# Detecting Axial Ratio of Microwave Field with High Resolution Using NV Centers in Diamond
## 3 Experiments and Results
### Wide-Field MW Magnetic Field Imaging with Ensemble NV Centers  
A single NV center is capable of detecting the strength of the particular circularly-polarized component of the MW magnetic field with high spatial resolution and high sensitivity. Combining  
Figure 4: Rabi frequency versus MW source power applied. The data are fitted by a linear function. Figure 3: (**a**) Rabi frequency measurement sequence. A first green pulse of 3 \(\upmu\)s was applied to initialize the NV electron spin to the \(m_{\mathrm{s}}=0\) state; then, the MW field was applied to interact with the NV electron spin; finally, the electron spin state was determined by simultaneously applying the laser pulse and photon counts read pulse (0.3 \(\upmu\)s). To avoid the drift effect of the NV center on determining the NV state, the spin state photon counts were normalized to another photon counts read pulse that was applied in succession to the first one after the NV center was polarized again. (**b**) Rabi oscillation signal driven by a resonant MW field of 1.44 GHz. Each data point on the diagram was obtained by repeating the measurement sequences one million times. The oscillation frequency of 0.63 MHz indicates an MW magnetic field strength of \(B_{\sigma^{-}}=0.22\) G at the NV spot. The fitting error yields a sensitivity of 1.7 \(\upmu\)T/\(\sqrt{\mathrm{Hz}}\). (**c**) Fourier transformation of (**b**). the wide-field microscopy system and ensemble NV centers, the detection speed can be crucially improved, and the spatial resolution is only confined to the optical diffraction limit, which is generally at the micron or sub-micron degree. For many practical applications, wide-field microscopy that offers MW field strength imaging at a short time scale and requires simpler experimental technologies could be highly beneficial. For wide-field detection of the MW magnetic field, the experiment was based on a home-built wide-field back focal imaging microscope system. As shown in Figure 5a, the green laser was focused at the back focal area of the object (\(NA=0.7\)) to illuminate the sample with collimated light. The red fluorescence emitted by ensemble NV centers at the focal plane was collected by the same objective and was separated from the source light by a dichroic mirror. Then, the light was filtered and focused on an EMCCD (Andor Ultra iXon 897, Oxford, UK) for imaging. The spatial resolution of the microscope system was about 700 nm, as the wavelength of fluorescence emitted from the NV center was roughly between 600 nm and 750 nm. The sample used in this experiment was a 2.6 \(\times\) 2.6 \(\times\) 0.3 mm\({}^{3}\) general single crystal (100)-oriented CVD diamond from Element Six company with inherent ensemble NV centers. The inherent NV concentration in the diamond was estimated to be about \(3\times 10^{14}\) cm\({}^{-3}\) (2 ppb) by comparing the fluorescence photon counts of the NV centers in the diamond to that of a single NV center in type-IIa diamond under the confocal microscope system. The fluorescence emitted from the diamond was ascribed to the NV\({}^{-}\) center according to its zero photon line at 637 nm detected by the spectrometer FHR 640 (data not shown). A metal micro-strip line with a width of 8 \(\mu\)m was fabricated on the top surface of the diamond to deliver the microwave field. Figure 5b is a fluorescence picture taken by EMCCD with an exposure time of 2 \(s\). The metal strip line on the top surface blocked the fluorescence from the NV center, as shown in Figure 5b. Rough detection of the MW magnetic field was carried out by comparing the signals with and without the resonant MW magnetic field applied [18]. At a zero magnetic bias field, the NV ground electron spin energy splitting was \(D_{gs}\sim\) 2.87 GHz. Taking pictures of the same region as in Figure 5b with an MW field of \(f=2.87\) GHz applied, the NV fluorescence brightness decreased in the vicinity of the metal strip line (data not shown). The signal contrast decreased with the distance between NV centers and the MW strip line.