Determining strain components in a diamond waveguide from zero-field optically detected magnetic resonance spectra of negatively charged nitrogen-vacancy-center ensembles
Mohammad Sahnawaz Alam, Federico Gorrini, Michał Gawełczyk, Daniel Wigger, Giulio Coccia, Yanzhao Guo, Sajedeh Shahbazi, Vibhav Bharadwaj, Alexander Kubanek, Roberta Ramponi, Paul E. Barclay, Anthony J. Bennett, John Patrick Hadden, Angelo Bifone, Shane Michael Eaton, Paweł Machnikowski
Laser-written optical waveguides in diamonds are a key technology to enhance coupling between defect centers and light, boosting applications in nanoscale sensing and quantum information processing. However, laser writing of photonic structures produces strain in the diamond lattice, modifying the properties of defect centers in poorly understood ways. This study demonstrates that optically detected magnetic resonance spectroscopy provides sufficient information to fully characterize the spatial distribution of strain in such a device, even without a constant magnetic field. The work yields an accessible tool that could be very useful for advancing diamond-based quantum technologies.
Laser-written optical waveguides in diamonds are a key technology to enhance coupling between defect centers and light, boosting applications in nanoscale sensing and quantum information processing. However, laser writing of photonic structures produces strain in the diamond lattice, modifying the properties of defect centers in poorly understood ways. This study demonstrates that optically detected magnetic resonance spectroscopy provides sufficient information to fully characterize the spatial distribution of strain in such a device, even without a constant magnetic field. The work yields an accessible tool that could be very useful for advancing diamond-based quantum technologies.
Determining Strain Components in a Diamond Waveguide from Zero-Field ODMR Spectra of NV− Center Ensembles
The negatively charged nitrogen-vacancy (NV−) center in diamond has shown great potential in nanoscale sensing and quantum information processing due to its rich spin physics. An efficient coupling with light, providing strong luminescence, is crucial for realizing these applications. Laser-written waveguides in diamond promote NV− creation and improve their coupling to light but, at the same time, induce strain in the crystal. The induced strain contributes to light guiding but also affects the energy levels of NV− centers. We probe NV− spin states experimentally with the commonly used continuous-wave zero-field optically detected magnetic resonance (ODMR). In our waveguides, the ODMR spectra are shifted, split, and consistently asymmetric, which we attribute to the impact of local strain. To understand these features, we model ensemble ODMR signals in the presence of strain. By fitting the model results to the experimentally collected ODMR data, we determine the strain tensor components at different positions, thus determining the strain profile across the waveguide. This shows that zero-field ODMR spectroscopy can be used as a strain imaging tool. The resulting strain within the waveguide is dominated by a compressive axial component transverse to the waveguide structure, with a smaller contribution from vertical and shear strain components.
Negatively charged nitrogen-vacancy (NV) centers and other color centers in diamonds have emerged as promising platforms for quantum communication, quantum information processing, and nanoscale sensing, owing to their long spin coherence times, fast spin control, and efficient photon coupling. Deterministic placement of individual color centers into nanophotonic structures is critical for scalable device integration, and ion implantation is the most viable technique. Nanofabrication processes, including diamond etching, are essential to realize these structures but can introduce crystal strain through lattice damage. In this work, we investigate the impact of ion implantation and nanofabrication-induced strain on the electronic spin levels of NV-centers. We demonstrate that the zero-field continuous-wave optically detected magnetic resonance (CW-ODMR) spectroscopy serves as a sensitive probe of local crystal strain. We report the presence of a shear strain feature in diamond substrates arising from the ion-implantation and nanofabrication processes, as evidenced by the asymmetric splitting black observed in the zero-field CW-ODMR spectrum of NV-centers.
Integrated photonic circuits promise to be foundational for applications in quantum information and sensing technologies, through their ability to confine and manipulate light. A key role in such technologies may be played by spin-active quantum emitters, which can be used to store quantum information or as sensitive probes of the local environment. A leading candidate is the negatively charged nitrogen vacancy (NV−) diamond color center, whose ground spin state can be optically read out, exhibiting long (≈1 ms) coherence times at room temperature. These properties have driven research toward the integration of photonic circuits in the bulk of diamond with the development of techniques allowing fabrication of optical waveguides. In particular, femtosecond laser writing has emerged as a powerful technique, capable of writing light guiding structures with 3D configurations as well as creating NV complexes. In this Perspective, the physical mechanisms behind laser fabrication in diamond will be reviewed. The properties of waveguides, single- and ensemble-NV centers, will be analyzed, together with the possibility to combine such structures in integrated photonic devices, which can find direct application in quantum information and sensing.
As promising quantum sensors, nitrogen-vacancy (N-V) centers in diamond have been widely used in frontier studies in condensed-matter physics, material science, and life sciences. In practical applications, weak laser excitation is favorable as it reduces the side effects of laser irradiation, such as phototoxicity and heating. Here we report a combined theoretical and experimental study of (near) zero-field optically detected magnetic resonance (ODMR) of N-V-center ensembles under weak 532-nm laser excitation. In this region, both the width and splitting of the ODMR spectra decrease with increasing laser power. This power dependence is reproduced with a model that accounts for the laser-induced charge neutralization of $\mathrm{N}\text{\ensuremath{-}}{V}^{\ensuremath{-}}$--N+ pairs, which alters the local electric field environment. These results are useful for the understanding and development of N-V-based quantum sensing in light-sensitive applications.
Quantum emitters, such as the negatively charged nitrogen-vacancy center in diamond, are attractive for quantum technologies, such as nano-sensing, quantum information processing, and as a non-classical light source. However, it is still challenging to position individual emitters in photonic structures while preserving the spin coherence properties of the defect. In this paper, we investigate single and ensemble waveguide-integrated nitrogen-vacancy centers in diamond fabricated by femtosecond laser writing followed by thermal annealing. Their spin coherence properties are systematically investigated and are shown to be comparable to native nitrogen-vacancy centers in diamond. This method paves the way for the fabrication of coherent spins integrated within photonic devices.
High Resolution Image Download MS PowerPoint Slide Ensemble negatively charged nitrogen-vacancy centers in diamond are promising quantum sensors. To optimize their sensitivity, it is crucial to increase the number of spins sampled and maximize their coupling to the detection system without degrading their spin properties. In this paper, we demonstrate enhanced quantum magnetometry via a buried laser-written waveguide in diamond with 4.5 ppm nitrogen-vacancy centers. The waveguide-coupled nitrogen-vacancy centers exhibit spin coherence properties comparable to those of nitrogen-vacancy centers in pristine diamond. Waveguide-enhanced magnetic field sensing is demonstrated in a fiber-coupled integrated photonic chip, where probing an increased volume of high-density spins results in 63 pT·Hz –1/2 of DC magnetic field sensitivity and 20 pT·Hz –1/2 of AC magnetic field sensitivity. This on-chip sensor realizes at least an order of magnitude improvement in sensitivity compared to the conventional confocal detection setup, paving the way for high-sensitivity quantum magnetometry with nitrogen-vacancy ensembles.