H. G. Walther

EXPERIMENTAL RESULTS OF PHOTOTHERMAL MICROSTRUCTURAL DEPTH PROFILING

Experimental results of thermal conductivity depth profiling of case‐hardened steel and mechanically loaded ceramics are presented, based on an inversion algorithm for a sample with piecewise linearly inhomogeneous depth profile of the thermal conductivity. The photothermally obtained profiles are interpreted or compared with results from conventional or destructive measuring techniques.

Theory of microstructural depth profiling by photothermal measurements

An analytical solution for the photothermally measurable surface temperature of a sample with piecewise linearly inhomogeneous depth profile of the thermal conductivity k is presented. Based on this solution an inversion algorithm using a sequence of one‐parameter fits is suggested in order to estimate the k profile. Numerical simulations demonstrate the performance of the approach and its insensitivity to random errors.

Definition, resolution, and contrast in photothermal imaging

Photothermal imaging can be described quantitatively by means of a point spread function (PSF), which represents the image of a buried thermal point defect. From this PSF conclusions can be drawn about amplitude and phase contrast, half‐width, and resolution of photothermal imaging. The theoretical findings are stated by measurements from an appropriately designed model sample.

Separation of optical thin-film and substrate absorption by means of photothermal surface deformation technique

The photothermal surface deformation (PTD) technique is shown to be feasible to separate absorption losses of single‐layer film and substrate absorption from each other by a completely contactless detection of the thermoelastic sample response only appropriately varying the focus radius of the heating beam. This offers a method for a depth‐resolved distinction of absorptive regions within a layered structure by the help of lateral resolved PTD absorption measurements. Additionally, both the sensitivity and feasibility of the apparatus is demonstrated by selected results at 10.6 μm.

Quantitative characterization of material inhomogeneities by thermal waves

Photothermal measurement techniques offer the possibility to determine thermal properties on and below the sample’s surface. Thus, subsurface thermal inhomogeneities such as defects, buried structures, and continuous profiles of thermal parameters become accessible by photothermal means. Our special interest is focused on the quantitative characterization of material modifications in near-surface layers that are induced by thermal (such as hardening) or mechanical (such as grinding) treatments of the surface as well as the reconstruction of a subsurface structure’s depth, size, and defect strength. Altogether, these investigations are aimed to develop photothermal techniques toward a true quantitative and noncontact inspection method for the nondestructive evaluation of opaque solids.

An attempt towards quantitative photothermal microscopy

Based on the description of photothermal image formation as a convolution between a defect distribution and the photothermal point spread function, a technique is presented by which subsurface thermal inhomogeneities can be quantitatively reconstructed from photothermal images. Numerical simulations demonstrate the performance and the limits of the suggested approach. Experimental results obtained from a low thermal contrast sample are presented and compared with the theory.

Proposal for photothermal characterization of boundaries between layer and substrate

The accurate measurement of frequency dependent photothermal phase angle curves allows the characterization of the boundary layer between substrate and coating. Regarding the measurable quantities phase extremum and the ratio of the abscissa for the zero crossing and the extremum we can classify the boundary type as a jump in material properties, as a delamination or as a diffusion zone between substrate and coating.

Analysis of the detectability of buried inhomogeneities by means of photothermal microscopy

Calculated photothermal contrast functions arising from lateral scanning of buried stripe‐ and disk‐shaped deviations in thermal conductivity k are presented. From the theoretical findings the measuring conditions are derived in terms of the modulation frequency for which optimal photothermal discrimination of subsurface thermal inhomogeneities can be achieved. The theoretical results have been verified experimentally be measuring model samples, in which localized inhomogeneities were embedded.

Development of a sensitive detection system based on the photothermal effect for biomolecular interaction studies

THe use of a photothermal detection system for the study of interacting biomolecules is described. Two different setups are presented to demonstrate the performance of the system by measurements of DNA/intercalator-samples immobilized on membrane supports used in molecular biological techniques.

Photothermal Investigations of Realistic Interfaces

The interpretation of photothermal measurements of layered systems demand the modelling of different kinds of interfaces. We found a method to receive the thermal wave reflectivity and the thermal contact resistance of the interface of a system layer/substrate. Furthermore, theoretical and experimental results of the investigation of thick interfaces (‘smeared transition’ between two materials) are given.