Schematics of the mechanisms giving rise to the signal. The surface deformation deflects the probe beam that is partially clipped by the knife edge before reaching the detector. The change in reflectivity with temperature also contributes to the signal.
Dependence of the phase delay with the normalized thermal diffusivity when only the photodeflection term is significant. The maximum slope of about 20 is observed around .
Experimental setup. An AOM chops the green pump beam at the desired frequency. The reflected red signal is partially clipped by the knife edge and collected by the PD. The CCD allows the beams position and size measurement. L: lenses and M: mirrors. L1 is a microscope objective. A LED is included to illuminate the sample and register with the CCD an optical image of the analyzed region.
Example of one measurement and fit. The measured signal is fitted by the functions accounting for the two contributions, the photoreflectivity and the photodeflection .The text box is common to (a) and (b) and shows the values of the least square fit. (a) Amplitude, (b) phase angle, and (c) square error as a function of the fitting critical frequency. This curve is used in order to determine the uncertainty in the fit.
Plot of the thermal diffusivity as a function of the phase delay at a fixed frequency of 150 kHz for a line scan across the treated area. The linear and quadratic fits to the data are also shown. Fit: x and y denote the phase delay and the diffusivity, respectively.
(a) Thermal diffusivity as a function of the position for a cross section. Inset: calibration curve, (b) optical micrograph recorded by the CCD camera, indicating the scanned region in the sample, and (c) optical micrograph for the etched sample, the base material structure (pearlite) is revealed.
Thermal diffusivity vs position for the scanned area shown in a 3D plot. Inset: corresponding 2D map for the same data.
Glossary of terms.
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