Abstract
Enhancing the measurement signal from solid state quantum sensors such as the nitrogen-vacancy (NV) center in diamond is an important problem for sensing and imaging of condensed matter systems. Here we engineer diamond scanning probes with a truncated parabolic profile that optimizes the photonic signal from single embedded NV centers, forming a high-sensitivity probe for nanoscale magnetic field imaging. We develop a scalable fabrication procedure based on dry etching with a flowable oxide mask to reliably produce a controlled tip curvature. The resulting parabolic tip shape yields a median saturation count rate of 2.1 $\pm$ 0.2 MHz, the highest reported for single NVs in scanning probes to date. Furthermore, the structures operate across the full NV photoluminescence spectrum, emitting into a numerical aperture of 0.46 and the end-facet of the truncated tip, located near the focus of the parabola, allows for small NV-sample spacings and nanoscale imaging. We demonstrate the excellent properties of these diamond scanning probes by imaging ferromagnetic stripes with a spatial resolution better than 50 nm. Our results mark a 5-fold improvement in measurement signal over the state-of-the art in scanning-probe based NV sensors.
Highlights
Sensing and imaging of nanoscale sample volumes is increasingly relevant for a wide range of topics, from nanomagnetism to structural imaging in biology and condensed matter [1,2], and requires high sensitivity and high spatial resolution
We develop a charge-state-registered pulsed-excitation measurement protocol to determine the photon-detection efficiency for photons emitted while the N-V resides in the metrologically relevant, negative charge state (N-V−)
The number of photons counted during a 594-nm excitation pulse of duration τprobe, follows a bimodal distribution described by a two-state fluctuator model for N-V chargestate dynamics [30] [Fig. 2(b)]
Summary
Sensing and imaging of nanoscale sample volumes is increasingly relevant for a wide range of topics, from nanomagnetism to structural imaging in biology and condensed matter [1,2], and requires high sensitivity and high spatial resolution. Optical initialization and readout of the N-V’s electronic spin [4,5] together with the ability to embed a single, near-surface N-V into all-diamond scanning-probe devices [6,7] have enabled the quantitative imaging of nanoscale systems such as skyrmions [8,9,10], domains in antiferromagnets [11,12], electron transport in graphene [13,14], and magnetism in two-dimensional (2D) materials [15] Despite these achievements, a wide range of even more challenging applications is currently beyond the sensitivity limits for practical scanning-probe-based studies [1,2], and points to the need for improved diamond scanning-probe technology. We further design the exit aperture of the device so as to direct the N-V emission into a small numerical aperture (NA), thereby minimizing reflections off the diamond-air interface at the output facet of the scanning probe This yields a near-surface N-V in a high collection efficiency, broadband, waveguided device, thereby satisfying both of the requirements outlined above. We demonstrate the spatial resolution and sensitivity of these devices by imaging the magnetic field from a ferromagnetic Co-Fe-B stripline
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