Applications

Intr oduction 5

1-4-2 Light Localization Can Occur at Biologically
Interesting Scale
We can focus light to a spot of a few hundred nanometers with con-
ventional optics with relative ease. This is a fairly unique property of
light in the EM spectrum. The long wavelengths of RF, microwaves,
and even terahertz wave preclude focusing at such scale. X-ray does
not suffer from such a limitation, but focusing X-ray requires rela-
tively elaborate schemes. Unlike the X-ray, optical waves are nonion-
izing EM waves, which will not impose health hazards, and therefore
are more favorable for bio applications.
The scale of a few hundred nanometers is biologically interesting
as organelles are typically of that size. A microscope with such resolu-
tion can provide good imaging of cells. Microfluidics is a good match
at this scale as well because this is a scale size at which fluidic con-
trols are still possible.
By using optical near-fields, it is also possible to achieve even bet-
ter length-scale or proximity sensitivity. The resonance-based biosen-
sors described in Chaps. 12 and 13 are good examples of optofluidic
devices that take advantage of this.

1-4-3 Light Can Manipulate Fluids and Objects
Suspended in Fluids
Despite the fact that the force that light can directly exert is relatively
weak, the extent of that force can be significant when it is exerted on
small objects. Optical tweezers (Chap. 15) is a growing research field
that capitalizes on this force to manipulate objects. Recently, there has
been significant progress made in the use of waveguides to exert
related types of controls (see Chap. 5).
Beyond direct force exertions (through momentum transfer),
there are other more subtle ways in which light can be used to manip-
ulate and move fluids and/or objects in fluids. The use of optically
induced heating and fluid vaporization as a means to manipulate
fluid is a new development that shows significant advantages for
optofluidics (Chap. 19).

1-5 Future
Optofluidics is a rapidly growing field. The permutations of optics
and microfluidics combinations are numerous and exciting to explore;
we can reasonably expect this field to continue its rapid growth over
the next decade. Optofluidics have brought about new and poten-
tially better ways to build or use established optical structures and
devices. Some of the growth directions in recent years have also been
remarkably unanticipated. For example, the optofluidic maskless
lithography technique (see Chap. 17) is unique and elegant in its imple-
mentation and applications.