Mirage effect spectrometry and light profile microscopy: Two views of an optical depth profile (abstract)

Photothermal depth profiling techniques are well adapted for the inspection of optically absorbing features on the length scale of 1–100 μm in a variety of media. However, the depth profiling mechanism intrinsic to thermal wave imaging is inherently ill posed [J. F. Power, AIP Conf. Proc. 463, 3 (1999)], and suffers obvious disadvantages such as sensitivity to experimental errors (requiring regularization) and subsurface broadening of the regularized depth profiles. Recently, through the introduction of light profile microscopy (LPM) an alternate method of optical inspection was made available for depth profiling optically absorbing, scattering, and luminescent structures on this length scale [J. F. Power and S. W. Fu, Appl. Spectros. 53, 1507 (1999); J. F. Power and S. W. Fu, U.S. Patent Pending]. LPM inspects a thin film under test by directing a laser beam through the material along the depth axis, parallel to a polished cross-sectional viewing surface. Luminescence and elastic scatter excited in the beam volume is imaged by a microscope aligned orthogonal to the beam axis. The images obtained by this method showed striking depth contrast in a variety of materials with subsurface interfaces and depth variations of luminescence yield. When implemented in dual beam mode [J. F. Power and S. W. Fu, U.S. Patent Pending; J. F. Power and S. W. Fu, (unpublished)] with an associated mathematical method, LPM may be used to quantitatively resolve depth variable optical absorption from light scattering and luminescence efficiency. In contrast to photothermal methods, the LPM technique is well posed. LPM was evaluated in tandem with mirage effect spectrometry (in normal deflection mode with bicell detection) [J. F. Power, S. W. Fu, and M. A. Schweitzer, Appl. Spectros. 54, 110 (2000)], to determine the effective use of each technique in analysis problems on complex materials. This study used samples with known depth variations of optical properties including homogeneous absorber layers, and structures composed of thin laminate assemblies of photodegraded polymers which could be disassembled and independently studied using UV-visible spectrophotometry. LPM shows consistent high sensitivity to sharp interfaces and suffers no degradation of spatial resolution with depth, while photothermal depth profiling shows substantial resolution loss. However, photothermal depth profiling exhibits a number of complementary advantages. These include a substantially enhanced sensitivity for depth profiling optical absorption (over LPM), and insensitivity to isolated optical defects and moderate levels of light scattering. The photothermal depth profiling method also had superior spatial averaging properties, which presented a smoothed picture of profiles containing regions of spurious, enhanced absorption or scattering, to which LPM is sensitive. Currently, inversion techniques based on the generalized singular value decomposition are being considered to evaluate the joint information available from both optical and photothermal probes. A full discussion of the relevant instrumental and mathematical issues will be presented.

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