68
J. Eur. Opt. Society-Rapid Publ. 21, 7( 2025)
STED powers were too high, resulting in damage to several areas of the sample. Therefore, we sought to modify a set of the proper parameters for the STED microscope system to increase contrast and resolution without any harmful effect on sample.
For the next measurement, we recorded images of a cross-shaped pattern on the NV centres substrate in two cases, based on changes to the optimum STED parameters. For both cases, we used similar parameter values except for the pixel size. To prevent damage to the sample, we reduced the excitation and STED power to 3 % and 5 % of their respective laser powers. To increase the speed of image recording, we decreased the line accumulation value to one. The pinhole size, excitation wavelength, STED wavelength, and frame size are the same for both cases( see Table 3).
Figure 10. PL spectra for different positions on the sample surface as well as the bottom of the sample. Fluorescence emission from NV centres shows two peaks at wavelengths 575 nm( NV 0) and 637 nm( NV �) for different positions. Green dotted boxes show ZPL for( NV 0) and( NV �) separately.
around 700 nm. The result is in good agreement with the typical emission wavelength spectrum of NV centres [ 70 ], showing that the NV centres are distributed evenly over the sample.
4.3.2 STED measurement
In the first step, we tried to find the optimum excitation and STED power, expressed as a percentage of the maximum excitation and STED laser power, to image the patterns of the sample. The maximum pulsed STED and excitation laser powers are 3 W and 150 lW atf rep = 40 MHz respectively. Initially, we imaged a random feature on the substrate. Based on the PL spectrum, a wavelength range between 600 nm and 700 nm was selected to collect the fluorescence emission with the detector. Further parameters are listed in Table 2.
Figure 11 indicates the image of the features in confocal and STED modes. The arrow in Figure 11 indicates each feature. Figure 12 shows the intensity profiles of a crosssection through the features. There is a drop in fluorescence emission intensity around the border of the features in STED mode. The dotted lines in the STED mode diagram indicate areas where the intensity drops to zero. These areas are not clear in the confocal mode diagram compared to the STED mode diagram. Thus, two features with a separation distance of 330 nm can be resolved in STED mode. Moreover, our result shows that the border of the features have a proper contrast in STED mode.
Although high line accumulation improves the signal-tonoise ratio, it causes a slow recording of the image. For 14 lines of accumulation in a frame size of 4.5 2 lm 2, we observed a slight stage drift that caused changes in the focal plane during data acquisition. Additionally, at the determined laser power, we found that both the excitation and
Case I
The image of the cross-shaped pattern in both confocal and STED modes was obtained using the determined values of Case I in Table 3( see Fig. 13). The arrow in Figure 13 indicates the location where the intensity profile was measured. Figure 14 shows the intensity profiles of the yellow line through the cross-shaped pattern. There is a drop in fluorescence emission intensity around the borders of the features in STED mode. The dotted lines in the STED mode diagram indicate the area where the intensity drops to zero. Again, in the STED image, the borders of the pattern are evident, whereas these areas are not clear in the confocal mode diagram.
Case II
Figure 15 exhibits images of the cross-shaped pattern in both confocal and STED modes for determined values of Case II in Table 3. The arrow in the Figure 15 indicates the location where the intensity profile was measured. Figure 16 shows the intensity profiles of the yellow line through the cross-shaped pattern. The fluorescence emission intensity in Case II falls sharply around the borders of the pattern compared to Case I. The dotted lines in the STED mode diagram indicate area where the intensity drops to zero. The width of the area is 2.1 lm. This result shows that reducing the pixel size of the image while maintaining the same dwell time( 20 ls) helps us record a highresolution image of the pattern. However, the scanning time in Case II increases two times compared to Case I.
5 Discussion of concepts for universal and versatile label-free-SRM metrology tool( s)
As might be obvious from the number of different label-free SRM methods we considered and investigated for the goal to develop and realise practical and( as much as possible) universal label-free SRM methods applicable in dimensional nanometrology, an optimum solution for this challenge might not exist, but according to the intended application, different methods might be required. Usually, the investigated materials and structures will not offer suitable