Thermal wave detection

In situations where absorption of the incident radiation by the transducing gas is troublesome a piezoelectric transducer (made from barium titanate, for example) can be attached to the sample (or sample cuvette in the case of liquids) to detect the thermal wave generated in the sample by the modulated light (8,9). The low frequency, critically damped thermal wave bends the sample and transducer thus producing the piezoelectric response. The piezoelectric transducer will also respond to a sound wave in the solid or liquid but only efficiently at a resonant frequency of the transducer typically of the order of 10 to 100 KHz (see Figure 4). Thus neither in the case of microphonic nor piezoelectric detection is the PA effect strictly an acoustic phenomenon but rather a thermal diffusion phenomenon, and the term "photoacoustic" is a now well established misnomer. 

To satisfy all the above requirements one needs to utilize lasers for both the generation and the detection of the thermal waves. The generation is, of course, straightforward. The detection is more involved, performed either by interferometric detection of the thermoelastic displacements of the sample surface or by laser detection of the local thermoelastic deformations of the surface. All the other methods for thermal-wave detection suffer from either being limited to low modulation frequencies or from needing contact to the sample. 

There have been some initial studies of thermal-wave detection using the techniques described above. Ash and his colleagues have performed some imaging experiments with the laser interferometric technique, (8-11) while Amer and his colleagues have used both the laser interferometric and a laser deflection (surface deformation) technique for spectroscopic studies on amorphous silicon. (12-13) These various investigations were all performed at low to moderate modulation frequencies (<100 kHz) only. 

That is, the acoustic waves simply carry or amplify the information describing the thermal-wave event. In thermal-wave imaging, the thermoacoustlc waves are thus used as a monitor to detect the presence of thermal waves scattered or reflected from thermal features. 

Applications. We have performed, with the system described above, high-resolution, thermal-wave imaging on many different materials. We have detected and Imaged subsurface mechanical defects such as microcracks and voids, grain boundaries, grains, and dislocations, and dopant regions and lattice variations in crystals. 

The technique of infrared thermography relies upon the detection of infrared radiation emitted from the surface of a structure. An infrared scanning unit converts electromagnetic thermal energy radiated from an object into electronic video signals and produces color-coded maps of isotherms (64). Differences in thermal waves on the surface of a material can be detected that will make certain flaws visible and allow detection of flaws with low-temperature differentials with respect to the surrounding area within... 

Fig. 1.35 Optothermal spectroscopy of solids and of molecules adsorbed at surfaces (a) propagation of thermal wave induced by a pulsed laser (b) deformation of a surface detected by the deflection of a HeNe laser beam (c) time profile of surface temperature change following pulsed illumination [90]...

Mandelis has reviewed photothermal TA techniques. Thermal waves may be optically induced in solid samples by modulated irradiation. These thermal waves then interact directly with the sample and such interaction is detected by suitable sensors. Acoustic waves may be simultaneously induced and detected. These techniques have specialized application to solid-state systems to determine thermal transport properties such as thermal conductivity, diffusivity/effusivity, and specific heat capacity. These techniques are of particular significance in the determination of mechanisms of solid-state phase transitions. 

The third part of the book is devoted to the medical and technical applications of liquid crystals. The colour of cholesteric liquid crystals changes with changes in temperature or other variables of state. This feature can be used in thermographic applications, for instance, thermal mapping of human skin for the diagnosis of circulatory diseases or the detection of tumors. In modern industrial applications this property is of value for direct temperature diagrams, detection of wave fields, locating faults in electronic devices and for thermally activated information displays. 

Photoacoustics is well established [36] for the investigation of optical and thermal properties of condensed matter, but it has only rather recently been applied to liquid crystals [37-41]. The photoacoustic technique is based on the periodic heating of a sample, induced by the absorption of modulated or chopped (electromagnetic) radiation. In the gas microphone detection configuration the sample is contained in a gas-tight cell (Fig. 6). The thermal wave produced in the sample by the absorbed radiation couples back to the gas above the sample and periodically changes the tempera-... 

If this stress gives rise to acoustic emission, this emission can be detected and an image of the discontinuity can be made. The thermal wave technique can be used to detect subsurface flaws in the material. The SLAM is an analytical technique based on this effect. 

Scanning laser acoustic microscopy (SLAM) (characterization) In SLAM, a pulsed laser introduces a thermal wave into the material. A discontinuity in the material through which the thermal pulse passes can give rise to acoustic emission, which is then detected. 

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