Page 53 - Optofluidics Fundamentals, Devices, and Applications

34 Cha pte r T h ree

3. It is straightforward to obtain a graded profile of refractive
index in L systems by taking advantage of diffusion between
2
miscible liquids possessing different refractive indices.
2
4. Since the L devices are formed inside a microfluidic channel,
the manipulation of the liquids used for optics in microchannels
is the same as that of liquids used for other purposes (separa-
tions, reagent storage, sample preparation, etc.). It is thus pos-
sible to design and cofabricate the channels for the optical parts
of integrated system, and for other parts simultaneously. This
2
feature facilitates integration and prealignment of L devices to
the relevant components on the same microfluidic platform.
This chapter has two objectives:
1. To discuss the basic construction of L devices, and the char-
2
2
acteristics of dynamic L interfaces formed between laminar
streams in microchannels
2. To give examples of optofluidic devices—L waveguides, L 2
2
lenses, L light sources, and bubble diffraction grating—to
2
demonstrate the design and operation of these devices.

3-2 Basic Design and Construction
of Liquid-Liquid Devices
2
Typically, L devices consist of multiple streams of liquids possessing
different optical properties (such as refractive indices) coflowing in a
single microchannel. Figure 3-1 shows a representative design of an L 2
device. It consists of multiple inlets for different liquids to flow into a
main channel. Depending on the application, this main channel can
have different geometries (a straight channel of uniform width is
2
shown in Fig. 3-1). To form and maintain the L interface, liquids are

Liquid 1 Microchannel wall To fluid outlet
Laminar interface
Liquid 2
Light
input
Light output

Liquid 3

To fluid outlet
2
FIGURE 3-1 Schematic representation of the typical design of an L device.