Page 82 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Optical Components 63

As can be seen, mode confinement decreases from 95% for a 2-μm
waveguide to 18% for a 0.5-μm waveguide, resulting in a significant
increase in overlap between the optical mode and the liquid, from 5%
to 82%. This, however, comes at the expense of an increase in mode
size to more than 3 μm because the waveguide becomes weakly
guided. Bending loss (not shown) is also increased drastically.
SCLC optofluidic waveguides can be integrated with other opto-
fluidic components to support variety of applications. Among these
applications, label-free biosensing is of increasing importance. A
powerful method for optical biosensing is interferometry. A wave-
guide interferometric biosensing explores variations in the effective
refractive index of a waveguide caused by biological analytes bound
to the surface. Worth et al. [5] demonstrated a polarimetric wave-
guide interferometer based on silicon nitride on SiO slab waveguide.
2
With their approach, they could measure the differential effective
index between the orthogonal waveguide modes, from which they
could distinguish between specifically and nonspecifically bound
particles.
The sensitivity and the tuning strength of an optofluidic device
exploiting SCLC waveguides can be greatly enhanced by its coupling
to an optical resonator. For example, Chao et al. [6] demonstrated
homogenous and surface sensing by using a microring resonator
(MRR) in SCLC waveguide configuration. The waveguide core was
made of polystyrene on SiO , and was covered by the solution to
2
be analyzed. With Q factor of ~20,000, their devices could detect
−7
effective index variations of ~10 . Binding of the specific biomol-
2
ecules could be traced with a detection limit of 250 pg/mm of
mass coverage on the sensor surface. De Vos et al. [7] demonstrated
the detection of protein concentrations down to 10 ng/mL using min-
iaturized (5-μm radius) silicon on insulator (SOI)–based MRR with
liquid clad. This result demonstrates that the SCLC waveguide is
promising for miniaturized optofluidic systems, as long as the limited
interaction of the optical mode with the liquid can be tolerated.

4-2-2 Liquid-Core Waveguide
The disadvantage of insufficient interaction between the liquid clad
and the optical mode propagating mostly in the core of the SCLC
waveguide can be overcome by the use of liquid-core waveguides
(LCW). The optical mode propagating in such waveguides is mostly
confined to the liquid core; therefore the interaction of light with the
liquid is enhanced tremendously.
Most of the early versions of LCWs were implemented by realiz-
ing a hollow-core structure surrounded by a solid clad. The hollow
core can then be filled with liquid, forming a liquid-core waveguide.
A major challenge in realizing such waveguides is the choice of
cladding materials. Similarly to the SCLC waveguides, the guiding