NONDESTRUCTIVE OPTICAL DEPTH PROFILING IN THIN-FILMS THROUGH ROBUST INVERSION OF THE LASER PHOTOPYROELECTRIC EFFECT IMPULSE-RESPONSE

The laser photopyroelectric effect measures an optical absorption dept h profile in a thin film through the spatial dependence of a heat flux source established below the film surface by light absorption from a short optical pulse. In this work, inverse depth profile reconstructio n was achieved by means of an inverse method based on the expectation- minimum principle (as reported in a companion paper), applied in conju nction with a constrained least-squares minimization, to invert the ph otopyroelectric theory. This method and zero-order Tikhonov regulariza tion were applied to the inversion of experimental photopyroelectric d ata obtained from samples with a variety of discrete and continuous de pth dependences of optical absorption. While both methods were found t o deliver stable and accurate performance under experimental condition s, the method based on the constrained expectation-minimum principle w as found to exhibit improved resolution and robustness over zero-order Tikhonov regularization.