Page 22 - Optofluidics Fundamentals, Devices, and Applications

Intr oduction 3

spherical—a curvature profile that is used in most commercially
available lenses.
It is also interesting to note that the usefulness of this advantage
extends beyond devices that have dynamically controllable fluidic
surfaces; this advantage also enables low-cost and easy fabrication of
optical components. For example, the toroid optical resonators dis-
cussed in Chap. 13 are able to achieve their high optical quality factor
through the melting and solidification of the resonators’ rims to create
smooth optical tracks.

1-3-2 Diffusion Can Create Controllable
Blend of Optical Properties
Miscible liquids and their interfaces can also be of significant use in
the optofluidic context [1].The solid-based structures failed to pro-
vide the property that can be created by the diffusion across the
interface of two liquids. Specifically, the diffusion process can create
a concentration and refractive-index gradient which is smooth and
controllable. The controllability and flexibility by which this gradient
can be adjusted through flow parameters, fluid choices, and the
device structures enable the creation of novel optical interconects. For
example, an optical splitter and wavelength filter based on the selec-
tive mixing of two fluid jets in a third fluidic medium has been dem-
onstrated (Chap. 3). Unlike a conventional beamsplitter, the split ratio
of the optofluidic beamsplitter can be dynamically tuned for any
given wavelength.

1-3-3 Fluid Can Be an Excellent Transport Medium
It is relatively easy to input, move, and manipulate fluid in an opto-
fluidic device. Pressure differential is a common and convenient
means. Electrokinetic approaches provide another set of flow con-
trol mechanisms (see Chap. 2 for more information). Over the past
few years, several optical approaches for manipulating fluid have
also been developed (Chaps. 5, 8, and 19). The optofluidic micro-
scope (OFM) (Chap. 11) capitalizes on this advantage by using
microfluidics as the means for sample input and microfluidic flow
as the scanning mechanism during image acquisition. The optoflu-
idic maskless lithography approach (Chap. 17) is another excellent
example of an optofluidic technology that makes good use of fluid
transport.
The easy transport of fluids benefits the field of optofluidics in
three other ways. First, we can use the change of fluid media in an
optofluidic device as a way to alter the properties of the device—thus,
allowing us to create adaptable devices (see Chaps. 7, 12, and 18 for
some excellent examples). Some of the properties that can be altered
this way include refractive indices, spectral absorption coefficients,
and scattering coefficients. The optofluidic lasers (Chap. 10), for