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arXiv:1705.09241

Fourier optical processing enables new capabilities in diamond magnetic imaging

Mikael P. Backlund, Pauli Kehayias, Ronald Lee Walsworth

Published May 25, 2017• 8 citations
arXivHTMLPDF

Abstract

Diamond-based magnetic field sensors have attracted great interest in recent years. In particular, wide-field magnetic imaging using nitrogen-vacancy (NV) centers in diamond has been previously demonstrated in condensed matter, biological, and paleomagnetic applications. Vector magnetic imaging with NV ensembles typically requires an applied field (>10 G) to separate the contributions from four crystallographic orientations, hindering studies of magnetic samples that require measurement in low or independently specified bias fields. Here we decompose the NV ensemble magnetic resonance spectrum without such a bias field by modulating the collected light at the microscope's Fourier plane. In addition to enabling vector magnetic imaging at arbitrarily low fields, our method can be used to extend the dynamic range at a given bias field. As demonstrated here, optically-detected diamond magnetometry stands to benefit from Fourier optical approaches, which have already found widespread utility in other branches of photonics.

DOI
10.48550/arxiv.1705.09241

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Diamond-based magnetic field sensors have attracted great interest in recent years. In particular, wide-field magnetic imaging using nitrogen-vacancy (NV) centers in diamond has been previously demonstrated in condensed matter, biological, and paleomagnetic applications. Vector magnetic imaging with NV ensembles typically requires an applied field (>10 G) to separate the contributions from four crystallographic orientations, hindering studies of magnetic samples that require measurement in low or independently specified bias fields. Here we decompose the NV ensemble magnetic resonance spectrum without such a bias field by modulating the collected light at the microscope's Fourier plane. In addition to enabling vector magnetic imaging at arbitrarily low fields, our method can be used to extend the dynamic range at a given bias field. As demonstrated here, optically-detected diamond magnetometry stands to benefit from Fourier optical approaches, which have already found widespread utility in other branches of photonics.
Magnetic sensing and imaging using ensembles of nitrogen-vacancy (N-VVV) centers in diamond recently have found application in disciplines as wide-ranging as condensed matter physics, biology, and paleomagnetism. However, distinguishing the signals from the four inequivalent N-VVV orientations requires an external magnetic field, which can hinder the study of some magnetic samples. The authors present an $a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l-o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ method to tease apart the signals at arbitrary bias fields, by exploiting the distribution of photoluminescence at the Fourier plane of a microscope. Their technique will help N-VVV-based magnetic sensing reach its full potential.
Nitrogen-vacancy (N\mathrm{N}N-VVV) centers in diamond are among the best-studied quantum-based sensors for a wide range of applications. Here, we demonstrate an optical polarization-based method for full vector magnetometry using ensembles of N\mathrm{N}N-VVV centers. The anisotropy of the electric dipole moments of the N\mathrm{N}N-VVV centers enables the assignment of the different magnetic field split transitions in the optically detected magnetic resonance spectra by appropriately setting the optical polarization in the excitation and/or detection of the N\mathrm{N}N-VVV photoluminescence signal. By further employing the optical-selection rules for the driving microwave field, we show how the full vector of the investigated magnetic field can be restored without the need to apply external bias fields. We discuss different N\mathrm{N}N-VVV-center orientation implementations and limitations toward common approximations used in the N\mathrm{N}N-VVV magnetometry literature for our approach.
On-chip magnetic field sensing with nitrogen-vacancy (NV) centers in diamond requires scalable integration of 3D waveguides into diamond substrates. Here, we develop a sensing array device with an ensemble of shallow implanted NV centers integrated with arrays of laser-written waveguides for excitation and readout of NV signals. Our approach enables an easy-to-operate on-chip magnetometer with a pixel size proportional to the Gaussian mode area of each waveguide. The performed continuous wave optically detected magnetic resonance on each waveguide gives an average dc-sensitivity value of (195±3) nT/Hz, which can be improved with lock-in-detection or pulsed-microwave sequences. We apply a magnetic field to separate the four NV crystallographic orientations of the magnetic resonance and then utilize a DC current through a straight wire antenna close to the waveguide to prove the sensor capabilities of our device. We reconstruct the complete vector magnetic field in the NV crystal frame using three different NV crystallographic orientations. By knowing the polarization axis of the waveguide mode, we project the magnetic field vector into the lab frame.
We demonstrate 4π4π4π-steradian vector magnetic field sensing using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond coupled to a microwave (MW) cavity. The MW cavity enhances the spin-photon coupling which enables efficient, high-contrast spin-state readout via MW interrogation and removes the need for bulky optical collection components. An applied AC bias magnetic field lifts the zero-field degeneracy of the four crystallographic NV orientations, allowing each orientation to be individually addressed and used for vector reconstruction of the magnetic field. The resulting magnetometer has a 40\% contrast (20x higher than typical for optical spin-ensemble readout) and achieves a single-axis sensitivity of 250 pT/Hz\sqrt{\mathrm{Hz}}Hz​ which is flat from DC to 1 kHz. Noise models of the composite spin-cavity system establish MW amplitude noise as the dominant noise source and predict a thermal noise limit of 2 pT/Hz\sqrt{\mathrm{Hz}}Hz​.
In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.
We present a novel method for full vector magnetometry using nitrogen-vacancy (NV) centers. In contrast to conventional optically detected magnetic resonance techniques, our method employs two distinct broadband microwave pulses and measures them after transmission through the NV sensor medium, thus capturing the line splitting of the ground state triplet due to the Zeeman effect. Two orthogonally polarized microwave pulses allow resolving all magnetic field components independently by reading out differently oriented NV centers. Simulated data is analyzed using deep neural networks, whose efficacy we expect to translate very well to experiments. Our method yields sensitivities between 5 pT/Hz5~\mathrm{pT}/\sqrt{\mathrm{Hz}}5 pT/Hz​ and 100 pT/Hz100~\mathrm{pT}/\sqrt{\mathrm{Hz}}100 pT/Hz​ across different magnetic field vector components, while achieving approximately nT\mathrm{nT}nT accuracy at a signal-to-noise (SNR) ratio of 70 dB70~\mathrm{dB}70 dB. By being capable of accurately measuring magnetic fields down to 25 μT25~\mathrm{μT}25 μT, the need for a bias field beyond Earth's magnetic field is eliminated.