Thermoelastic modeling: application to super-resolution in photothermal and thermoelastic microscopy

Photothermal and thermoelastic microscopies are nondestructive methods using optical excitation and detection. In photothermal microscopy, the photoreflectance is used to detect the dynamic component of the surface temperature. In our microscope, the normal component of the thermoelastic displacement is also detected with a laser probe, leading to thermoelastic images. Both methods are used to image surface and subsurface inhomogeneities of the investigated object. A thermoelastic model has been developed to calculate the temperature and the displacement fields in the bulk and at the surface of an isotropic solid. Modeling is applied to the case of limited size optical excitation, corresponding to super-resolution. Theoretical temperature profiles show that the resolution essentially depends on the radius of the excitation beam. Conversely, the thermoelastic displacement provides a lower resolution. Finally, experimental devices are presented. Some images of test samples are shown to place in evidence the different resolutions obtained with thermal and thermoelastic methods in the super-resolution case. An extrapolation of this study should allow to fix the values of the experimental parameters to optimize a microscope using a nanometer sized source.