Polarization-assisted Vector Magnetometry in Zero Bias Field with an Ensemble of Nitrogen-Vacancy Centers in Diamond F. Mu¨nzhuber, J. Kleinlein, T. Kiessling,∗ and L. W. Molenkamp Physikalisches Institut Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany We demonstrate vector magnetometry with an ensemble of nitrogen-vacancy (NV) centers in diamondwithouttheneedforanexternalbiasfield. Theanisotropyoftheelectricdipolemoments oftheNVcenterreducestheambiguityoftheopticallydetectedmagneticresonancesuponpolarized visible excitation. Further lifting of the remaining ambiguities is achieved via application of an appropriatelylinearlypolarizedmicrowavefield,whichenablessuppressionofspin-statetransitions of a certain crystallographic NV orientation. This allows for the full vector reconstruction of small 7 (≤ 0.1mT) magnetic fields without an external bias field having to interfere with the magnetic 1 structure. 0 2 PACSnumbers: 61.72.jn,07.55.Ge,81.05.ug n a J I. INTRODUCTION where g is the electron g-factor, µ the Bohr magneton B 4 and S the projection of the spin along the quantization z The negatively charged nitrogen-vacancy (NV) center axis. (Ageneraldescriptionforarbitrarilyorientedfields l] in diamond has attracted enormous research interest in can be found e.g. in Ref.10.) Accordingly, only the ener- al the last decade because of its formidable versatility in getic position of |±1(cid:105)-states changes as a function of the h photonic applications. In addition to its suitability as external field. s- quantum information processing tool1, quantum cryp- Direct transitions from |0(cid:105) → |±1(cid:105) can be introduced e tography element2,3 and frequency standard4, the defect via the application of a microwave (MW) field, which m is a prospective candidate for the realization of quan- matches the splitting between the states in energy. Be- . tum sensors for a wide band of physical parameters, cause the splitting is sensitive to the magnetic field at at such as temperature5,6, acceleration7, pressure8, elec- the site of the NV center, the local magnetic field can m tric9, and in particular magnetic fields10,11. The single be probed via the MW frequency needed to induce an - atom-like defect embedded in a controllable, but nearly ODMR Signal. d non-interacting solid state environment enables the con- Two approaches for NV center based magnetometers n struction of magnetometers with an unprecedented com- can be found in the literature. Using a single NV center o bination of spatial and magnetic field resolution. Pre- allows a spatial resolution given by the Bohr radius of c [ viously proposed schemes build on established scanning the defect, which is on the nm scale19–23. In this config- probe technology and add the magnetic field sensitivity uration only the projection of the magnetic field vector 1 of an atomic gas sensor, which promises insight into new along the NV axis can be measured. Determining the v physics12–15. other components of the field vector thus requires more 9 8 Magnetometry with NV centers is based on optically than one sensor. 0 detected magnetic resonance (ODMR) spectroscopy of The second approach employs an ensemble of NV cen- 1 thefield-sensitivespinstatesofthedefect16,17. Themul- ters18,24,25. The NV centers are oriented along the four 0 tiplicity of the electronic ground state of the NV cen- crystallographicaxeswithinthediamondlattice. Agiven 1. ter is S = 1. The three resulting eigenstates along magnetic field will obviously have different projections 0 the quantization axis, which is set by the crystallo- along these distinct axes. With all four possible NV ori- 7 graphicorientationoftheNVaxis,areusuallylabeledas entations present in the observation volume, all vector 1 |m =−1,0,1(cid:105). Due to crystal field splitting the |±1(cid:105)- components can be measured with the same probe in v: staStesareenergeticallyshiftedfromthe|0(cid:105)-state. Optical one run. i excitation of the center as well as subsequent optical re- The high information density of the resulting ODMR X combination are spin conserving. As crucial ingredient, spectrum is, however, not straightforward to analyze. A r the optical intensity of the |±1(cid:105)-states is weaker com- bare spectrum as depicted in Fig. 1 (a) contains no in- a pared to the |0(cid:105)-state because of an additional recom- formationonwhichpairoftheresultingeightresonances bination channel in the infrared for this configuration18. belongs to which NV orientation. Without further input Therefore,theintensityoftheopticalphotoluminescence abouttheexternalfield,thecorrectreconstructionofthe (PL)signalcanserveasameasurefortheNVspinstate. magnetic field vector is impossible26. ForanexternalmagneticfieldB parallelorantipar- In principle, an additional bias field that has different ext allel to the NV axis, the energetic shift of the states is projectionsalongthefourNVaxescanbeusedtoidentify described by the Zeeman term the resonances24. However, it may not always be desir- abletoexposetheinvestigatedsampletoamagneticbias field. ∆E =gµ B S (1) Zeeman B ext z In the following we demonstrate how the ambiguity of 2 MW frequency (GHz) II. POLARIZATION SELECTIVE VISIBLE 2.75 2.80 2.85 2.90 2.95 3.00 EXCITATION OF NV CENTERS al n g orm. R - Si The first prerequisite for the appearance of a specific NM (a) D resonanceintheODMRspectrumisopticalexcitationof O theNVcentersalongtheassociatedcrystallographicaxis. 180 1.00 (b) g) We use a 532nm diode laser system to excite the NV e d centersinastandardconfocalgeometry. Theorientation ( 90 Pol. kLaser|| [100] 0.99 ofthelinearpolarizationplaneofthelaserbeamistuned 0 bytherotationofaλ/2waveplateintheexcitationpath. (c) g)180 We achieve a spatial resolution of 700nm by employing de an infinity-corrected, long working distance microscope ( 90 0.98 Pol. kLaser|| [110] odbetjeeccttiiovne apnadthb.yTuhsiengsaamppilnehiosleplaascesdpaitnialthfieltceernitnerthoef 0 (d) eg)180 0.97 atra3nDslvaeticotnorstmagaeg,nwethiocnhaistuhsreedetdoimraesntseirosncaaln, ctlhoesesda-mlopolpe d ( 90 position. The PL signal from the NV centers is filtered Pol. 0 kLaser|| [111] 0.96 spectrally by a 690nm±40nm band pass. 2.80 2.85 2.90 2.95 MW frequency (GHz) Theexcitationefficiencyiscruciallydeterminedbythe coupling strength between the electric field of the laser kLaser|| [100] kLaser|| [110] kLaser|| [111] beam and the dipole moments of electron orbitals of the NV center, which are oriented perpendicular to the NV axisandtoeachother. Forinstance,thedipolemoments V V d1andd2ofthe[111]-orientedNVaxispointalong[211] N N VN and [011]27,28. N TheprobabilityΓtoexciteaNVcenterisproportional to the projection of the electric field vector (cid:126)ε of the in- coming laser beam on its dipole moments d(cid:126)29: FIG. 1. (a) NV ensemble ODMR spectrum in random mag- neticfieldwithrandomorientationoflaserpolarization. The field orientation is chosen such that each of the eight pos- sible resonances is clearly resolved. (b) ODMR signal as a function of the MW frequency and the orientation angle of the linear polarization excitation plane. A horizontal cut re- (cid:12) (cid:12)2 sembles an ODMR spectrum as in (a). The pairwise appear- Γ∝(cid:12)(cid:126)ε·d(cid:126)(cid:12) (2) (cid:12) (cid:12) ance of resonances under variation of the polarization angle is characteristic for light incident along the direction of the [100]-crystallographic axis. Magnetic field (|B(cid:126)| ≈ 4mT) has random orientation. (c) Same as (b) for light incident along the [110]-axis. The contribution from two NV orientations For laser incidence parallel to the [100] axis, the po- can be clearly assigned, the two other remain indistinguish- larization can be rotated in the (100)-plane via the λ/2 able. Magnetic field is oriented coarsely along [110]. (d) wave plate in the excitation path. The field vector (cid:126)ε is Sameas(b)forlightincidentalongthe[111]-axis. Eachofthe fourNVorientationsgivesanuniqueresponsetothevariation then described by of the laser polarization. Magnetic field is oriented coarsely along [110]. Below: Schematic view of the diamond lattice along the [100]- (left), [110]- (center), and [111]- (right) crys- tallographic axis for better visualization. 0 (cid:126)ε=Esinθpol (3) Ecosθ pol the resonances can be lifted by exploiting the NV center withθ theanglebetweenthepolarizationandaspecific pol selectionrulesforboththeexcitationinthevisibleregion axis in the (100)-plane. Accordingly, the probability Γ is and the induced transitions by the MW field. a function of the angle of laser polarization: 3 MW frequency (GHz) 2.80 2.85 2.90 2.95 (cid:12)(cid:12)(cid:12) 0 0√ (cid:12)(cid:12)(cid:12)2 (a) g)180 1.00 Γd1 ∝(cid:12)Esinθpol 1/ √2 (cid:12) de 0.99 (cid:12) (cid:12) ( 90 (cid:12) Ecosθpol −1/ 2 (cid:12) ol. P 0.98 E2 E2 (cid:16) π(cid:17)2 0 = 2 (sinθpol−cosθpol)2 = 4 cos θpol+ 4 , (b) g)180 0.97 e d (cid:12) √ (cid:12)2 ol. ( 90 0.96 (cid:12)(cid:12) 0 −2/√ 6 (cid:12)(cid:12) P 0 0.95 Γd2 ∝(cid:12)(cid:12)(cid:12)EEcsoinsθθppooll 11//√66 (cid:12)(cid:12)(cid:12) 2.80 MW 2fr.8e5quency 2(.G90Hz) 2.95 E2 E2 (cid:16) π(cid:17)2 = (sinθ +cosθ )2 = sin θ + , 6 pol pol 12 pol 4 (a) B ext I ∝Γ +Γ d1 d2 E2 (cid:32) (cid:16) π(cid:17)2 sin(cid:0)θ + π(cid:1)2(cid:33) BMW ∝ cos θ + + pol 4 . (4) 4 pol 4 3 (b) Consequently, the intensity I of the optical transitions changes upon rotation of the linear polarization of the FIG. 2. (a), (b) ODMR signal obtained from a [111] ori- excitation. ented diamond with laser illumination along [111]-axis and Lookingontoa(100)-facetofdiamond,onecandistin- |B(cid:126)|≈4mTasafunctionoftheMWfrequencyandtheangle guish(Fig.1below)pairsoftheNVaxesthathaveparal- ofthelinearlypolarizedexcitationlaseratdifferentlocations lelprojectionsintotheobservationalplane. Accordingly, above the waveguide. The positions as well as the orienta- the resonances of these orientations appear and quench tions of the magnetic fields are indicated in the schematic pairwise during the rotation of the linear polarization of view of the coplanar waveguide below. The material of the the exciting laser beam. The number of possible NV ori- waveguideisastandardhighfrequencycircuitmaterialmade entations to which a resonance can be assigned is there- of ceramic-filled PTFE composites. The width of the signal fore reduced to two. Vice versa, a specific orientation of line is 500µm. thepolarizationallowsfor selectiveNVcenterexcitation of certain crystallographic orientations. plane NV axis and one or more additional NV axis. In This principle can be further improved by using sam- this specific case and more general for small magnetic ples of diamond that show (110)- or (111)-facets. If the fields, the spectral positions of the resonances heavily incidentlaserbeamisparalleltothe[110]-axis,twoofthe overlap. This renders a convincing evaluation of the four orientations are parallel to the observational plane. ODMR spectrum very challenging. For this reason, it According to Eq.2, theangle of polarization can be cho- is highly desirable to further reduce the number NV ori- sen such that the signal from those orientations is en- entations which contribute to the ODMR spectrum. tirely suppressed. These can, therefore, unambiguously be identified from their response to the variation of the excitation polarization, as shown in Fig. 1 (c). However, III. POLARIZATION SELECTIVE MICROWAVE an assignment of the two remaining resonances to the EXCITATION OF NV CENTERS two out-of-plane orientations remains unfeasible. AsdemonstratedinFig.1(d), illuminationofthedia- We can further enhance the directional selectivity by mondalongthe[111]-axisallowstoovercomethisdeficit. utilizingtheselectionrulesofthemicrowave-inducedNV Three NV orientations are nearly parallel to the obser- spin transitions. The Hamiltonian HMW of the NV cen- vational plane and the angles of laser polarization for ter spin subjected to a driving magnetic field BMW of which they are preferentially excited are at intervals of frequency ω in the presence of a static magnetic field ∆θ = 60◦. Only the out-of-plane orientation is con- B takes the basic structure (in the rotating wave ap- stantly excited at any angle of polarization, which ren- ext proximation)30 ders it easy to identify. In principle, all eight resonances canbeassignedunequivocallytotheircorrespondingNV orientations, i.e., all components of a magnetic field vec- ∆ (cid:15) 0 − − tor can be reconstructed (except their sign) without the HMW =(cid:15)∗− 0 (cid:15)+ (5) need of a bias magnetic field. 0 (cid:15)∗ ∆ + + In practice, this is complicated if the magnetic fields have equal or nearly equal projections along the out-of- where∆ =ω±±ωdescribestheenergeticdetuningof ± L 4 theMWfrequencyfromtheLarmorfrequencyoftheNV spin ωL± = D± gµ(cid:126)BBext,axial, which itself is set by the (a ) 1 .4 m T || [1 1 1 ] 2300 BD 1 tBttchiirooeyexnnsts(cid:15)taaobxlfei∝fistthewo(ledefBestMnsthpaeWtltihiNtcet±Vimn|i0gaBc(cid:105)geDMnantne(We=tdri)c2t(.chffi8ooee7rmld|Gs±pmaoH1lna(cid:105)ozle)nlsntgfiatasentt.lhdedseIsnt)aht.hrteehTepihdsqreruopiavjtiecnrecatnttuniizrobsaeiny--, arb. un) 0 D 1B0ext (2µ0T ) 30 010eval (µT) Evaluated ± x y l ( m we immediately recognize that only MW fields which a a hdanauvcOeeDtarMaxnR-siotsiirognnysa-clb.oemtwpeoennenttheineitgheensNtaVtesbaansids tchaunsicnatursoe- MR sign 0 .1 gnetic fie D ld Guiding the MW via a coplanar waveguide to the di- O (m amond, both the magnetic as well as the electric field T ) component of the MW emitted from the waveguide are 0 m T (b ) linearly polarized. The impact of the linear MW polar- 0 .0 1 ization is already obvious from the data shown in Fig. 1. 2 .8 0 2 .8 5 2 .9 0 2 .9 5 0 .0 1 0 .1 1 DuetotherelativeorientationoftheMWfieldtothefour M W fr e q u e n c y ( G H z ) E x t. m a g n e tic fie ld ( m T ) possible NV axes, the ODMR transitions are noticeably different in amplitude. FIG. 3. (a) Magnetic field response of a single NV orienta- To further demonstrate this interplay, we exploit the tion,isolatedbycombinationofselectiveMWandlaserexci- spatial stray field distribution near the signal lead of the tation. Theexternalfieldisappliedalongthe[111]-direction. waveguide structure. Depending on whether one probes ThecontributionfromthethreeotherNVorientationstothe directly on top of the signal or in the gap between signal ODMRspectraisnegligible. (b)Robustnessofthelineshape and ground lines, the polarization of BMW is either par- analysis. At small magnetic fields (B <50µT) the influence of the transverse crystal field limits the field resolution. The allel to the waveguide surface or points out of the plane. inset shows how the reliability can be further increased by Thisdramaticallychangestheefficiencywithwhichtran- higher frequency sampling, when the transverse crystal field sitions of the out-of-plane NV orientation can be driven. plays a minor role (see text). InFig.2wepresentacomparisonofODMRsignalsac- quired at the aforesaid positions. We apply an in-plane external magnetic field in order to evidence the effect on Before employing this method in a real vector mag- the different transitions. This keeps the contribution of netometry experiment, we investigate its sensitivity. To the out-of-plane NV centers at their zero-field value (the this end, we apply an external magnetic field along a physics of the remaining splitting are discussed below). given NV axis and test the accuracy of the line shape Their spectral position therefore correspond to the situ- analysis. ation of a low external field measurement, for which the The response of the selectively excited NV centers is problem of overlapping resonances occurs. shown in Fig. 3 (a). For strong enough external field the For polarization parallel to the surface we see in spectrum explicitly confirms the excitation of only one Fig.2(a)thattheout-of-planeorientationdominatesthe orientation, because we can observe merely two transi- spectrum. The contributions of the remaining in-plane tions instead of four, six, or eight. For very small fields NV orientations are strongly reduced. The weakest sig- we recognize a further splitting, which is not related to nalisobservedfromtheNVorientationthatexperiences the applied magnetic field and persists even at zero-field the strongest spectral shift. This is immediately under- value. This splitting is well understood and arises from stood from Eq. 5. The largest shift corresponds to the the transverse component of the crystal field to the NV largest projection of B onto one of the NV axes. As centers31. ext B is collinear with the polarization vector of BMW in The experimental spectra in Fig. 3 (a) can be per- ext this configuration, this means that the BMW and BMW fectly reproduced by a model function consisting of two x y components are minimized. Therefore the MW drives Lorentzians with negative amplitudes. Because of the transitions of this NV orientation the least. transversecrystalfieldsplittingthespectralshiftoftheir The situation changes dramatically for out-of-plane minima does not follow the external field linearly down MW fields. The formerly dominating contribution from to very small magnetic fields32. The spectral position of the out-of-plane orientation is then virtually eliminated, the transitions can be described by whereas the in-plane orientations show comparable in- tensities. (cid:113) Combined with the controlled rotation of the exciting ∆ν = E2 +(gµ B /h)2 (6) trans B ext laser polarization, we are now in a position to selectively addresssingleNVorientationsofanensembleofNVcen- where E2 corresponds to the strength of the trans- trans ters,therebyliftingtheinherentambiguitieswhichother- verse crystal field. wisearisewhenworkingwithanensembleofNVcenters. Takingtheaboveintoaccount,ouranalysisresultsina 5 (a) (c) = 0 deg = 70 deg = 140 deg = 140 deg Pol Pol Pol Pol ) n. u b. r (a = 70 deg R signal Pol b. un.) M = 0 deg ar OD Pol nal ( 2.8 2.9 3.0 R sig MW frequency (GHz) M D O 0 deg (b) 140 deg 70 deg scantrajectory 10 µm 2.80 2.85 2.90 2.95 2.80 2.85 2.90 2.95 2.80 2.85 2.90 2.95 MW frequency (GHz) MW frequency (GHz) MW frequency (GHz) FIG. 4. (a) ODMR spectra as a function of laser polarization with out-of-plane MW field for suppressing contributions from out-of-planeNVcenters. Thespectraforeachangleoflaserpolarizationareoffsetforbettervisualization. Theangleswiththe highest selectivity are 0◦ (green), 70◦ (blue), and 140◦ (red). (b) Schematic view of the diamond surface and the structure on top of it. The scan trajectory, a scale bar, and the relative orientation of the in-plane NV axis to the structure are indicated. The color scheme links the NV orientation to the belonging angle of excitation polarization from (a). (c) ODMR spectra as a function of spatial position and angle of laser polarization. The step size between each measurement point is ∆x=500nm. We recognize the individual evolution of the spectra at each angle, confirming the complete selectivity of the excitation. On therightsideareconstructionofthemagneticfieldvectorisshown,reproducingthesupposeddecayandrotationofthestray field. The length as well as the color scheme corresponds to the field strength. The direction of the arrows corresponds to the direction of the field vector if translated to the trajectory in (b). The size of the circle corresponds to the detection area. verygoodagreementbetweentheactualappliedfieldand fields range from 1.20mT to 1.23mT and the sampling thefieldvaluesobtainedfromthelineshapeanalysis. As rateissetto100Hz. Thisenablesafieldresolutiondown a rough estimate, we achieve a field resolution of better to a few µT. thanB =50µTataspectralresolutionof1MHzandan integration time of 200ms. FromFig.3(b)oneobservesadecreasingaccuracyand IV. SPATIALLY RESOLVED VECTOR a trend to overestimate the actual field at small fields. MAGNETOMETRY This is understood from the structure of Eq. 6.33 For small fields, the E-part dominates the spectral position Asademonstrationofthecapabilitiesofourtechnique and the contribution due to B is vanishing. For in- we investigate the stray field of a ferromagnetic stripe ext stance, an external field of B = 10µT is expected to structured on the surface of a processed sample of [111] result in a shift of ∆ν =280kHz for an undisturbed NV diamond.35 Thegeometryofthestructureischosensuch center according to Eq. 1. In contrast, having an addi- that change in amplitude and direction of the static B- tional transverse field splitting of E = 5MHz as in our field vector occur on a length scale comparable to our diamond (comp. to Fig. 3 (a)), Eq. 6 yields a shift of resolution limit. only ∆ν =8kHz. We start by identifying the angles of laser polariza- This effect limits the achievable field resolution at tion that show the highest degree of selective excitation. small fields in our measurements. To circumvent this As shown in Fig. 4 (a), these are found at 0◦, 70◦, and issue, homogenization and relaxation of the local lattice 140◦. The deviation from the expected 60◦ intervals environmentarerequired,whichhavebeendemonstrated arise from artifacts induced by optical elements in the previously to be feasible34. The inset in Fig. 3 (b) shows setup. The beam splitter used for guiding the excita- thatsmallerchangesintheexternalfieldcanberesolved, tion onto the optical axis of microscope objective and as soon as the resonance shift occurs approximately lin- detection path, which is mandatory in confocal optical early with the change of the magnetic field. The applied setups, has slightly different reflectivities for the s- and 6 (a) (b) 2 µm 2 µm 2 mT 0 mT max min FIG. 5. Comparison between measured (a) and simulated (b) stray field distribution of the micromagnet. We recognize the white space in the measured data to be caused by the strong fields and field gradients at these grid points, rendering a measurementoflocalmagneticfieldunfeasibleinthegivenconfiguration. Thescandirectionsarenotparalleltothesymmetry axisofthestructureinordertoplacethemicromagnetattheappropriatepositionrelativetothewaveguide. Thereforethegrid of the experimental results is slightly rotated. The background in part (a) is a b/w image from the diamond surface, wherein the microstructure is marked. p-components of the incident laser beam. Therefore, the ing rotation of the field vector in order to resolve the beamsplitteractsforcertainanglesofpolarizationasan sign-ambiguity. This last remaining question could be additional rotator. addressed by the application of circularly polarized MW Theapplicationofexternalreferencefieldsenablesthe fields, which selectively excite transitions between the determination of the orientation of the in-plane NV axes |0(cid:105)→|+1(cid:105)- and the |0(cid:105)→|−1(cid:105)-states28. relative to the laboratory frame and the microstructure. Wecanfinallyexpandthemethodtoatwodimensional Fig. 4 (b) indicates their relative alignment and further scan to investigate the complete stray field distribution. marksthetrajectoryalongwhichweperformvectormag- TheresultisshowninFig.5(a). Thevectorfieldrotates netometry that is shown in Fig. 4 (c). around the tip of the structure. The comparison with a We sample a trajectory of length of l=12µm in steps numerically simulated stray field distribution of such a of ∆x = 500nm for each angle of the laser polarization. structure36 as is shown in Fig. 5 (b) strongly supports The low implantation depth (d=10nm) of the NV cen- the reliability of our method. ters assures that the emitted signal stems only from the It further reveals some drawbacks of the procedure. region directly beneath the microstructure. For this rea- Whentheobservationspotischosenclosetothemagnet, sonweassumeaperfectin-planemagneticstrayfieldand twoproblemsoccur. First,thestrengthofthelocalmag- neglect the out-of-plane component in the further analy- neticfieldshiftstheenergeticpositionofthe|0(cid:105)→|±1(cid:105)- sis. transitions out of the sampled frequency interval. Sec- Usingthemodelfunctiondescribedintheprevioussec- ond, the field gradient along the observed defect ensem- tionwedeterminetheabsolutevaluesof∆ν foridentified ble leads to a strong broadening of the resonances in the laserpolarizationangles. Beinginsensitivetosignof∆ν, ODMR signal. The spectra in Fig. 4 (c) at the strongest the resulting value for the local field component along a energetic shift already mildly indicate these effects. specific NV orientation also only corresponds to the ab- Both issues can actually be avoided. Adjustment of solutevalueofcomponentandremainsambiguous. Only the sampled frequencies combined with nano-patterning upon the comparison of the three in-plane NV orienta- of diamonds37,38 will allow for further improvements of tion, the number of possible reconstructions is reduced the presented principle of magnetometry. At the same down to two. These vector reconstructions are identical time,theutilizationofnano-structureddiamondsamples except for their sign. will help to increase the overall spatial resolution. This allows us to map the stray field from our mi- To summarize, we demonstrated how the ambiguities crostructure. The reconstructed field vectors are shown ofanODMRspectrumofanensembleofNVcenterscan at the right edge of Fig. 4 (c). We assume an evolv- be lifted by the application of properly polarized optical 7 excitation. Thelinearpolarizationofthelasercanreduce a ferromagnetic microstructure. The method can be ap- thenumberofexcitedNVorientationtotwoincaseofin- plied as well to AC magnetometry schemes where higher cidence along the [111]-crystallographic axis. With help sensitivitylimitscanbereached. Thetemperaturerange of linearly polarized MW fields, a single NV orientation in which the principle is employable extends from liquid canbechosentocontributetotheODMRspectrum. 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