Page 85 - Optofluidics Fundamentals, Devices, and Applications

66 Cha pte r F o u r

liquid cladding. The liquids are introduced into the channels of a
microfluidic network designed to sandwich the flowing core liquid
between flowing slabs of the cladding liquid. The core/clad boundary
can be controlled by manipulating the rate of flow of the liquids,
allowing the tunability of the optofluidic waveguides. More infor-
2
mation about the L waveguides is given in Chap. 3.
4-2-3 Hybrid-Core Waveguide
The optofluidic waveguides described up to now could be clearly
defined either as solid-core waveguides or as liquid-core waveguides.
In recent years, however, a new class of waveguides is emerging,
where the waveguide core includes structures on the micro-nanoscale,
with mixed regions of solid and air. The air regions can be filled with
liquid, realizing special waveguides with a hybrid solid/liquid core.
We thus use the term hybrid core waveguides (HCW) to describe them.
Here we focus on a specific and attractive example of HCW, the slot
waveguide.
The slot waveguide was first demonstrated by Xu et al. [25]. It
was realized by etching a 100-nm vertical slot into a 540-nm wide,
250-nm thick silicon waveguide core, on top of SiO lower cladding.
2
The authors demonstrated a significant drop in effective index of
the horizontal mode, leading to the conclusion that a significant
portion of the mode was confined to the narrow slot. The operation
concept of the slot waveguide can be explained as follows. If an
optical mode with its electrical field (E) coincide with the horizontal
axis is excited in this waveguide, a discontinuity in electric field is
expected around the slot, whereas the electric displacement (D)
across the slot boundary is continuous. Because the electric dis-
placement is given by D = ε E = n E, the discontinuity in the electric
2
field is given by:

E ⎛ n ⎞ 2
slot = silicon
E ⎜ ⎝ n slot ⎠ ⎟
silicon
For air core waveguide, this ratio can go as high as 12. Even if the
slot is to be filled with water, a high ratio of 7 is expected, making this
waveguide very attractive for applications where small mode size
and large overlap between the liquid and the optical mode is of inter-
est. The slot waveguide was also realized with Si N as a core material
3 4
[26]. This material platform is less attractive in terms of field confine-
ment because of the lower refractive index contrast, but on the other
hand it can operate in the visible range, thus offering an important
advantage for many biosensing applications. Si N slot waveguides
3 4
were realized with dimensions in the order of a single-micron width
and 300-nm height. Typical slot widths are ~200 nm. Nitride-based