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

拟议的研究的目的是探索使用二次谐波产生（SHG）的非线性光学技术开发一个子表面成像方法，能够无损检测和评估的腐蚀和开裂的涂漆金属和复合层状材料。 实验安排将利用飞秒光脉冲在800-2000 nm的光谱范围内从钛：蓝宝石激光器和放大器系统和光学参量放大器泵浦它的二次谐波产生。 激光束将聚焦到样品中的一个点上; SHG信号将在反向传播几何中收集，由相敏探测器感测，并由计算机记录。 通过光栅扫描入射激光束，收集SHG信号，并将归一化信号绘制为x-y平面中位置的函数，将逐点生成特定深度（z）的二维（2-D）x-y图像。 通过调整焦斑的z位置，可进入样品内的不同深度（z）。 针对不同z值的一系列2-D x-y扫描将提供样品的3-D亚表面断层图像。 待研究的样品包括铆钉、半导体和电介质晶片、涂漆石英板、腐蚀或/和划伤的金属板，以及来自飞机机翼、桥梁和其他结构的老化金属样品。 所提出的方法的意义是：能够获得3D图像的深度~500 mm，更高的空间分辨率~20 mm，和潜在的金属腐蚀速率估计腐蚀厚度测量。 该项目将对土木、机械、电气和光学材料系统中的裂缝、腐蚀和缺陷的检测产生影响，并将研究和教育活动整合到一个为弱势少数群体和美国主流社会新来者服务的学术机构中。

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.