Page 84 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Optical Components 65

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FIGURE 4-3 Scanning electron micrograph images of hollow-core ARROWs with
rectangular (left) and arch-shaped (right) cross sections. (D. Yin, J. P. Barber, E. J.
Lunt, A. R. Hawkins, and H. Schmidt, “Optical characterization of arch-shaped
ARROW waveguides with liquid cores,” Opt. Exp., 13, (2005), 10564–10569.)

the sacrificial layer and the fabrication process. ARROW waveguides
having cross sections ranging from few microns to few 10s of microns
were realized. Figure 4-3 shows an SEM picture of rectangular (left)
and arch shaped (right) ARROW waveguides. Pictures were reprinted
from Ref. 14. Such waveguides were recently demonstrated for appli-
cations such as fluorescence [15] and surface-enhanced Raman scatter-
ing (SERS) detection [16]. Two review papers describing the ARROWs
were recently published [17,18].
Another type of LCW that is not based on guiding by TIR is the
Bragg fiber, first demonstrated by Fink et al. [19]. The cladding of
these fibers is made of dielectric mirrors surrounding the hollow core.
The hollow core can be filled with liquids (although it was not dem-
onstrated so far). Light cannot escape through the cladding because
of the high reflectivity of the dielectric Bragg mirrors. The Bragg mir-
rors can be designed to be omnidirectional, that is, providing high
reflection for all angles of incidence. A slightly different version of the
Bragg fiber is the hollow-core photonic crystal fiber, described by
Russell [20]. This fiber is made of a hollow core, typically in the range
of few microns to 10s tens of microns. The hollow core is surrounded
by a two-dimensional periodic structure made of air holes in silica,
realizing a photonic band-gap and preventing the escape of light
from the hollow core. With this configuration, liquids were injected
into the hollow core to demonstrate light and particle guiding through
the liquid-filled core [21], and detection of surface-enhanced Raman
scattering from molecules in solution with silver nanoparticles [22].
Both the Bragg fiber and the photonic crystal fiber offer excel-
lent control over photonic properties and low propagation loss,
but cannot be monolithically integrated with on-chip optofluidic
systems. An alternative type of LCWs, based on total internal
2
reflection, is the liquid-liquid (L ) waveguide demonstrated by the
Whitesides group and others [23,24]. The L approach allows the manip-
2
ulation of light in waveguides that comprise a liquid core and a