Differential Terahertz Imaging Methods for Enhanced Detection of Subsurface Features, Flaws, and Damage

Picometrix proposes to demonstrate the feasibility of using differential time domain terahertz imaging methods to enhance the contrast and detectability of features such as kissing disbonds and cracks that in conventional THz imaging only weakly reflect or scatter the THz pulses. The goal of the project is to develop methods of shearographic loading of the samples, and use the penetrating THz pulses to detect the subsurface deformation of the defects in the differential THz images with better contrast than traditional THz imaging. In a "kissing" disbond there is a region where the two sides of the material are not adhered, but the space between the two sides are essentially in perfect optical contact. When the space between the two interfaces is so optically "thin," the reflections of the THz pulses from the top and bottom surfaces cancel each other out. The defect signature is only weakly detectable compared to when the spacing is greater than the minimum THz wavelength (approx. 50-150 microns), the shearographic loading will microscopically deform defects, changing the small THz reflections in the loaded vs. unloaded state. The differential images should subtract all background clutter and highlight the microscopic subsurface distortion of the defects under loading.