Page 83 - Optofluidics Fundamentals, Devices, and Applications

64 Cha pte r F o u r

mechanism of the LCWs is based on TIR. As such, the refractive index
of the clad needs to be lower than that of the core to avoid “leakage”
of the optical mode. Moreover, in order to support the growing
effort of miniaturization of optofluidic systems, large refractive
index difference between the core and the clad is desired. For water-
based LCWs, with core refractive index of ~1.33, achieving high
refractive index contrast is very challenging. In fact, the refractive
index of most solids rarely falls below 1.3. For example, glass, which
is frequently used as cladding material, is not suitable as the cladding
of water-based LCWs because its refractive index is higher than the
liquid. An attractive cladding material for LCWs is Teflon AF, because
of its low refractive index (n ~1.29). Various LCWs with Teflon AF as
cladding material were demonstrated, with applications in Raman
spectroscopy [8–9], fluorescence spectroscopy [10], and capillary
electrophoresis [11]. Unfortunately, it is difficult to spin coat Teflon
AF on substrates because it does not adhere well to most substrates.
This technical obstacle may be overcome by surface treatment (e.g.,
by oxygen plasma). An alternative approach for realizing low-refractive-
index cladding material is by using subwavelength nanoporous
material. Because the dimensions of the pores are much smaller com-
pared to the optical wavelength, scattering loss is minimized and the
refractive index can be tuned by controlling the volume fraction of
the pores. Based on this concept, a planar one dimensional wave-
guide having cladding material with effective refractive index rang-
ing from 1.15 to 1.27 was demonstrated [12]. The nanoporous dielec-
trics were made by the sacrificial porogen approach, in which an
organic macromolecular phase is selectively removed from a phase-
separated organic/inorganic polymer hybrid, resulting in nanoscopic
pores having a diameter in the range of 10 to 15 nm.
A different type of LCW is the antiresonant reflecting optical wave-
guide (ARROW). These waveguides were recently introduced as a
promising approach for the realization of hollow-core integrated optics
with very small core volumes. In contrast to the previous examples
the guiding of light in these waveguides is not based on TIR. Instead,
the ARROWs employ multiple dielectric cladding layers, and rely on
the antiresonance of the transverse wave vector component for each
layer, which yields quasi-guided modes [13]. Although these modes
are leaky, a properly designed ARROW waveguide can guide light
with loss as low as 1.1 dB/cm in the visible wavelength regime [14].
ARROW waveguides are typically fabricated by surrounding a sacrifi-
cial core with silicon dioxide and/or silicon nitride layers. The sacrifi-
cial layer is then removed by selective wet etching. The layers are
grown to specific thicknesses such that ARROW-based optical confine-
ment is obtained. Typical layer thickness is in the range of 100 to 200 nm.
A variety of sacrificial materials can be used, including photosensitive
polymers and metals. Different waveguide profiles, for example, rect-
angular, trapezoidal, and arch-shaped can be realized, depending on