Abstract
We propose a hybrid laser system consisting of a semiconductor external cavity laser associated to an intra-cavity diamond etalon doped with nitrogen-vacancy color centers. We consider laser emission tuned to the infrared absorption line that is enhanced under the magnetic field dependent nitrogen-vacancy electron spin resonance and show that this architecture leads to a compact solid-state magnetometer that can be operated at room-temperature. The sensitivity to the magnetic field limited by the photonshot-noise of the output laser beam is estimated to be less than . Unlike usual NV center infrared magnetometry, this method would not require an external frequency stabilized laser. Since the proposed system relies on the competition between the laser threshold and an intracavity absorption, such laser-based optical sensor could be easily adapted to a broad variety of sensing applications based on absorption spectroscopy.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, the optical detection of the magnetic resonance between the electronic triplet S = 1 spin states of the negatively charged nitrogen-vacancy (NV) color center in diamond and the measurements of the Zeeman shifts induced by an applied magnetic field has been used in a variety of solid-state magnetometers [1]. Due to the special properties of the NV center, these systems can be operated in ambient conditions to detect a broad range of magnetic fields created by both physical and biological systems [2, 3]. By raster scanning a single NV spin over a magnetized substrate and detecting the spin-dependent luminescence emitted by this atomic-like defect, the stray magnetic field created by the sample magnetization can be mapped with nanometer spatial resolution [4, 5]. Compared to a single spin, the magnetic field sensitivity of an ensemble of NV centers contained in a macroscopic single-crystal diamond sample is increased by where N is the number of NV centers used as magnetic sensors [6]. Due to this enhancement, continuous-wave magnetometry based on an NV ensemble has recently reached a sensitivity level of about [7, 8]. However this technique is constrained by the collection efficiency of the NV luminescence and by parasitic background light that spectrally overlaps the broad luminescence of the NV center with wavelength extending from 637 nm to about 800 nm.
The spin state of the NV center can also be determined by the infrared (IR) optical transition associated with the singlet S = 0 spin state [9–11]. The corresponding scheme for detecting the perturbation by an applied magnetic field is based on the absorption of a signal beam tuned to this IR transition centered at wavelength [12]. The magnetic-field dependent signal is then free from any background and the relevant photon detection efficiency can be almost ideal [13]. Nevertheless, the low optical depth of the IR transition at room temperature, even for a dense ensemble of NV centers, needs to be compensated by a multi-pass configuration as it has been demonstrated for NV center magnetometry based on visible luminescence monitoring [14]. This enhancement scheme can be implemented by placing the NV doped diamond in an optical cavity resonant with the IR signal beam [15, 16]. A shot-noise limited sensitivity of was recently achieved using a miniaturized Fabry-Perot cavity [17] which could even be realized in integrated diamond photonics [18, 19].
In order to circumvent the previously mentioned drawbacks of luminescence based magnetometers, it was proposed to operate the NV center transition between the triplet spin states in the stimulated emission regime [20–22] so that population inversion in the NV center levels provides the optical gain of a laser. By setting the laser at its threshold, sensitivities of about are anticipated [21]. Nevertheless the stimulated emission from the NV centers can be strongly affected by the excited state absorption (ESA) phenomena and by the photoconversion between the negatively-charged state , with the previously described spin triplet structure, and the neutral charge state [23]. These parasitic effects can make the implementation of NV center magnetometry based on the visible optical laser amplification challenging [24].
Here we propose to combine the IR absorption method and the laser threshold magnetometry method by considering a hybrid laser architecture which integrates the diamond sample containing the NV centers in an external-cavity laser. The optical gain in the laser is provided by an independent semiconductor material which is optically pumped. The laser threshold of the whole system is then sensitive to the applied magnetic field via the losses on the IR transition induced by the spin resonance of the NV centers. In this scheme, the ESA in the gain medium becomes irrelevant and has a marginally negative effect on the IR signal absorption efficiency. Using a rate equation model of the photodynamics of the NV center that takes into account its two charge states, we evaluate the magnetic field sensitivity of this hybrid laser system. Finally, we discuss the possible advantages of this sensor architecture for practical applications.
2. Model of the spin-dependent NV center dynamics
The NV center consists of a nitrogen impurity linked to an adjacent vacant lattice site. In the negatively charged state NV which consists of six electrons associated to the dangling bonds around the lattice vacancy, four of these electrons populate the lowest energy states [25]. The remaining two electrons create both spin triplet S = 1 states and spin singlet S = 0 states that are associated to optical transitions within the bandgap of diamond. In the spin triplet manifold of the ground electronic state , the magnetic interaction between electron spins induces a zero-field splitting of between the mS = 0 and spin projection sublevels along the intrinsic quantization axis that is defined by the N-to-V axis of the defect inside the crystal as shown in Fig. 1(a)
According to selection rules determined from group-theory methods [25, 26], the optically electronic transitions between the triplet sublevels of the electronic ground state and the corresponding triplet sublevels of the excited electronic level are mainly spin-conserving. Due to a non-radiative decay path from the excited states through the metastable singlet states and and then a preferential return to the mS = 0 ground state(Fig. 1(a), green laser optical excitation of the triplet sublevels polarizes the electron spin of the NV center into the mS = 0 sublevel [27]. The non-radiative leakage to the metastable S = 0 state also induces a lower luminescence efficiency of the sublevels compared to mS = 0 so that the occupation probability in this ground state spin manifold of compared that of the mS = 0 can be determined by monitoring the photoluminescence (PL) intensity. These properties enable the optically detected magnetic resonance (ODMR) signal that can be induced by applying a microwave field resonant with the mS = 0 to transition. Since a magnetic field applied to the NV centerinduces Zeeman shifts that lift the degeneracy of the sublevels, the magnetic field amplitude can be determined by measuring these Zeeman shifts in the ODMR microwave frequency spectrum [28].
The detection of spin polarization can also be realized by measuring the transmission of a signal IR beam that probes the absorption on the transition between the singlet metastable states and [12]. Under green light continuous optical pumping and in the absence of resonant microwaves, the NV centers are pumped into the mS = 0 ground sublevel leading to a reduced occupation rate of the metastable singlet state . In this off-resonance regime the IR signal transmission is maximal. For magnetic fields applied along one of the NV axis, when the microwave field frequency is resonant with frequency , where is the NV gyromagnetic ratio and BNV the projection on the NV axis of the applied magnetic field, the population is transferred into the ground state. The occupation rate of the state increases and the magnetic field dependent spin resonance can be detected as a lower transmission of the IR signal beam.
A rate equation model is used to describe the photodynamics of the triplet and singlet states and to estimate the optical losses induced by the magnetic resonance between the sublevels of the ground state on the signal beam that propagates through the NV doped diamond sample [15, 19]. In order to take into account the photoionization process [29–32] between and two supplementary levels associated to the neutral charge state [32] are added to this level scheme, as shown in Fig. 1(b). In our configuration, the photoionization and the ESA only reduce the numerical value of the inferred IR absorption cross-section (see Appendix 1) but are not an intrinsic limitation as it is the case for laser threshold magnetometry based on the visible transition.
The spin sublevels mS = 0 and of the state of the center are labelled 1 and 2 whereas 3 and 4 are the corresponding spin sublevels of the excited state . The ground and excited states of the singlet IR transition are respectively labelled 6 and 5. Finally, 7 and 8 are the ground and excited states. The radiative or non-radiative relaxation rate from α to β levels is ; the values of these parameters are given in Appendix 1 and similar measurements can be found in [33, 34]. The relaxation rate from 2 to 1 can be neglected since the associated spin-relaxation time, longer than at room temperature [35], is much longer than all other decay processes. When excited in the upper singlet state, the system can only decay to the lower singlet state and thus . Finally, the optical transition are spin conserving and thus . The optical depth of a diamond plate of thickness e doped with the NV centers (see the Fig. 1(b) insert) is obtained from the steady state solution of the following system calculated at each position indexed by z in the diamond sample:
where is the population density of state α. The pumping rates are related to the pump Ig and signal Is optical intensities through , , , and , where the cross-sections σβ are given in Appendix 1. The system is considered as closed and where NNV is the density of the NV centers contained in the diamond sample. The pump (green) and signal (IR) intensities at the output of the diamond sample are then obtained by:After integration along z, these equations determine the optical depth τ for the IR signal beam as a function of the green-light Wg and microwave WMW pumping rates:
3. Hybrid architecture for NV laser magnetometry
The proposed hybrid architecture shown in Fig. 2(a) is based on a vertical external cavity surface emitting laser (VECSEL). The gain medium is a half- vertical cavity surface emitting laser (VCSEL) consisting of semiconductor multiple InGaAs/GaAs quantum wells grown on a perfectly reflecting Bragg mirror both centered at λs [36]. The output coupling mirror (M) of the laser cavity has a transmission coefficient T. A diamond thin plate containing a high concentration of NV centers is inserted inside the cavity. The semiconductor quantum wells are pumped using a laser at and the NV centers are spin polarized by illuminating the diamond sample with a green laser at wavelength. The diamond plate operates as an intracavity etalon leading to single-mode operation of this external cavity semiconductor laser. The extra losses due to the NV absorption in the diamond plate, and thus the threshold and the efficiency of this hybrid laser depend on the spin state of the NV centers that are driven by the microwave field. Consequently, the output power Pout of the laser can be modified by the magnetic field B applied on the NV centers. As previously explained, the IR losses are increased when the microwave field is on-resonance, leading to a higher threshold and a lower efficiency compared to the off-resonance case as shown in Fig. 2(b). Using these features the magnetic field dependent spin resonance can be detected by monitoring the IR laser output power.
4. Parameters of the VECSEL
The main requirements on the IR laser are (i) to operate in the regime of high-finesse cavity in order to increase the effective path of the IR signal in the diamond plate [15–17], (ii) to be low-noise since the magnetic field sensitivity is directly related to the IR optical signal noise and (iii) compactness. The VECSEL-based architecture is therefore a good candidate especially when the cavity length is chosen to reach the class A regime of laser operation (corresponding to a cavity lifetime longer than population inversion lifetime) enabling a photon shot-noise limited amplitude noise operation [37].
The parameters of the hybrid laser magnetometer are deduced from those given in [36] which describes a shot-noise limited semiconductor VECSEL emitting at a wavelength of , close to λs. In the class A regime, the output power Pout of the IR laser is given by:
where Psat is the pumping saturation power, and r the rate of the pumping power Pp above the laser threshold Pth: where ϵ are the losses introduced by the intracavity etalon for a round trip inside the cavity, and η is the proportionality factor that relates the optical gain obtained after one round trip in the cavity to the pumping power Pp.With an intracavity etalon that ensures single-mode operation, the laser realized in [36] has a threshold power of and provides an output power of for of pump power applied to the VECSEL. Considering an output coupling mirror with transmission , we then infer from Eq. (4) a saturation power of . Without the intracavity etalon, the output power is for the same pump power. Since in this case ϵ = 0, we deduce by combining Eq. (4) and Eq. (5). If we consider again the case of the etalon in the laser cavity, we have at the threshold so that .
5. Intracavity diamond etalon and magnetic field sensitivity
We now consider that the intracavity etalon consists of a diamond sample doped with NV centers, without any anti-reflection coating on the input and output facets. The etalon is illuminated using an additional green laser which polarizes the NV spins in the mS = 0 sublevel of the ground electronic state and also feeds the metastable singlet level (6) shown in Fig. 1(b). Taking into account the optical thickness of the diamond plate, the absorption of the IR beam due to the singlet transition of the NV centers then corresponds to additional intracavity optical losses
where the factor 2 accounts for the round trip inside the cavity and is an enhancement factor of the losses which is induced by the high refractive index of the diamond plate (see Appendix 2). The laser pumping rate then becomes: where η and ϵ have the values previously determined. The parameter ξ corresponds to the useful losses of the diamond sample that determine the efficiency of the laser response to the applied magnetic field, as: where is the resonance frequency of the microwave field with the dependence to the applied magnetic field. If we assume that the spin resonance has a Lorentzian lineshape, the maximum of is reached for where and are the optical depths with respectively the microwave field being either on-resonance or off-resonance (see Appendix 3). This maximum value is then given by where and is the full width at half maximum of the spin resonance. We assume here that the linewidth of the electronic spin resonance (ESR) is limited by the spin dephasing time and by the spin polarization relaxation rate Γ taking into account populations dynamics [38] and related to Wsat the microwave saturation rate by , neglecting the optical pump power broadening we thus can write where ΩR the Rabi frequency is related to the microwave pumping rate by . Taking into account the pumping rate given by Eq. (7), we can then determine the maximal response of the laser-based magnetometer: with . Assuming that the laser output noise is at the limit of photon shot-noise we have where is the measurement bandwidth. The equivalent magnetic noise of the sensor can then be deduced from Eqs. (10), (11) and (12).6. Results
The simulations of the equivalent magnetic field noise are based on the laser parameters given in section 4 apart from the diamond etalon with thickness (note that we take into account parasitic losses due to diamond by taking ). We also assume that the laser emission is tuned to the NV center IR transition.
We now consider two configurations with different realistic densities of NV centers. Config. 1 refers to and [39] whereas Config. 2 to and [6]. The length of the cavity and the curvature of the output mirror are such that the waist of the laser mode is . The diamond etalon is located as close to the waist position as allowed by the pumping beam. We assume a green pump intensity corresponding to a mean power of .
We first show in Fig. 3(a) the population N1 as a function of the microwave pumping rate for the two configurations. By fitting the results by with A, B and Wsat as free parameters we can deduce the microwave saturation rate, for Config. 1: and for Config. 2: . We then are able to plot in Fig. 3(b) the equivalent magnetic field noise versus the microwave Rabi frequency for the two studied configurations. In both cases, the equivalent magnetic field noise reaches an optimum coming from the trade-off between the increase of the contrast and the broadening of the ESR. The following optimal Rabi frequencies are used in the rest of the work: for Config. 1 and for Config. 2. Further optimization of the results are shown in Fig. 4 where the equivalent magnetic field noise is plotted as a function of the transmission of the output mirror for several pumping rates r. For both configurations an optimum output coupling is found depending on the IR absorption. Figure 4 also shows that by operating the laser close to its threshold (here ), the equivalent magnetic field noise can be strongly reduced, reaching for instance almost for the parameters of Config. 2. This value could be reduced to by using both pulsed optical and microwave pumping to avoid ESR broadening [38] and considering a higher Rabi frequency .
Indeed, at its threshold, the laser becomes highly sensitive to the intracavity optical losses and thus to magnetic field fluctuations similarly to the behavior of visible laser threshold magnetometry [21]. Finally comparison between the two configurations of Fig. 4 shows that the trade-off between the NV center density and the spin dephasing time associated with Config. 2 leads to an improved sensitivity. Note that once fundamental limits are achieved, the sensitivity scales as [28]. For the considered diamond thickness and waist size the spin projection noise determined by the total number of NV centers participating in the measurement is smaller than [28]. This noise can therefore be neglected compared to the shot-noise limit set by the laser output photon flux. Note the spin projection noise limit could be reached by operating closer to the laser threshold.
7. Conclusion
We have shown that magnetometry based on the IR absorption associated to the singlet states of the NV center can be implemented by integrating a diamond sample containing the NV centers inside an external half-VCSEL cavity. This scheme does not require a narrow linewidth stabilized IR laser as in realizations based on multi-pass absorption in a resonant passive cavity [17]. Compared to previous proposals consisting of a diamond laser using the NV centers for optical amplification, the detrimental effects of both the parasitic ESA by the triplet excited state and the photoconversion to the NV0 charge state are also circumvented since the optical gain is obtained from an independent system. Moreover, the use of a semiconductor material makes it possible to consider electric-current pumping which is of great interest for practical implementations avoiding the pump/signal configuration [17, 22]. Our simulations show that a photon shot-noise limited sensitivity of about (and even if the ESR linewidth is limited by the spin dephasing time) can be reached for realistic parameters.
Appendix A--Photophysical parameters
Table 1 gives the values of the photophysical parameters used in the simulations. Since we considered the transition between the two charge states NV and NV0, we updated the value of the IR absorption cross-section which was previously inferred from experimental data [15]. For this purpose, we used the same method consisting in adjusting the value of σs to obtain the experimental value of the single-pass IR transmission reported in [12].
Appendix 2--Effective optical depth of the diamond plate
The maximum of transmission of the diamond plate is given by
where and are the Fresnel coefficients associated to the index of refraction of diamond nd. As , in the first-order of approximation, we have on one hand , on the other hand where corresponds to an effective optical depth taking into account the multiple passes due to Fresnel reflections within the diamond plate. First-order calculations allow us to write which gives with representing the absorption enhancement factor due to Fresnel reflections.Appendix 3--Spectral profile of the optical depth
We assume a Lorentzian shape for the ESR, thus we can write
where and is the frequency of the microwave. We have thusThe maximum of sensitivity is obtained for which gives on the one hand
and on the other handThis maximal value of the optical depth is used to determine the optimal value of given in Eq. (9).
Funding
European Union Seventh Framework Programme (FP7/2007-2013) DIADEMS (611143); German Federal Ministry of Education and Research (BMBF) Quantumtechnologien Program (FKZ 13N14439); CNRS PICS project MOCASSIN.
Acknowledgments
We acknowledge Isabelle Sagnes for fruitful discussions on the VCSEL design. The work of JFR, FB, and TD is performed in the framework of the joint research lab between Laboratoire Aimé Cotton and Thales R&T. YD acknowledges the support of the Institut Universitaire de France and the Alexander von Humboldt Foundation.
References
1. L. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, and V. Jacques, “Magnetometry with nitrogen-vacancy defects in diamond,” Rep. Prog. Phys. 77, 056503 (2014). [CrossRef] [PubMed]
2. R. Schirhagl, K. Chang, M. Loretz, and C. L. Degen, “Nitrogen-Vacancy centers in diamond: Nanoscale sensors for physics and biology,” Ann. Rev. Phys. Chem. 65, 83–105 (2014). [CrossRef]
3. F. Casola, T. Sar, and A. Yacoby, “Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond,” Nat. Rev. Mater. 3, 17088 (2018). [CrossRef]
4. G. Balasubramanian, I. Y. Chan, R. Kolesov, M. Al-Hmoud, J. Tisler, C. Shin, C. Kim, A. Wojcik, P. R. Hemmer, A. Krueger, T. Hanke, A. Leitenstorfer, R. Bratschitsch, F. Jelezko, and J. Wrachtrup, “Nanoscale imaging magnetometry with diamond spins under ambient conditions,” Nature 455, 648 (2008). [CrossRef] [PubMed]
5. J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, P. Cappellaro, L. Jiang, M. V. Gurudev Dutt, E. Togan, A. S. Zibrov, A. Yacoby, R. L. Walworth, and M. D. Lukin, “Nanoscale magnetic sensing with an individual electronic spin in diamond,” Nature 455, 644 (2008). [CrossRef] [PubMed]
6. V. M. Acosta, E. Bauch, M. P. Ledbetter, C. Santori, K.-M. C. Fu, P. E. Barclay, R. G. Beausoleil, H. Linget, J. F. Roch, F. Treussart, S. Chemerisov, W. Gawlik, and D. Budker, “Diamonds with a high density of nitrogen-vacancy centers for magnetometry applications,” Physical Review B 80, 115202 (2009). [CrossRef]
7. J. F. Barry, M. J. Turner, J. M. Schloss, D. R. Glenn, Y. Song, M. D. Lukin, H. Park, and R. L. Walsworth, “Optical magnetic detection of single-neuron action potentials using quantum defects in diamond,” Proceedings of the National Academy of Sciences 113, 14133–14138 (2016). [CrossRef]
8. J. M. Schloss, J. F. Barry, M. J. Turner, and R. L. Walsworth, “Simultaneous broadband vector magnetometry using solid-state spins,” Phys. Rev. Applied 10, 034044 (2018). [CrossRef]
9. L. J. Rogers, S. Armstrong, M. J. Sellars, and N. B. Manson, “Infrared emission of the NV centre in diamond: Zeeman and uniaxial stress studies,” New J. Phys. 10, 103024 (2008). [CrossRef]
10. V. M. Acosta, A. Jarmola, E. Bauch, and D. Budker, “Optical properties of the nitrogen-vacancy singlet levels in diamond,” Phys. Rev. B 82, 201202 (2010). [CrossRef]
11. P. Kehayias, M. Doherty, D. English, R. Fischer, A. Jarmola, K. Jensen, N. Leefer, P. Hemmer, N. Manson, and D. Budker, “Infrared absorption band and vibronic structure of the nitrogen-vacancy center in diamond,” Phys. Rev. B 88, 165202 (2013). [CrossRef]
12. V. M. Acosta, E. Bauch, A. Jarmola, L. J. Zipp, M. P. Ledbetter, and D. Budker, “Broadband magnetometry by infrared-absorption detection of nitrogen-vacancy ensembles in diamond,” Appl. Phys. Lett. 97, 174104 (2010). [CrossRef]
13. S. Ahmadi, H. A. R. El-Ella, A. M. Wojciechowski, T. Gehring, J. O. B. Hansen, A. Huck, and U. L. Andersen, “Nitrogen-vacancy ensemble magnetometry based on pump absorption,” Phys. Rev. B 97, 024105 (2018). [CrossRef]
14. H. Clevenson, M. E. Trusheim, C. Teale, T. Schröder, D. Braje, and D. Englund, “Broadband magnetometry and temperature sensing with a light-trapping diamond waveguide,” Nature Phys. 11, 393–397 (2015). [CrossRef]
15. Y. Dumeige, M. Chipaux, V. Jacques, F. Treussart, J. Roch, T. Debuisschert, V. Acosta, A. Jarmola, K. Jensen, P. Kehayias, and D. Budker, “Magnetometry with nitrogen-vacancy ensembles in diamond based on infrared absorption in a doubly resonant optical cavity,” Phys. Rev. B 87, 155202 (2013). [CrossRef]
16. K. Jensen, N. Leefer, A. Jarmola, Y. Dumeige, V. M. Acosta, P. Kehayias, B. Patton, and D. Budker, “Cavity-enhanced room-temperature magnetometry using absorption by nitrogen-vacancy centers in diamond,” Phys. Rev. Lett. 112, 160802 (2014). [CrossRef] [PubMed]
17. G. Chatzidrosos, A. Wickenbrock, L. Bougas, N. Leefer, T. Wu, K. Jensen, Y. Dumeige, and D. Budker, “Miniature cavity-enhanced diamond magnetometer,” Phys. Rev. Applied 8, 044019 (2017). [CrossRef]
18. O. Gazzano and C. Becher, “Highly sensitive on-chip magnetometer with saturable absorbers in two-color microcavities,” Phys. Rev. B 95, 115312 (2017). [CrossRef]
19. L. Bougas, A. Wilzewski, Y. Dumeige, D. Antypas, T. Wu, A. Wickenbrock, E. Bourgeois, M. Nesladek, H. Clevenson, D. Braje, D. Englund, and D. Budker, “On the possibility of miniature diamond-based magnetometers using waveguide geometries,” Micromachines 9, 276 (2018). [CrossRef]
20. A. Faraon, C. M. Santori, and R. G. Beausoleil, “Color centers affected by magnetic fields to produce light based on lasing,” US Patent p. US 2014/0072008A1 (March 13,2012).
21. J. Jeske, J. H. Cole, and A. D. Greentree, “Laser threshold magnetometry,” New Journal of Physics 18, 013015 (2016). [CrossRef]
22. V. G. Savitski, “Optical gain in nv-colour centres for highly-sensitive magnetometry: a theoretical study,” Journal of Physics D: Applied Physics 50, 475602 (2017). [CrossRef]
23. S. D. Subedi, V. V. Fedorov, J. Peppers, D. V. Martyshkin, S. B. Mirov, L. Shao, and M. Loncar, “Laser spectroscopy of highly doped NV- centers in diamond,” Proc. SPIE 10511, 105112D (2018).
24. J. Jeske, D. W. M. Lau, X. Vidal, L. P. McGuinness, P. Reineck, B. C. Johnson, M. W. Doherty, J. C. McCallum, S. Onoda, F. Jelezko, T. Ohshima, T. Volz, J. H. Cole, B. C. Gibson, and A. D. Greentree, “Stimulated emission from nitrogen-vacancy centres in diamond,” Nat. Commun. 8, 14000 (2017). [CrossRef] [PubMed]
25. M. W. Doherty, N. B. Manson, P. Delaney, F. Jelezko, J. Wrachtrup, and L. C. L. Hollenberg, “The nitrogen-vacancy colour centre in diamond,” Phys. Rep. 528, 1–45 (2013). [CrossRef]
26. J. Maze, A. Gali, E. Togan, Y. Chu, A. Trifonov, E. Kaxiras, and M. Lukin, “Properties of nitrogen-vacancy centers in diamond: the group theoretic approach,” New J. Phys. 13, 025025 (2011). [CrossRef]
27. G. Thiering and A. Gali, “Theory of the optical spinpolarization loop of the nitrogen-vacancy center in diamond,” arXiv:1803.02561 (2018).
28. J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker, P. R. Hemmer, A. Yacobi, R. Walsworth, and M. D. Lukin, “High-sensitivity diamond magnetometer with nanoscale resolution,” Nat. Phys. 4, 810 (2008). [CrossRef]
29. Y. Dumeige, F. Treussart, R. Alléaume, T. Gacoin, J.-F. Roch, and P. Grangier, “Photo-induced creation of nitrogen-related color centers in diamond nanocrystals under femtosecond illumination,” Journal of Luminescence 109, 61–67 (2004). [CrossRef]
30. N. B. Manson, J. P. Harrison, and M. J. Sellars, “Nitrogen-vacancy center in diamond: Model of the electronic structure and associated dynamics,” Phys. Rev. B 74, 104303 (2006). [CrossRef]
31. N. Aslam, G. Waldherr, P. Neumann, F. Jelezko, and J. Wrachtrup, “Photo-induced ionization dynamics of the nitrogen vacancy defect in diamond investigated by single-shot charge state detection,” New Journal of Physics 15, 013064 (2013). [CrossRef]
32. I. Meirzada, Y. Hovav, S. A. Wolf, and N. Bar-Gill, “Negative charge enhancement of near-surface nitrogen vacancy centers by multicolor excitation,” arXiv:1709.04776 (2017).
33. L. Robledo, H. Bernien, T. van der Sar, and R. Hanson, “Spin dynamics in the optical cycle of single nitrogen-vacancy centres in diamond,” New Journal of Physics 13, 025013 (2011). [CrossRef]
34. N. Kalb, P. C. Humphreys, J. J. Slim, and R. Hanson, “Dephasing mechanisms of diamond-based nuclear-spin memories for quantum networks,” Phys. Rev. A 97, 062330 (2018). [CrossRef]
35. M. Mrózek, D. Rudnicki, P. Kehayias, A. Jarmola, D. Budker, and W. Gawlik, “Longitudinal spin relaxation in nitrogen-vacancy ensembles in diamond,” EPJ Quantum Technology 2, 22 (2015). [CrossRef]
36. G. Baili, M. Alouini, D. Dolfi, F. Bretenaker, I. Sagnes, and A. Garnache, “Shot-noise-limited operation of a monomode high-cavity-finesse semiconductor laser for microwave photonics applications,” Opt. Lett. 32, 650–652 (2007). [CrossRef] [PubMed]
37. G. Baili, M. Alouini, T. Malherbe, D. Dolfi, I. Sagnes, and F. Bretenaker, “Direct observation of the class-b to class-a transition in the dynamical behavior of a semiconductor laser,” EPL Europhysics Letters 87, 44005 (2009). [CrossRef]
38. A. Dréau, M. Lesik, L. Rondin, P. Spinicelli, O. Arcizet, J.-F. Roch, and V. Jacques, “Avoiding power broadening in optically detected magnetic resonance of single NV defects for enhanced dc magnetic field sensitivity,” Phys. Rev. B 84, 195204 (2011). [CrossRef]
39. Y. Kubo, C. Grezes, A. Dewes, T. Umeda, J. Isoya, H. Sumiya, N. Morishita, H. Abe, S. Onoda, T. Ohshima, V. Jacques, A. Dréau, J.-F. Roch, I. Diniz, A. Auffeves, D. Vion, D. Esteve, and P. Bertet, “Hybrid quantum circuit with a superconducting qubit coupled to a spin ensemble,” Phys. Rev. Lett. 107, 220501 (2011). [CrossRef] [PubMed]
40. J.-P. Tetienne, L. Rondin, P. Spinicelli, M. Chipaux, T. Debuisschert, J.-F. Roch, and V. Jacques, “Magnetic-field-dependent photodynamics of single NV defects in diamond: an application to qualitative all-optical magnetic imaging,” New Journal of Physics 14, 103033 (2012). [CrossRef]
41. T.-L. Wee, Y.-K. Tzeng, C.-C. Han, H.-C. Chang, W. Fann, J.-H. Hsu, K.-M. Chen, and Y.-C. Yu, “Two-photon excited fluorescence of nitrogen-vacancy centers in proton-irradiated type Ib diamond,” J. Phys. Chem. A 111, 9379–9386 (2007). [CrossRef] [PubMed]