KEYWORDS: Polishing, Silica, Color centers, Scanning electron microscopy, Profiling, Electron beams, Surface finishing, Defect detection, Solar thermal energy, Oxygen
Polishing of optical components produces defects in the subsurface resulting in a decrease in their performance. These defects can be large, like cracks, but also punctual, like oxygen vacancies or non-bridging bonds. Under electronic excitation, punctual defects can produce luminescence (also called cathodoluminescence) and/or can trap electrical charges. In a first work, we have shown that the cathodoluminescence technique is a good way to detect punctual defects (color centers) in silica. In this work, we have improved our cathodoluminescence technique in order to measure the depth profiling of color centers. We have also used the “Charge Contrast Imaging” technique in environmental SEM to observe directly sub-surface scratches generated by polishing.
In the field of the development of LIL and LMJ fusion class high power lasers, CEA has made important efforts to understand and improve laser induced damage threshold of fused silica optics at the wavelength of 351 nm. Since several years, we have focused on optimizing the grinding, polishing and post polishing processes to overcome the existing performances with various industrials and academics partners. In this paper, we describe our understanding of the nature of the polished silica interface and our approach to rich our damage threshold goal. Our efforts were mainly put on reducing the cracks region extension and removing or optimizing the polishing top layer. We give also some details on the influences of each of the polishing process from rough material grinding to post processing. We demonstrate that some order of magnitude in laser damage initiation density can be gained by combining appropriate fabrication steps.
The polishing of optical components introduces defects in subsurface that can contribute to the damage initiation and the decrease of optical performance. These defects can trap electric charges or could be precursors to color center. We have proposed to study subsurface defects generated by the surface finishing by using trapping of charges and cathodoluminescence measurements. These two techniques are complementary: the cathodoluminescence gives information on defects having radiative decay and the grounded current can also be sensitive to other defects having non-radiative decay. It is shown that they are able to distinguish the surface finishing of natural silica. The defects are mainly concentrated in the first micron layer. We have also found that the repartition of defects on the surface in not homogeneous.
Polishing of optical components produces defects in the subsurface resulting in a decrease in their performance. These defects can be large, like cracks, but also punctual, like oxygen vacancies or non bridging bonds. Under electronic excitation, punctual defects can produce luminescence (also called cathodoluminescence) and/or can trap electrical charges. We have developed a new technique for investigating the trapping of charge and the cathodoluminescence in insulators by using a Scanning Electron Microscope (SEM). Then, it is possible to control the depth of electron penetration by changing the electron beam energy. Therefore, we hope that this technique allows measuring the depth profile of subsurface punctual defects due to the polishing. In this work, we have investigated amorphous silica prepared with different surface finishing.
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