Optical far‐field super‐resolution microscopy with local probes
Authors: Chen Xiangdong , Sun Fangwen 
Published: December 2016

The local fields, such as magnetic, optic and temperature, can be used to characterize the properties of physical and biological materials. These local fields could be measured with closely placed quantum emitters. The imaging process were usually accomplished by placing the probe on a scanning tip. However, the moving of tips might change the local field distribution. Another promising method is to place an array of probes close to the sample and detect the probes with super‐resolution optical far‐field microscopy. In this way, the relative distance between the probe and sample is fixed, and the scanning process would not affect the results of detection. The resolution of fluorescence based optical far‐field microscopy has been improved to several nanometers in recent years. It is possible to detect the nanoscale physical local effects with optical far‐field microscopy.
In this work, we used the nitrogen vacancy (NV) center in diamond as probes to detect the local optical field passing through aluminum structures. By improving the spatial resolution of optical microscopy with NV center, we can detect the shape of structures with resolution below diffraction limit. This method has the potential to detect other materials, which enable it to be a universal super‐resolution microscope.

It has been proved that the charge state conversion of NV center can be pumped by lasers with different wavelengths. The green laser can initialize the center to the negative charge state with fidelity about 75%, while red laser can initialize the center to neutral charge state with fidelity about 95%. To realize high spatial resolution microscopy, we used a Gaussian shaped 637 nm laser beam to initialize the charge state, and then a doughnut shaped 532 nm laser beam to switch the charge state of NV, as shown in Fig.1 (a). Therefore, the doughnut shaped 532 nm laser beam will change the charge state of NV from NV
0
to NV
‐
. Only the charge state of NV at the center of 532 nm laser beam will not be changed by doughnut laser beam (Fig. 1 (b)). The charge state was finally detected by a weak 589 nm laser. By detecting the fluorescence of NV
‐
, the intensity of fluorescence would show a dark spot, which indicated the position of NV with resolution below diffraction limit. This super‐resolution microscopy was named as charge state depletion (CSD) microscopy.

In order to improve the resolution of microscopy, one way was to increase the power of 532 nm laser. Instead, we applied another 780 nm Gaussian shaped laser beam, as in Fig. 1(a). The 780 nm laser can accelerate the charge state conversion pumped by 532 nm. Therefore, the resolution of microscopy was significantly improved by applying the 780 nm laser, as shown in Fig 1(c). And the power of 532 nm laser can be reduced by a factor about 10 times by applying 0.4 mW 780 nm laser. The resolution of 50 nm can be obtained with 0.14 mW 532 nm laser. As the 780 nm laser might cause less photon damage than 532 nm laser, this method could be applied to the biological imaging.

In further step, we used the super‐resolution microscopy with NV center in bulk diamond to detect the structure of nanoscale materials, as shown in Fig.2. Aluminum was deposited on the surface of diamond plate. The laser beams were used to pump NV center through the aluminum structures. The local optical field below diamond surface
was
affected by the shape of aluminum material. And the charge state conversion of NV center was pumped by the local optical field below diamond surface. By applying CSD microscopy, we can detect the charge state of NV center with high spatial resolution. Subsequently, the structure of aluminum was imaged with spatial resolution higher than that with confocal microscopy, as in Fig.2 (c)‐(e). This method can be used to detect properties of other nanoparticles.

In summary, we demonstrated the super‐resolution microscopy with NV center based on charge state conversion. The power of laser was decreased at least one order by applying an additional 780 nm laser. A universal super‐resolution microscopy was developed by using the NV center ensemble in bulk diamond as local optical field probes.