Subsurface Imaging of Material Cracking and Corrosion Under Paints Using Femtosecond Second Harmonic Generation

Cuny City College, New York NY

Investigators

Abstract

The objective of the proposed research is to explore the use of the nonlinear optical technique of second-harmonic generation (SHG) for developing a sub-surface imaging approach that is capable of non-destructive detection and evaluation of corrosion and cracking of painted metals and composite layered materials. The experimental arrangement will make use of femtosecond light pulses in the 800-2000 nm spectral range from a Ti:sapphire laser and amplifier system and an optical parametric amplifier pumped by it for second harmonic generation. The laser beam will be focused onto a spot in the sample; SHG signal will be collected in back-propagation geometry, sensed by a phase-sensitive detector, and recorded by a computer. A two-dimensional (2-D) x-y image for a particular depth (z) will be generated point-by-point by raster scanning the incident laser beam, collecting the SHG signal, and plotting the normalized signal as a function of position in the x-y plane. Different depths (z) within the sample will be accessed by adjusting the z-position of the focal spot. A series of 2-D x-y scans for different z-values will provide the 3-D sub-surface tomographic images of the sample. The samples to be investigated include rivets, semiconductor and dielectric wafers, painted quartz plates, corroded or/and scratched metal plates, as well as aged metal specimens from airplane wing, bridge, and other structures. Significance of the proposed approach are: ability to obtain 3-D images up to a depth of ~500 mm, higher spatial resolution of ~20 mm, and potential for estimation of metallic corrosion rate from corrosion thickness measurements. The project will have an impact on sensing of cracks, corrosion, and defects in civil, mechanical, electrical, and optical material systems, and integration of research and education activities in an academic institution serving the disadvantaged minorities and newcomers to the mainstream of American society.