Bulk Optical Sensing Techniques

Until now most efforts in the development of miniaturized analyzers are based on the use of photometric and electrochemical detection. During the workshop many examples will be given. It can be easily shown that miniaturizing the devices with respect to diameters of channels, injected volumes, etc. asks for a more than proportional reduction in the detector volume [6]. In this respect the important development of (ultra-) microelectrodes has to be mentioned. Because electrochemical techniques, but also optical ones like surface plasmon resonance, are essentially based on surface phenomena they are critically dependent on the condition of the sensing surfaces. As these surfaces can often easily be affected in an irreversible way, extreme care in handling is required to get reproducible results. The search for other techniques related to bulk properties of the samples should therefore be continued. These may comprise optical as well as magnetic or enthalpimetric principles. 

In its simplest version the QPM technique for second harmonic generation (SHG) consists in alternatively reversing the polar axis direction in successive thin layers of the same thickness J of a crystalline material. This leads to a reversal of sign of in successive layers of the layered material and one can easily see that ii d = ml = mnl/ k then the conversion efficiency increases with the number of layers. This actually amounts to periodically modulating in the nonlinear optical source = yP )EE in Equation [9] with a wave vector that precisely matches and cancels Isk in the propagation direction note that the linear refractive index is unaffected and remains the same as in the bulk crystal while I2 grows as (Figure 3). In a certain sense the quasiphase-matched systems can also be viewed as a nonlinear photonic crystal. 

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