Digital holographic characterization of multilayered structures by thermal scanning

Imaging hidden or occluded objects is crucial for their identification and characterization in various fields, including material science, biomedical imaging, and nondestructive testing. Digital holography, which provides both amplitude and phase information, is especially effective for detecting deformation in stressed samples. When combined with external stimuli, such as thermal stressing, it becomes a powerful tool for imaging, identifying, and characterizing hidden or occluded inhomogeneities within the investigated sample. In this work, we present a technique based on digital holography, coupled with axial thermal scanning for identification and localization of hidden layers in translucent samples. The method scans the sample volume along the axial direction using a moving heat source, creating a spatiotemporal refractive index distribution within the object. A Fourier transform digital holographic interferometer setup captures the changes in the probe beam interacting with this refractive index distribution as digital holograms. The phase distribution from numerically reconstructed digital holograms reveals information specific to the scanned region, leading to the imaging and localization of the inhomogeneities in the sample volume. These objects can then be characterized based on their thermo-optic properties. We demonstrate the proof of concept and present results for the detection of layers and subsurface structures within multilayered samples.