External instruments

All the investigated devices have their actual sensing and actuating units, respectively, directly integrated on chip. The optofluidic chip itself is then connected to external instruments using the previously described optical and fluidic systems. Elements such as the light source and photodetectors are not integrated on-chip but provided externally. A typical overall arrange-ment for an optofluidic experiarrange-mental setup is illustrated in Fig. 3.18. The optofluidic chip is placed in the center of the setup. Glass fibers and PTFE tubings provide the physical connections into the device. As mentioned ear-lier, an external light source is used in all the experiments. The chip itself is placed on a mounting stage underneath an optical microscope. For the actual functionality of the system typically no visual inspection is neces-sary. Nevertheless, in the development phase a permanent verification of the ongoings inside the device is essential. Using a microscope the obtained results can be correlated to the actual behavior on-chip and are hence ver-ified. For data recording, a digital camera can be assembled on top of the

Syringe pump for supply of samples

External light source

Digital camera

Light microscope

Photodetectors

Oscilloscope Fluidic

outlet Optofluidic chip

Fluidic inlet

Figure 3.18: Schematic of a typical optofluidic experimental setup. The optofluidic chip is placed in the center underneath a light microscope having a digital camera attached. A syringe pump and PTFE tubings, an external light source and glass fibers, standard photodetectors, and an oscilloscope complete the setup.

microscope and can be connected to a PC. If a controlled flow of liquid is essential, a high-precision syringe pump can be integrated in the setup.

For applications not demanding specific flow rates the injection can be done manually as well. The actual results can either be obtained through the mi-croscope images or through light signals coupled into additional glassfibers.

Those glass fibers are then connected to standard photodetectors. The use of photomultiplier tubes was intentionally avoided for two reasons. Atfirst, standard photodetectors are much more cost effective than photomultiplier tubes. And secondly, photodiodes hold the potential for integration into the optofluidic chip whereas photomultiplier tubes don’t. The electrical signals are then displayed on an oscilloscope.

In general, much attention is paid to the selection of the external

com-3.4 Peripheral connections 49 ponents. The peripheral system is tried to be kept as small and simple as possible. Furthermore, expensive instruments are avoided to allow a cost effective system to be built up. Finally, the main components are selected such that a future integration into the optofluidic chip is possible. Laser diodes and photo diodes are components which can already be integrated on-chip. For systems asking for a pumping system one could think of im-plementation of an electro-osmotic drivenflow. Such pumping sources can be integrated and are much space optimized compared to the bulky external syringe pumps. In that sense, in future a cost effective, compact, hand-held system can be aimed for.

Chapter 4

On-chip light modulation

This chapter compares two basically different approaches for the realization of optofluidic light modulation units. After explaining the device designs both modulators are examined experimentally. Finally the advantages and disadvantages of each approach are discussed.

The work presented in this chapter has been published in [36,38,57,58].

4.1 Introduction

Optical principles are embedded in different ways for actuation as well as sensor elements [59] on microfluidic platforms. One example isfluorescent labeling, which is exploited for the visualization and examination of various biological and chemical reactions [60, 61]. For this kind of analysis, optics plays an important role in both excitation and detection. Optofluidics allows to integrate the two disciplines offluidics and optics on a single chip where liquids not only coexist with light but take over the role of solid optical el-ements. They are directly integrated in the functional optical path. Once coupled into the device [62], light can interact with liquids for a vast variety of applications. Active elements, such as coherent light sources [63, 64], as well as passive ones, such as waveguides and lenses [16, 65–68], have already been successfully implemented. Compared to its solid counterparts, such optofluidic devices provide an enormously increasedflexibility as well as tunability. Liquids can either be easily exchanged or streams of liquids in a microfluidic channel can be changed in size by alternating their inlet velocities resulting in continuously reconfigurable characteristics of the de-vices.

Especially for on-chipfluorescence excitation a sophisticated optical ar-rangement is crucial. The examined amount of sample in microfluidic de-vices is extremely low calling for a precise spatial control of possible stimuli (e.g., light for the excitation of fluorescence). Furthermore, a temporarily

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confined time slot for the exposure of the analyte to light results in reduced photo bleaching issues. This has led to the investigation of various on-chip light modulation principles. Xuet al.[69] have developed a system compris-ing a senscompris-ing as well as switchcompris-ing unit based onfiber optics. Songet al.[70]

published an integrated 2x2 optical switch based on pneumatically opened and closed air gaps. Campbell et al. [71] have developed a multi-layer 2x2 optical switch exploiting the different reflection/transmission properties of two liquids. Both groups apply mechanically moving parts for the light switching process. Although providing good switching characteristics, the use of mechanically moving parts automatically induces wearing and stick-ing issues. Seow et al. [72] proposed an optofluidic switch without any moving parts based on an L2-waveguide. In that work, flow rates are ad-justed carefully to alter the width of the liquid core resulting in two possible light switching states. This principle based on an L2-waveguide does not employ any moving elements but depends on very stable flow conditions.

In the following sections, two on-chip optofluidic, contact-free light modulation units, both of them working without any moving elements are presented. The first modulator exploits the laminarflow conditions in a mi-crofluidic channel to build up an L2-waveguide. Hydrodynamic pressure differences are then used to steer this waveguide. In a second realization the light modulation is performed without the need of a permanentflow through the device which allows more stable light configurations to be achieved.