Laser Speckle Interferometry for Measuring Three-Dimensional Mesoscopic Deformations in Polycrystalline Surfaces

A speckle interferometric microscope has been constructed to simultaneously measure mesoscopic deformations in all three directions at a surface. The purpose is to understand mesoscale mechanics in polycrystalline materials using direct observation. The microscope uses three different wavelength lasers to separate dimensional data (out-of-plane and two in-plane directions) while capturing the intensity of speckle reflected from a surface as a function of position and time. Images are captured before and after deformation, with an intermediate image taken before deformation but with a mirror in one arm of each interferometer tilted. Post-processing exploits the mirror tilts to produce carrier fringes that isolate deformation data in Fourier space and increase sensitivity by orders of magnitude over traditional interferometry. Data are taken with 1 $\mu $m spatial resolution, and 10 nm deformations have been resolved. Experiments underway are using the system to study polycrystalline creep.