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to adapt to the materials under test. An alternative for technical applications could be alternative marker structures, which could be added as a thin layer on top of the device substrate or included in the substrate material itself, as long as this inclusion does not severely deteriorate the manufacturing process or device performance of the intended nanoscale structure or device. As alternative markers which could be suitable for this approach, e. g. quantum dots or NV centres in artificial diamond have been applied [ 56, 57 ]. Alternatively, doping of quartz substrates( as an example) with ions suitable for laser devices, such as Neodymium( Nd) would be promising alternatives to fluorescence marker labelling.
In the following, we will shortly recap some promising approaches for label-free or label-surrogate SRM methods and will discuss their potential for universal SRM-based nanometrology.
3.2.1 STED on NV centres
STED is one of the techniques that produce sub-diffraction resolution by squeezing the PSF through saturated depletion of the excited state of the fluorescence dye [ 58, 59 ]. A donut-shaped beam at STED wavelength is used to obtain effective PSF in practice. Here, non-linearity comes from a change in the population of involved states, which is affected by a single-photon process [ 59 ]. With this technique, the spatial resolution can be improved down to below 10 nm [ 60 ].
3.2.2 SAX
The saturation excitation microscopy technique( SAX) was developed based on extracting saturated emissions [ 61 ]. This technique improves the spatial and axial resolution using a confocal microscopy setup by modulating the excitation intensity temporally and detecting the harmonic modulation of the fluorescence signal from the centre of the laser focus [ 61 ]. As saturated emission requires high excitation intensity, the photobleaching effect for fluorescence samples is an inevitable problem for imaging. Due to this fact, scattering from individual plasmonic nanostructures is an alternative candidate as a label-free agent in SAX. They provide strong scattering, high photostability, and exceptional localization precision for label-free microscopy [ 62 ]. When an individual plasmonic nanoparticle, like a gold nanosphere, is examined for scattering measurements, it shows saturable and reverse saturable absorption. This behaviour occurs near the resonance wavelength of the nanoparticle [ 63 ]. Lee et al. indicated that the combination of gold nanoparticles and SAX can improve spatial resolution from a label-free agent [ 64 ]. An advantage of this technique is that photobleaching is not common when the intensity of the excitation is high. The lateral and axial resolutions also improved simultaneously, and significant resolution enhancement was observed compared to previous SAX methods using fluorescence markers [ 64 ].
3.2.3 SUSI Enhancement |
of |
the |
surface-to-volume |
ratio |
of |
the |
plasmonic nanostructures gives rise to increasing the local |
electric field and providing optical nonlinear interaction, known as nonlinear plasmonics [ 65 ]. The nonlinearity allows the control of scattered light from plasmonic nanoparticles by another light source which is called all-optical plasmonic switch. Suppression of scattering imaging( SUSI) is a technique which was developed based on( STED) microscopy systems to create a switch between scattering and nonscattering states of plasmonic nanoparticles for nanoparticle imaging [ 66 ]. In this technique, scattering from gold nanoparticles at close to resonance wavelength is controlled by another beam in a confocal microscope setup. A donutshaped beam at a different wavelength is used to suppress the scattering of particles. When the control beam intensity is increased, the scattering signal of nanoparticles reduces, and a high-resolution image of nanoparticles is obtained. In this way, SUSI can enhance the resolution of gold nanoparticles to k / 9 [ 66 ].
For a range of applications, such as sensors, there is a need for the development of the metrology and imaging of periodic diffractive plasmonic nanostructures. SUSI can be a promising method for imaging this kind of a sample. For spherical plasmonic particles, polarization should not affect the switch process [ 66 ]. However, polarization plays an active role in scattering of plasmonic nanostructures. Therefore, a prior study is needed to determine optical properties of the sample such as the precise resonance angle, resonance wavelength and sensitivity of the resonance. In practice, these parameters can be determined using ellipsometry techniques with a standard model and sample.
For this ellipsometric prior study, first, a gold grating with a period of 200 nm, a thickness of 60 nm, and line widths( CD) of 70 nm was prepared. For that 3 nm of Cr followed by 60 nm of gold was deposited on a 150 mm fused silica substrate by thermal evaporation. This is followed by 10 nm of chromium deposited by ion beam sputtering. In the next step, a respective resist pattern is generated using a Vistec 350OS electron beam writer with character projection apertures [ 67 ]. This pattern is then transferred to the chromium layer by ion beam sputter etching using argon ions and then into the gold layer using a mixture of argon and oxygen. Finally, the chromium layer is removed by reactive ion etching using chlorine chemistry. The dimensional parameters of the gratings were designed using calculations by the JCMsuite 6.0.10 finite element solver aiming to maximize the variation of the amplitude ratio( W = tan �1(| r p / r s |)) and phase shift( D = arg(| r p / r s |)) of the reflection coefficients of light polarized parallel( r p) and perpendicular( r s) to the plane of incidence. The sample was measured using a Woollam M-2000DI ellipsometer at angles of incidence from 60 ° to 75 °. Figure 4 shows the measured and calculated W and D spectra for plane of incidence perpendicular to the grating lines.
The real and imaginary part of the refractive index as a function of wavelength can be obtained through data inversion analysis [ 68 ]. Also, sensitive parameters can be identified( not shown here) by analysing the fW 0 ¼ @ W and f 0 @ P ¼ @
@ P derivatives, where P denotes the individual parameters. As a result, essential information such as initial polarization, resonance wavelength, and the sensitivity of resonance