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

这项研究的目的是探索利用二次谐波(SHG)的非线性光学技术来开发一种亚表面成像方法，该方法能够无损地检测和评估涂装金属和复合材料层状材料的腐蚀和开裂。该实验装置将利用钛宝石激光器和放大系统产生的800-2000 nm光谱范围内的飞秒光脉冲和由其泵浦的光学参数放大器产生二次谐波。激光束将聚焦到样品中的一个光点上；倍频信号将以反向传播几何形式收集，由相敏探测器检测，并由计算机记录。通过对入射激光进行光栅扫描、收集倍频信号并绘制归一化信号作为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。通过调整焦点的z位置，可以访问样本内的不同深度(Z)。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。待调查的样品包括铆钉、半导体和电介质晶片、涂漆的石英板、腐蚀或/和划痕的金属板，以及飞机机翼、桥梁和其他结构的老化金属样品。提出的方法的重要意义在于：能够获得深度达~500 mm的三维图像，更高的空间分辨率(~20 mm)，以及从腐蚀厚度测量估计金属腐蚀速率的潜力。该项目将对民用、机械、电气和光学材料系统的裂缝、腐蚀和缺陷的检测，以及为弱势少数群体和美国社会主流新人服务的学术机构的研究和教育活动的整合产生影响。

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.