Subsurface damage detection of optical elements by analyzing the photobleaching properties of quantum dots

Tagging the lapping and polishing slurries with quantum dots (QDs) is a promising method for detecting the distribution and depth of subsurface damage (SSD) in optical element at the same time. However, QDs are prone to photobleaching which affects the detection accuracy of SSD distribution and depth. The photobleaching properties of QDs with different structure and composition are different. This paper proposes a SSD detection method of optical element by analyzing the photobleaching properties of QDs. C QDs, CdSe QDs, CdSe/ZnS QDs, InP/ZnS QDs, and CuInS2/ZnS QDs were selected to tag SSD of optical element. A fluorescence detection system for SSD was developed to collect fluorescence images of QD-tagged optical elements under various excitation intensities, excitation time, and excitation numbers. The photobleaching properties of the QDs in SSD of optical elements were quantitatively analyzed by two parameters: (1) fluorescence conversion efficiency under various excitation intensities and (2) fluorescence stability index under various excitation time and excitation numbers. The distribution and depth of SSD were detected to comprehensively analyze the influence of the photobleaching properties of QDs on the SSD detection accuracy. The results show that the anti-photobleaching properties of C QDs, InP/ZnS QDs, CdSe/ZnS QDs, and CuInS2/ZnS QDs were strong, and they can detect SSD distribution and depth with high accuracy; while the anti-photobleaching property of CdSe QDs was poor, and they can detect SSD distribution and depth with low accuracy, making them unsuitable for tagging SSD. For CdSe/ZnS QDs, they can be used to detect SSD accurately only when the excitation intensity was higher than 33 mW. This paper lays a foundation for investigating the formation mechanism of SSD and detecting SSD non-destructively and accurately.