Page 212 - Optofluidics Fundamentals, Devices, and Applications

Adaptive Optofluidic Devices 187

8-2-2 Adaptive Optofluidic Lenses
Fast development of optoelectronics in the past decades keeps push-
ing miniaturization of imaging systems to the new limits. Traditional
imaging systems with variable focal length incorporate a pair of
lenses, with a variable distance between them. This approach is too
cumbersome in many application areas where bulky optics with
moving parts are intolerable. A need for the capability of adjusting
the focal length without bulky optical components is essential for
numerous applications.
A significant step toward miniaturization of adaptive lenses was
made in the 1980s with the advent of liquid crystal (LC), which
allowed a successful implementation of LC-based lenses [62–66].
Such lenses are based on electro-optic effect, which allows control
over the refractive index of the constituent medium. Such lenses suf-
fer from many limitations: first, very strong electromagnetic fields are
required to produce a noticeable change in the refractive index with
electro-optic effect; second, LC is a birefringent optical medium and
so LC lenses are polarization sensitive. Moreover, costly fabrication
makes LC lenses impractical in many application areas and limits
such lenses to small apertures.
Optofluidic lenses overcame many of the limitations imposed on
LC lenses. Liquid-filled and polymer-based optofluidic lenses allow
focal tuning from −∞ to +∞, fast response times, easy low-cost fabrica-
tion, and a whole range of optical aperture sizes—from meters down
to several millimeters. Lenses based on liquid-filled shells or pneu-
matically deformed polymers that have recently been reported are
described throughout the chapter.
Two major types of optofluidic lenses evolved—liquid-filled and
pneumatic lenses. Liquid-based lenses commonly offer very wide
focal tuning but very limited aperture. Pneumatic lenses, on the con-
trary, allow large apertures for high performance optics, at the
expense of the focal tuning range. So the two types represent trade-
off between aperture size and focal tuning range and span a wide
range of application areas including vision devices [67–69], micros-
copy [70–73], photography [74], optical data storage [75], bioengi-
neering and medicine devices [76,78], biochemical and temperature
sensing [79–81], and lab-on-a-chip devices [70,82,83]. These lenses
can be driven mechanically [48,77,84–92], chemically [93,94], ther-
mally [95–97], by electrowetting effect [74,98,99], and by radiation
pressure [100]. These diverse means of manipulation and control
make optofluidic lenses versatile and appealing in a number of appli-
cation areas mentioned herein.

Liquid-Filled Adaptive Elements
The very old idea of using liquid-based optical lens [54,55,101–103]
has been recently revisited for portable adaptive lenses with
mechanical actuators [84,104–107]. Micro-lens array integrated into