J. Eur. Opt. Society-Rapid Publ. 22, 21( 2026) 199
Figure 1.( A) Plasma jet etching of fused silica sample prepared with 19 10 indentations using different indentation forces( HV0.5 and HV0.6). The indentation positions are further classified according to the Ry-Cx scheme. The orientation of the PJE wedge relative to the indentation matrix is depicted by the purple gradient.( B) Etch profiles along the wedge in y-direction with corresponding row positions of the indentation after eight subsequent PJE steps.
low signal intensity, it is challenging to resolve these weak signals and the smallest defects. In a previous study, this issue was addressed by introducing a pre-treatment step involving 30 % potassium hydroxide( KOH) etching at 60 ° C prior to OCT imaging. This wet chemical etching process was found to reduce imaging artifacts and enhance signal contrast, thereby improving the detectability of SSD features [ 61 ].
However, etching rates using 30 % KOH at 60 ° – 0.02 nm / s for polished fused silica and 0.14 nm / s for ground fused silica – necessitate prolonged processing times in this preparation step [ 61 ]. Furthermore, the application of KOH requires various safety precautions to overcome its potential risks with respect to working safety and environmental impact. The application of a plasma-based dry etching process features three orders of magnitude higher etching rate( about 450 nm / s) and can reduce the handling risks of wet chemicals mentioned before. In addition, plasma jet etching( PJE) can be locally applied at surface sites of interest, and specificetchdepthprofiles can be generated [ 63 ]. Therefore, an atmospheric-pressure chemically reactive plasma jet was applied for SSD preparation. The Vickers indentation, sharing many characteristics of ordinary indentation, exhibits a typical crack morphology that has been commonly used to study the generation of cracks in optical materials [ 6, 16, 64 – 66 ]. Typically, the analysis of surface and subsurface crack morphology induced by the Vickers indenter is performed using destructive methods. Elfallagh et al. [ 16 ] reported 3D FIB( focused ion beam) tomographic analysis of the SSD induced around Vickers indents in soda-lime-silicate glass. After the Vickers indentation tests, Yang [ 64 ] measured the SSD of ground ULE-glasses( ultra-low expansion) using the ball-dimple technique. Alternatively, the SSD was investigated by observing the cross-section of the indentations using a scanning electronic microscope [ 65 ]. A few research studies have also been conducted to examine the characteristic changes of Vickers-indented glass using non-destructive methods. Sung et al. [ 67 ] used digital holographic tomography( DHT) to study the refractive index changes under Vickers indentations in silica, soda-lime, and non-alkaline aluminoborosilicate glasses. Lacondemine et al. [ 68 ] directly observed the displacement field and microcracking in glass using X-ray tomography during an in situ Vickers indentation experiment. However, the characterization of SSD in Vickers indentation of fused silica glass using OCT has not been reported in detail to date. In this study, we focus on the quantitative characterization of SSD induced by Vickers indentations in fused silica, using a hybrid approach of PJE and OCT. The stepwise PJE process not only enables controlled material removal while avoiding health risks but is suitable for validating OCT results. Furthermore, PJE improves the imaging conditions for OCT by reducing surface-related artifacts. By integrating both techniques, we aim to establish a reliable method to determine the full depth and morphology of SSD in brittle optical materials. This combined approach is further assessed against empirical estimations and enables the application for precision SSD evaluation in optical manufacturing process chains.
2 Material and methods
2.1 Sample manufacturing using Vickers indentation
Corning Fused Silica HPFS7980 5F( Corning, Inc., denoted by FS) with dimensions of 40 40 10 mm 3 was used in this investigation. The sample surfaces were polished to P3 quality according to the DIN ISO 10110-7 standard. A Vickers testing machine( KB 30 S, KB Prüftechnik GmbH) was used to generate a matrix of Vickers indentations on the FS surface( see Fig. 1), consisting of 19 rows and 10 columns( denoted by R and C). Preliminary tests conducted with this Vickers indenter showed that no SSD was detected up to the application of HV0.4. Therefore, HV0.5 and HV0.6 were used in this study to generate targeted Vickers indentations. Higher loads were avoided, as they produced extensive cracking and non-confined damage,