Page 256 - Optofluidics Fundamentals, Devices, and Applications

230 Cha pte r Ni ne

9-5 Fluidic Lens for Lab-on-a-Chip and
Micro-Total-Analysis Systems
So far we have discussed the applications of fluidic lenses in imaging
optics. In this section, we introduce another class of fluidic lens appli-
cations. Here fluidic lenses are reduced to two-dimensional devices
to bend light beams that are confined to the plane of the substrate.
Great benefits can be derived from the ability to reduce a three-
dimensional optical system to a fully functional, miniature two-
dimensional system. The goal of such miniature two-dimensional
systems is to preserve the original functionality and to add new func-
tionality while reducing system size, cost, and complexity by orders
of magnitude. In-plane, two-dimensional non-image-forming optical
systems are useful for a number of applications, including optical
telecommunications [81] and lab-on-a-chip devices. In such systems,
there is a need to relay light from position A to position B in a specific
fashion: for example, by collimating the beam for a longer travel dis-
tance, by shrinking or expanding the beam to a specific diameter, or
perhaps by changing the numerical aperture of the system. A two-
dimensional miniaturized nonimaging system offers the benefits of
simplified light manipulation and drastically reduced path length in
a robust, integrated fashion. Such systems can be employed in lab-
on-a-chip devices for fluorescence detection assays, light scatter mea-
surements, or absorption measurements.
Miniaturized two-dimensional (in-plane) optical systems gener-
ally consist of small optical elements created in the plane of the sub-
strate. Such systems will typically require at least two materials to
create the refractive index contrast needed for refraction (Snell’s law).
Many systems, especially some of the pioneering work in two-dimen-
sional lenses for miniaturized optical chips, employ some air-material
interface to satisfy this need [81–85]. Light exiting a rectangular wave-
guide end facet with outward curvature into air will be focused. Sim-
ilarly, a small pocket of air, tenths of microliters in volume or less,
with inwardly curved sidewalls can also serve to focus an incident
light beam.
For many materials, the index contrast in such an air-material
system can be quite high, yielding strong lenses but unacceptably
high reflective and scattering losses. A more moderate refractive
index contrast will be desired, leading to the need to fill the tiny
above-mentioned spaces with some higher-index material. Fluidic
lenses and other optical elements are thus often created [81,86–88], as
the tiny nature of these spaces readily lends itself to capillary filling,
and a fluid-filled space circumvents the problem of unintentional
interfacial air gaps forming between two solid materials. Fluids can
further offer the capability of lens tuning [89–92] and can help reduce
light scatter by effectively reducing sidewall roughness [90], as dis-
cussed briefly in the following paragraphs.