Longitudinal light profile microscopy (LLPM) is introduced in this work as a new optical method for depth profiling the properties of thin-film materials. The method uses irradiation of an optically polished cross section of a prepared thin film sample, by a laser beam propagating along the depth (longitudinal) axis of the material. An observation microscope, aligned along an orthogonal axis to the sample cross section, transfers an image of the light profile propagating along the material's longitudinal (depth) axis to a recording camera. While depth attenuation of the optical beam in the sample is dominated by light absorption, and turbid scatter (which our analysis neglects), light profile images recovered by the microscope use contrast mechanisms based on luminescence and elastic or inelastic light scatter. Blur contributions to the images arising from axial thickness of the light profile “object” are shown to be minimal in our microscope. Our experimental setup, moreover, is constructed from relatively inexpensive, easily available components. A number of different image contrast mechanisms, including luminescence and elastic scattering contrast, were demonstrated on materials with known optical properties, including continuous media and laminates. The sample dimensions and depth-dependent image features were directly observable and unambiguous to identify. Images recovered on the basis of elastic scattering showed unusual contrast for optical interfaces in materials which were transparent at the analysis wavelength. The method holds promise for providing a plethora of new depth-resolved imaging mechanisms.

Impulse mirage effect spectroscopy is developed in this work as a nondestructive method for depth profiling the optical properties of samples which are nearly thermally homogeneous with depth. Both a theory and an experimental methodology are presented. An inverse scattering theory of the experimental photothermal deflection signal is derived, based on a previous theory of the impulse mirage effect, which takes into account the effect of Fresnel diffraction on the probe beam. To reconstruct the depth profile of heat source density generated by light absorption in an unknown sample, we have applied our inverse theory to the experimental impulse response, using a regularized minimum square error reconstruction algorithm based on our previously published expectation minimum principle. Because this reconstruction problem is ill posed, it was necessary to identify and compensate for all experimental bias errors significantly affecting the fidelity of the depth profiles. A procedure for obtaining the overall best-fit model of the depth profile given the minimum prior experimental information is presented. These procedures have produced an agreement between the experimental and theoretically predicted mirage effect response to within typical root-mean-square error levels of 0.5% or less.

Light profile microscopy (LPM) is a recently developed technique of optical inspection that is used to record micrometer-scale images of thin-film cross-sections on a direct basis. In single beam mode, LPM provides image contrast based on luminescence, elastic, and/or inelastic scatter. However, LPM may also be used to depth profile the optical absorption coefficient of a thin film based on a method of dual beam irradiation presented in this work. The method uses a pair of collimated laser beams to consecutively irradiate a film from two opposing directions along the depth axis. An average profile of the beam's light intensity variation through the material is recovered for each direction and used to compute a depth-dependent differential absorbance profile. This latter quantity is shown from theory to be related to the film's depth-dependent optical absorption coefficient through a simple linear model that may be inverted by standard methods of numerical linear algebra. The inverse problem is relatively well posed, showing good immunity to data errors. This profilometry method is experimentally applied to a set of well-characterized materials with known absorption properties over a scale of tens of micrometers, and the reconstructed absorption profiles were found to be highly consistent with the reference data.

Mirage effect spectrometry is experimentally evaluated in this work as a technique of optical depth profiling in thin films where no prior information is available about the sample properties. An apparatus suitable for performing quantitative measurements is described. High-precision experimental alignment procedures are introduced along with a new method for precise optical correction of the detector signal for experimental frequency response nonuniformities. Reconstructions were made of the heat source density and absorption coefficient depth profile in materials with known depth dependence. These included samples approximating weighted delta function arrays, and depth-continuous media known to obey Beer's law to a good approximation. The properties of these samples were examined independently by using a technique of depth-sensitive light microscopy. Mirage effect depth profiles reconstructed on samples containing discrete absorbers were effectively regularization limited, indicating that resolution is limited by random error in the data rather than experimental bias. Depth profiles obtained in continuously absorbing media show a good agreement with those obtained by reference methods.

Light profile microscopy (LPM) is a recently developed technique of optical inspection that is used to record micrometer scale images of thin film cross-sections on a direct basis. This technique uses a novel right-angle imaging geometry that shows outstanding contrast for subtle interface structures and morphologies that are invisible to conventional methods of inspection. When laser sources are used for sample illumination, image contrast is provided by luminescence and elastic and/or inelastic scatter. When a white-light excitation source is used for LPM, primary contrast is obtained from elastic scatter, while secondary contrast results from refraction, secondary transmission, and secondary reflection from material phases. We term this mode of inspection broadband light profile microscopy (BB-LPM). It is implemented with a compact, easily aligned apparatus and minimal sample preparation, and it shows outstanding interface contrast similar to laser LPM. In this work we demonstrate BB-LPM as a method for direct imaging of the layers structures of a variety of thin film samples of industrial and manufacturing interest.