Page 207 - Optofluidics Fundamentals, Devices, and Applications

182 Cha pte r Ei g h t

Electrowetting was also employed to actuate liquid in ON/
OFF switches [29,30,40]. In switched fluorescence device [29,30],
fluorescent oil is immersed in water on top of a waveguide with
hydrophobic cladding. The waveguide is used to excite the fluo-
rescent oil with a UV light in the ON state. As voltage is applied to
the water, the fluorescent oil layer gets displaced. The new con-
figuration of the water and cladding layers reflects the UV light
internally across the entire waveguide area. In this OFF state, no
optical path exists that would allow violet light to reach the fluo-
rescent oil layer.

8-1-2 Grating-Based Switches
Diffraction gratings are widely used in optics for beam splitting and
spatiotemporal filtering. Beams diffracted by a grating with period Λ
travel at different angles given by:
λ
sinα = m (8-1)
m Λ
Here λ is the wavelength of the normally incident beam, and m is an
integer, called the diffraction order.
Optofluidics enabled design and fabrication of tunable gratings.
Two major families are tunable geometry and tunable refractive index
gratings. An example of the first is self-assembled flowing lattices of
bubbles [41,42], discussed in detail in Chap 3.
Another family of tunable gratings is based on refractive index
tuning. A 1 × 4 free-space optical switch based on transmission blazed
grating was recently reported [43,44] (see Fig. 8-1e and 8-1f). Blazed
grating submerged into a liquid exhibits highly efficient diffraction of
an incident beam into m’th diffraction order when the fluid satisfies
blazed grating condition [45]:
Δ
m = ln (8-2)
λ
Here l is the height of the grating profile, and Δn is the difference
between the refractive indices of the fluid and the material used to
construct the grating. Introducing a liquid with refractive index that
satisfies Eq. (8-2) results in a beam deflection (diffraction) by an angle
given by Eq. (8-1).
It should be noticed that for this type of grating the period of the
phase modulation of the optical field incident on the grating is
modulo Λ when Eq (8-2) is satisfied. For such grating a high dif-
fraction efficiency of order m and consequently low insertion losses
are observed [46].
The design of the 1 × 4 switch was based on salt solutions in water,
which provided refractive indices in the range between 1.33 and 1.41
depending on the salt concentration, which allowed switching
between four different angles for the given geometry. One of the