J. Eur. Opt. Society-Rapid Publ. 21, 7( 2025) 63
Figure 4. W and D ellipsometry spectra calculated by simulation( blue line) and measured( orange line) at an angle of incidence of 70 ° for CD = 70 nm.
features with respect to design parameters can be obtained before using SUSI for periodic plasmonic nanostructures. Future work would be feasible to investigate the capabilities of the SUSI technique for imaging plasmonic nanostructures in terms of determined parameters with the aid of spectroscopic ellipsometry, which may give us dimensional information about the nanostructure’ s size and shape.
3.3 Discussion of methods
Among all the super-resolution techniques, STED microscopy is a promising and precise optical far-field technique for nanoscopy. It produces a high-resolution image by modifying the PSF without requiring for any complex mathematical analysis compared to SIM, SUSI, and SAX. For SIM, the post-processing method can be complex for the imaging of a label-free sample. Although SUSI is a promising method for label-free samples as well, it utilizes a complex deconvolution method for coherent emission of samples to improve spatial resolution. For SAX and SSIM, extracting high order nonlinear demodulation components to increase the resolution is a main task. Additionally, in practice, signal-to-noise ratio limits the resolution. In general, the key concept of using STED for a label-free sample is finding a material that can be optically transferred between the four-level system [ 60 ].
NV centres are a potential candidate for this task. They can be imaged by STED microscopy with a spatial resolution that is 10 – 20 times better than the diffraction limit due to high photostability at room temperature [ 69 ]. Embedded in diamond substrates, NV centres have great potential as alternative labels in SRM if they exhibit a high density and homogeneous distribution throughout the surface, providing homogeneous fluorescence [ 58 ]. A considerable amount of research on NV centres in diamond has investigated the application of NV centres as an individual emitter in quantum information, biosensing, and bioimaging [ 70 ]. However, imaging of nanostructures on a NVcentre substrate has not been investigated before.
In Section 4, we will concentrate on investigations of NV centres in artificial diamond as an example four-level system to replace fluorescence labelling.
4 Experimental
4.1 NV centres in artificial diamond
This section describes the preparation and features of the NV test samples:
Diamond nanocrystals( MSY 0-0.25 and 0-0.35) are acquired from Pureon Ò, with a median size of 125 and 180 nm, respectively. Samples are processed as shown in Figure 5.
Nitrogen impurities are naturally present in NDs with concentrations in the 10 – 200 ppm range, so that NV centres concentration can be increased by introducing additional vacancies in the lattice with ion irradiation damaging. To this scope, proton irradiation is performed on 125 nm NDs to enhance the concentration of NV centres and maximize their photoluminescence properties. More precisely, samples are dispersed in isopropanol, sonicated, drop-casted on a silicon wafer substrate( ~ 0.5 0.5 cm 2) and dried, so forming a ~( 30 ± 10) lm thick layer. A55mm 2 2MeVH + ion beam at the AN2000 accelerator facility of the INFN National Laboratories of Legnaro is employed. Beam current ranged 800 – 1000 nA. A fluence of 2 1016 cm �2 is delivered, being in a range of values that were found optimal in order to maximize the concentration of NV centres while preventing quenching effects associated with over-damaging [ 71 ]. To evaluate the damage level due to the irradiation, a Monte-Carlo simulation was carried out using SRIM software, following the procedure shown in [ 71 ], which proved how 2 MeV protons determine an almost flat damage density across the whole thickness of the NDs layer deposited on the silicon substrate.
High temperature thermal annealing in N 2 flow is then performed for 2 h at 800 ° C to promote the formation of NV centres by allowing the generated vacancies to get coupled with the N impurities. The same process is performed on unirradiated NDs for comparison. In this case, the treatment is merely performed to reorganize the crystalline phases and to graphitize the outer layers of amorphous carbon phases surrounding NDs core [ 72 ]. Surface purification is finally performed by thermal oxidation in air environment at 500 ° C for 8 h. Indeed, in previous work