# Battery Characterization via Eddy-Current Imaging with Nitrogen-Vacancy Centers in Diamond
## 2 Experimental Apparatus  
A schematic of the experimental setup is shown in Figure 1. Linearly polarized light from a continuous-wave green laser at 532 nm is continuously pumping the NV centers in a diamond sample. The laser power can be adjusted with a polarizer and a half-wave-plate. NV centers in the excited state decay back to the ground state both via radiative and nonradiative transitions, with red photoluminescence (PL) emitted in the former case. The fact that NV centers in the \(m_{s}=\pm 1\) magnetic sublevels have significantly higher probability to undergo an intersystem crossing (ISC) [12], i.e., a nonradiative transition from the triplet to the singlet excited state, results in the majority of NV centers being optically pumped into the \(m_{s}=0\) spin projection. In order to minimize the noise caused by intensity fluctuations in the laser beam, we use a light-power stabilization loop incorporating a photodiode (PD2), a proportional-integral-differential (PID) controller, and an acousto-optic modulator (AOM).  
A dichroic mirror and a notch filter are used to filter out red PL from the green light. The diamond used in this setup is a type-Ib, (111)-cut, high-pressure-high-temperature-grown sample with dimensions \(3.0\times 3.0\times 0.4\) mm\({}^{3}\). The initial nitrogen concentration was specified as less than 110 ppm. Then nitrogen-rich sample was electron-irradiated and annealed [11]. The diamond is glued on the top plane of a parabolic concentrator used to collect fluorescence. A radio-frequency (RF) coil is made from copper wire of 0.1 mm diameter. It has five turns and is wound around the diamond to provide a modulation field to both the diamond and battery. The modulation field is provided by the lock-in amplifier (LIA) used for the AC magnetometric scheme. The frequency of the field (maximum 4 MHz) is also used as the LIA reference for detection.  
A 3D translation stage is used to fast-scan the battery in front of the diamond. The battery is glued on top of a 24 cm long polyvinyl chloride (PVC) rod attached to a motorized three-dimension translation stage controlled by a computer. A variable background field is  
Figure 1: Experimental setup. 1–4: mirrors; 5: dichroic mirror; 6: lens; 7: parabolic concentrator; 8: RF coil; 9: diamond; 10: battery; 11: notch filter. PBS: polarizing beamsplitter; AOM: acousto-optic modulator; PD1,2: photodiodes; PC: personal computer; PS: power supply.  
provided by a custom-made electromagnet (EM). This field is required for the microwave-free magnetic field detection method. The EM consists of approximately 200 turns wound with a rectangular cross-section (\(1.4\times 0.8\,\mathrm{mm}^{2}\)) wire around a 5 cm-diameter bore. The coil is wound on a water-cooled copper mount, and produces a background field of 2.9 mT per ampere supplied. Diamond, battery and RF coil are all placed inside the bore of the EM. A LIA detects the amplitude (\(R\)) and phase (\(\theta\)) of PL modulation. The LIA is connected to the computer, and \(R\) and \(\theta\) are recorded along with the position of the battery (the 3D translation stage).