We present a promising method for creating high-density ensembles of nitrogen-vacancy centers with narrow spin-resonances for high-sensitivity magnetic imaging. Practically, narrow spinresonance linewidths substantially reduce the optical and RF power requirements for ensemble-based sensing. The method combines isotope purified diamond growth, in situ nitrogen doping, and helium ion implantation to realize a 100 nm-thick sensing surface. The obtained 10 17 cm −3 nitrogen-vacancy density is only a factor of 10 less than the highest densities reported to date, with an observed spin resonance linewidth over 10 times more narrow. The 200 kHz linewidth is most likely limited by dipolar broadening indicating even further reduction of the linewidth is desirable and possible.The nitrogen-vacancy (NV) center in diamond is a versatile room-temperature magnetic sensor which can operate in a wide variety of modalities, from nanometerscale imaging with single centers [1, 2] to sub-picotesla sensitivities using ensembles [3]. Ensemble-based magnetic imaging, utilizing a two-dimensional array of NV centers [4][5][6], combines relatively high spatial resolution with high magnetic sensitivity. These arrays are ideal for imaging applications ranging from detecting magnetically tagged biological specimens [7,8] to fundamental studies of magnetic thin films [9]. A key challenge for array-based sensors is creating a high density of NV centers while still preserving the desirable NV spin properties. Here we report on a promising method which combines isotope purified diamond growth, in situ nitrogen doping and helium ion implantation. In the 100 nmthick sensor layer, we realize an NV density of 10 17 cm −3 with a 200 kHz magnetic resonance linewidth. This corresponds to a a DC magnetic sensitivity ranging from 170 nT (current experimental conditions) to 10 nT (optimized experimental conditions) for a 1 µm 2 pixel and 1 second measurement time.Magnetic sensing utilizing NV centers is based on optically-detected magnetic resonance (ODMR) [10][11][12]. In the ideal shot-noise limit, the DC magnetic sensitivity is given by [9] in which h/gµ B = 36 µT/MHz, C is the resonance dip contrast, η is the photon collection efficiency, δν is the full-width at half maximum resonance linewidth, n N V is the density of NV centers in imaging pixel volume V , and t is the measurement time. From Eq. 1, it is apparent that to minimize δB ideal for a given linewidth δν, one would like to maximize the NV density n N V . Increasing n N V , however, can also increase δν. For example, lattice damage during the NV creation process can create inhomogeneous strain-fields [13]. More fundamentally, eventually NV-NV and NV-N dipolar interactions will contribute to line broadening. This dipolar broadening, δν dp , is proportional to the nitrogen density n N [14,15]. Since n N V is typically proportional to n N , we can divide δ ν into two components, δν = δν 0 + δν dp = δν 0 + An N V , to obtainin which δν 0 depends on factors independent of NV density (e.g. hyperfi...
