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The macroscopic centro-symmetry of polymer films can be broken by subjecting the sample to a strong static electric field, generated e.g. by surface electrodes or from a corona discharge. These so-called poled polymers often show a large piezo- and/or pyroelectric effect, making them the material of choice in a large variety of applications, such as microphones, pressure sensors or infrared detectors. To ensure uniform poling across the thickness of the polymer film (typically 5..100mm), experimental methods for obtaining a depth-profile of the piezo- or pyroelectric coefficient are necessary. For pyroelectric polymer films, where a temperature variation induces a change in surface charge, the Laser-Intensity Modulation Method (LIMM) is an easily implemented way of measuring pyroelectric distributions with a resolution of approximately 1mm. With a focused laser beam, the same technique can be used to implement a scanning pyroelectrical microscope. An intensity-modulated diode laser illuminates an opaque front electrode on the polymer film, creating a temporal and spatial thermal wave inside the sample (Fig. 1). An increase in the modulation frequency results in a decreased penetration depth of the thermal wave. Since the pyroelectric signal is dominated by regions where the thermal wave amplitude is high, ist depth-profile can be measured by scanning the frequency over several orders of magnitude. Absolute calibration of the pyroelectric depth-profiles requires the knowledge of the laser-induced temperature change at the surface of the sample. Small temperature changes can be detected with a bolometric technique, where the resistance of the aluminium surface electrode is measured.
Figure 1: Schematic setup of a LIMM experiment (after [4]). The thermal wave method is only one of several techniques developed for investigating pyroelectric depth-profiles. For example, pulsed lasers have been used instead of modulated cw lasers to generate a time-dependent temperature step or pulse in the sample. Refer to for further references to pyroelectric measurement techniques. In a LIMM experiment one measures the pyroelectric current I~ as a function of the modulation frequency w . The depth-profile of the pyroelectric coefficient p(x) (x: coordinate in space) is buried in a integral equation for I~, which is a Fredholm integral equation of the first kind. It is possible, a solution to find for this mathematical problem by a approximative technique. This method is discussed in detail in the manual for this experiment. Experimental steps:
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