Unbiased iterative reconstruction of polarization and space-charge profiles from thermal-wave experiments

The thermal-wave technique or laser-intensity modulation method is an important tool for the non-destructive probing of space-charge and polarization profiles in electrets. Analysing the experimental data requires solving a Fredholm integral equation which is known to be an ill-conditioned problem. This paper presents an iterative approach that is capable of reconstructing inherently unsmooth distributions. The deviations from the true profiles are slightly smaller than those obtained with Tikhonov regularization, while the computational burden is not a limiting factor on modern personal computers. The optimum number of iterations is estimated using the randomized generalized cross-validation technique. Results are shown for a number of model distributions, as well as for experimental data from a layered polyvinylidene fluoride film sandwich.