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Figure 7. Light microscope image of the NV centres substrate.( a) Patterns fabricated in the corner of the sample are shown in the red rectangle.( b) Different shapes of fabricated patterns on the NV-centre substrate.
along the line in Figures 6b1 – 6d1 are shown in Figure 6e. It is evident that the structured illumination image acquired using hyperspectral imaging shows the best spatial resolution. An improvement of 1.6 times was achieved compared to the diffraction limited spatial resolution. The set up is now fully optimised with the structured illumination line spacing. With diffraction limited line spacing, a spatial resolution improvement of 2 times would be achievable.
4.3 NV centres as test four-level system for STED-like SRM
In this section, first, we investigated the distribution of NV centres in a diamond substrate by measuring photoluminescence( PL) spectra for the existence of NV centres in the substrate as well as their homogeneities and for comparing the spectral resolution as proposed by Balasubramanian et al. [ 70 ]. Next, we imaged two random structures with a separation distance of 330 nm, and a cross-shaped pattern with 2.1 lm in size on the surface of a substrate with the NV centres in STED and confocal mode using a STED microscopy system. As a result of our tests, STED mode already provided higher-resolution images of the structures on the substrate surface than the confocal mode.
The sample is an artificial diamond substrate with embedded NV centres( cf. Sect. 4.1). The dimensions of thesampleare3 3 0.5 mm 3. The fabricated patterns in the sample corner were designed and fabricated by lithography( Fig. 7). To evaluate the PL spectrum of the sample, we used Raman spectroscopy in PL mode for different positions as well as from the bottom side of the sample.
We used a STED microscopy system( Abberior Instruments, Göttingen, Germany) to obtain high-resolution NV-centre images. The STED microscope is equipped with pulsed laser lines with wavelengths of 561 nm and 775 nm for excitation and saturation depletion of the excitation state, respectively. A vortex phase plate is inserted into the optical path of the 775 nm laser to generate a donutshaped beam. Both laser beams are overlapped and focused onto the sample using an oil-immersion objective with NA = 1.4. Finally, the emission signal of the NV-centre is recorded under reflection confocal detection( Fig. 8).
Theoretically, there are two modes to investigate and find the resolution for STED techniques. Based on using a pulsed or a CW laser, these modes are called pulsed mode and CW mode [ 77 ]. For pulse mode, the lateral resolution is given by [ 60 ]: d c
r ¼ q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n1 �) r ffi k rffiffiffi 1
; ð1Þ
2 bpn 2n bpn n 1 þ d c k where n ¼ I m
I sat is the saturation factor, I Sat is the saturation intensity( half of the population is quenched at the STED saturation intensity), I m is the maximum intensity at the donut crest, d c is the full width at half maximum( FWHM) of the confocal PSF, n is the refractive index, k is the STED wavelength, and b quantifies the steepness of the minimum intensity.
We adjusted a set of practical parameters as shown in Table 1 which are defining for the STED microscopy system. Depending on the type of sample, these parameters help us obtain an image with high contrast and resolution. In the next section, we will discuss the effect of each parameter on the imaging of the patterns.
4.3.1
Measurement of photoluminescence spectrum on NV centre samples
To obtain the PL spectrum of the sample, Raman spectroscopy in PL mode was used. The range of wavelengths was selected between 550 and 800 nm. To determine the homogeneity of the NV-centre population in the sample, five different positions of the sample were chosen to compare the similarity of their spectrum( see Fig. 9). For each position, the optical parameters of the setup were kept constant. The defect excited by green light( 532 nm) and the power of the laser was adjusted to 5 mW. Moreover, other laser powers were tested. The chosen power had a good signal-to-noise ratio without any harmful effect on the sample. An objective with 50 magnification was used for focusing the laser beam on the sample. The signal from the other side of the sample was obtained to determine the presence of NV centres on the bottom side of the sample.
PL spectra for different positions on the sample surface as well as from the bottom of the sample were measured