Fossilized Amber
Papers on this topic may be found by searching for keyword 'amber' in Prof. Federici's papers. Below we show some nice images of fossilized amber.
The figure below shows a visible image of ancient (10 MCE) termites embedded in amber. While these specimens are readily apparent with visible light, not all amber specimens are transparent to visible light.
Below is a terahertz computed tomography image of the same sample highlighting one of the termite's heads.
The intrinsic stresses of amber have also been characterized using polarized terahertz radiation. Below is shown a visible image of a piece of Mexican amber with plant debris inclusions. Also shown is a birefringence transmission image through the sample in the visible range. To find out more about these images, one can google 'photoelasticity'. Note the distortion of the stress map contours near the plant debris inclusions. This is expected since one would expect intrinsic stress to concentrate on defects or inclusions in an otherwise uniform material.
The images below show visible as well as terahertz stress map images of Mexican Amber sample. (a) visible birefringence image (b) calculated contour map of stress in the terahertz range for comparison with visible stress image. Note that the contours of (a) and (b) are very similar. Even though the electromagnetic frequencies of the visible and terahertz waves are orders of magnitude different, in both electromagnetic regimes one is measuring essentially the same stress.(c) stress direction image (d) normalized birefringence.
