From Photoacoustic Microscopy to Thermal-Wave Imaging
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MRS BULLETIN/OCTOBER 1996
separate image buffers in a computer so that when the scan is completed, each may be displayed on the screen as an image of the area scanned. This procedure has been termed scanned photoacoustic microscopy (SPAM). Any irregularities in the absorption of the light or in the conduction of heat away from the heated point on the sample will show up as patterns in the image. By exploiting the fact that these images depend on the subsurface thermal properties, it is possible to obtain information about subsurface structure, including the presence of cracks, delaminations, etc. An example of
such a pair (magnitude and phase) of images of two fatigue cracks in an aluminum alloy test specimen is shown in Figure 2. The magnitude image (see Figure 2a) displays the presence of the cracks through the absorption of the laser beam in open regions of the cracks (lighter regions). On the other hand, the phase image (see Figure 2b) of the same cracks shows subsurface features where the crack planes slant away from the vertical direction under the surface. These show up as lighter regions in the phase image on one side or the other of the cracks. Because of the acoustic properties of the gas and the response characteristics of the microphone, the frequency range of such experiments is typically from a few 10s of hertz to a few 10s of kilohertz. However acoustic waves are also generated within the sample itself by thermoelastic effects in the region where the energy is absorbed from the heating laser beam. These elastic vibrations of the sample can also be detected by a suitable transducer. The transducer could be a conventional piezoelectric transducer or some form of optical-surface displacement probe. An example of the latter is shown schematically in Figure 3, for which the displacement of the vibrating surface expansion "bump" in the vicinity of the heating beam focal region is detected by a second laser (probe) beam and quad-cell position sensor. The vibration of the bump causes the periodic deflection of the probe beam, which is
Intensity-Modulated Laser Heating Beam Microphone
Glass window
Seal
Figure 1. Schematic diagram of the gas cell used for photoacoustic microscopy.
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From Photoacoustic Microscopy to Thermal-Wave Imaging
Magnitude
Phase
54 \i
Figure 2. Photoacoustic images of two fatigue cracks in an aluminum alloy test specimen: (a) magnitude and (b) phase.
Intensity-Modulated Laser Heating Beam
Quad-Cell Position Sensor
Probe beam
detected in two orthogonal directions by the position sensor. As was the case for the gas-cell photoacoustic method, vector lock-in detection is usually employed as a means of reducing the noise bandw i d t h . This probe technique, with a simple position detector, is effective because it operates very close to the source of the vibration where the amplitude is largest. When one wishes to detect the vibration at some distance from the source, say on the back side of the sample, a more sensitive optical probe should be used, typically some form of in
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