expansion with a heterodyne laser interferometer (laser probe). Demodulation is obtained with specific electronics. The magnitude and phase of the surface vibration are given with a second lock-in amplifier (lock-in amplifier 1) and stored in a microcomputer that also drives the scanning units. With this multi-acquisition microscope, the typical duration of an experiment in order to obtain a set of five low noise images is about 15 minutes. The resolution of the SThEM is given by the size at the photothermal source (radius of the optical beam: 5 /xm here). 4.1. Application to the study of thin films The first example concerns the observation of subsurface thin layers. In order to demonstrate the capacity for subsurface investigation we successively vapour deposited a 200 nm thick SiC>2 and 100 nm thick aluminium layers onto a polycrystalline nickel substrate (Fig. 8a). The bright strip on the right part of the image (Fig. 8b) reveals the presence of the subsurface SiC>2 layer which is optically invisible. This image has been obtained at 220 kHz modulation frequency of the excitation beam. The image contrast corresponds to about 25° phase shift. As the SThEM makes it possible to observe the subsurface we decided to use it for the detection of thin films delamination. We used a 1 /xm thick DLC film deposited on a steel substrate. Several lines of Vickers indentations were performed under an applied load of 4.5N. A different spacing (25 to 140 pim) between indentations has been taken for each line. The SEM and thermoelastic images of the indentations spaced 25 /xm are shown in Fig. 9. Due to the film delamination, an optically invisible bright area between the indentations (Fig. 9a) was observed by the SThEM at 100 kHz operating frequency (Fig. 9b). It is an indication of the excessive heating resulting from the film delamination. The latter is due to the tensile residual stresses which develop around each indentation. The bright area (film delamination) could not be detected both in the case of a single indentation or when the spacing between indentations was higher than 40 /xm. In the latter case