Page 86 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Optical Components 67

slot waveguides were also used for the realization of MRRs for bio-
sensing applications [27]. Resonance shift of 212 nm/RIU (refractive
index unit) was reported. By using a similar platform, a label-free
biosensing of bovine serum albumin (BSA) and anti-BSA was also
2
demonstrated, with sensitivity limit in the range of 16 to 28 pg/mm .

4-3 Optofluidic Components for Manipulation
of Optical Signals
In parallel to the rapid progress in optofluidic waveguides there is a
growing effort to develop variety of other optofluidic components,
with a prime goal of manipulating and processing optical signals.
The integration of photonic components with liquids on the micro-/
nanoscale paves the way to widen and enhance their optical function-
alities, forming eventually a new class of optofluidic components for
manipulating optical signals. Components such as tunable filters,
switches, splitters and combiners, and beam deflectors were recently
demonstrated. This section describes some of the recent work in the
field, with a specific focus on tunable optofluidic filters. Other com-
ponents, for example, switches, splitters, and beam-steering devices
are covered in Chap. 8.

4-3-1 Optofluidic Filters
Optical filters are the subject of scientific and technological effort for
many years, with applications in microscopy, avionics, spectroscopy,
optical communication, sensing, astronomy, machine vision, laser range
finders, and environmental monitoring, to name a few. Optofluidics is a
promising approach for the realization of optical filters because (a) it
offers a wide tunability range, much larger than can be achieved by most
other physical effects, and (b) it allows the interaction of analytes carried
by the liquid with the optical filter, thus enabling on-chip realization of
optofluidic-filtering devices and systems. Two of the dominant mecha-
nisms used for the realization of optical filters are absorption and interfer-
ence. Optofluidic-absorption filters can be easily realized by introducing
absorptive liquid into the filtering region. The spectral absorption prop-
erties of the liquid determine the spectral response of the filter. The func-
tion of tunability can be acquired by replacing the liquid with another
liquid, having different spectral absorption properties. By mixing of liq-
uids it is possible to achieve continuous tuning of such filters. Macro-
scopic liquid absorption filters were already demonstrated many years
ago [28,29]. For example, Ref. 29 describes an optical cell with a variable
path length designed for use in conjunction with liquid filters. A path
length change from 1 mm to 14 mm changes the cutoff wavelength
by typically 30 nm. The miniaturization and on-chip integration of
absorption-based liquid filters holds great promise for the realization of
flexible, high-performance, and integrated optofluidic systems.