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

提出的研究目标是探索使用非线性光学技术的二次谐波产生（SHG）来开发一种地下成像方法，该方法能够无损地检测和评估涂漆金属和复合层状材料的腐蚀和开裂。实验安排将利用Ti：蓝宝石激光器和放大器系统以及由其泵浦的光参量放大器产生的800-2000 nm光谱范围内的飞秒光脉冲来产生二次谐波。激光束将聚焦在样品中的一个点上；SHG信号将以反向传播几何形状收集，由相敏检测器检测，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平面上位置的函数，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，可以访问样品内部的不同深度(z)。一系列不同z值的二维x-y扫描将提供样品的三维亚表面层析成像。待调查的样品包括铆钉、半导体和介电晶圆、涂漆的石英板、腐蚀或/和划痕的金属板，以及来自飞机机翼、桥梁和其他结构的老化金属样品。该方法的意义在于：能够获得深度为~500 mm的三维图像，具有~20 mm的更高空间分辨率，并且可以通过腐蚀厚度测量来估计金属腐蚀速率。该项目将对土木、机械、电气和光学材料系统的裂缝、腐蚀和缺陷的感知产生影响，并在为弱势少数民族和美国主流社会的新来者服务的学术机构中整合研究和教育活动。

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.