Photoacoustic Microscopy of Ceramics Using Laser Heterodyne Detection
In recent years a variety of thermoacoustic techniques have been used to image surface and near surface features in ceramics. These include early gas cell methods  as well as the Scanning Electron Acoustic Microscopy (SEAM) technique  and more recently the Mirage or Optical Beam Deflection (OBD) methods [3,4]. In the gas cell and Mirage methods, the effect of the outgoing thermal wave on the air boundary at the sample surface is sensed, respectively, by a microphone and by a laser beam. In SEAM the specimen vibration or acoustic wave is sensed directly by a contacting transducer. The gas cell and Mirage methods generate pure thermal images but require long times to generate useful images of areas a few millimeters square. SEAM, on the other hand, is a high signal/noise technique due to the exceptional sensitivity of piezoelectrics and thus requires shorter imaging times. However, SEAM is sensitive to both the thermal wave signal and the local mechanical response of the specimen to the thermal wave and convolutes the two responses. The author has recently demonstrated that the entire image can be dominated by the mechanical response alone . Thus SEAM image interpretation is considerably clearer for such cases. This thermomechanical mechanism is now fairly well understood and its analysis will be presented elsewhere. SEAM is thus an excellent method for imaging ceramics. Using Coordinate Modulation (CM) with SEAM 5mm x 5mm areas of ceramics have been imaged for surface and subsurface defects in 2 minutes. CM requires the electron beam to be dithered a small amount instead of being intensity modulated, the usual approach. This increases image contrast and will be described later.