A high-sensitivity fiber-coupled diamond magnetometer with surface coating
Shao-Chun Zhang, Hao-Bin Lin, Yang Dong, Bo Du, Xue-Dong Gao, Cui Yu, Zhi-Hong Feng, Xiang-Dong Chen, Guang-Can Guo, Fang-Wen Sun
AA high-sensitivity fiber-coupled diamond magnetometer with surfacecoating
Shao-Chun Zhang,
1, 2
Hao-Bin Lin,
1, 2
Yang Dong,
1, 2
Bo Du,
1, 2
Xue-Dong Gao, Cui Yu, Zhi-Hong Feng, Xiang-Dong Chen,
1, 2
Guang-Can Guo,
1, 2 and Fang-Wen Sun
1, 2, a) CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026,P.R. China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China,Hefei, 230026, P.R. China National Key Laboratory of ASIC, Hebei Semiconductor Research Institute, Shijiazhuang 050051,P.R. China (Dated: 25 February 2021)
Nitrogen-vacancy quantum defects in diamond offer a promising platform for magnetometry because of their remark-able optical and spin properties. In this Letter, we present a high-sensitivity and wide-bandwidth fiber-based quantummagnetometer for practical applications. By coating the diamond surface with silver reflective film, both the fluo-rescence collection and excitation efficiency are enhanced. Additionally, tracking pulsed optically detected magneticresonance spectrum allowed a magnetic field sensitivity of 35 pT / √ Hz and a bandwidth of 4 . . At the nanometer scale,benefiting from their angstrom-scale size, single NV centershave been used for applications with ultra-high-resolution in living biology systems , nuclear magnetic resonance ,and magnetic resonance force microscopy . At the microm-eter scale, sensor employing ensemble of NV centers usu-ally provide improved sensitivity at the cost of spatial res-olution, which facilitates a wide-field magnetic imaging forbio-magnetic structures , integrated circuits , and even thegeological samples . At the millimeter scale, bulk ensem-ble magnetometers with large active volumes offer ultra-highsensitivity , but most of the techniques have the require-ment of cavity or parabolic lens for increasing fluores-cence collection efficiency, leading the sensors unsuitable forusing at the few-millimeter scale.The recently developed fiber-optic probes coupledwith NV centers enabled a compact approach at the scale ofa few hundred micrometers. Coupling the NV center to anoptical fiber integrated with a two-wire microwave transmis-sion line enable a sensitivity of 300 nT / √ Hz . The subse-quent double-fiber coupling approach allows efficient noisecancellation and a sensitivity of 35 nT / √ Hz . Addition-ally, by optimizing fiber collection system, the best sensi-tivity of 310 pT / √ Hz in the frequency range of 10 − . On the other hand, recent study with amatched micro-concave mirror to a sphered optic-fiber endhas achieved over 25 times more fluorescence collection fromNV enriched micrometer-sized diamond . In particular,thanks to the simplicity and robustness, such a fiber-opticquantum sensor has successfully realized in vivo detection .Here, we pasted a bulk diamond on the optical fiber tip, a) Electronic mail: [email protected] and a reflective film was coated on the five surfaces exposedto the air through silver mirror reaction, as shown in Fig. 1,which suppressed the green laser and red fluorescence fromrefracting into the air. Such a strategy can not only enhancethe fluorescence collection efficiency, but also the excitationefficiency of NVs in diamond. In this case, we present a fiber-based quantum magnetometer for practical applications withhigh sensitivity and wide bandwidth. With pulsed opticallydetected magnetic resonance (ODMR) implementation, opti-cal and microwave (MW) power broadening of the spin reso-nances can be avoided, which allows a further improvement insensitivity. By optimizing the initialization time of NV centerand then tracking pulsed ODMR, a magnetic field sensitivityof 35 pT / √ Hz and a bandwidth of 4 . , our work has higher sensitivity, widerbandwidth and more ways to optimize the sensitivity, like dou-ble quantum magnetometry and spin-bath driving . Theseresults make a significant advance in transiting lab-based sys-tems to practical applications at the millimeter scale. (b) DiamondAir optical fibertipfiber coreDiamondAir optical fibertipfiber core silver films (a)
FIG. 1. (a-b) Schematic of the fluorescence collection and excita-tion efficiency with (a) and without (b) the silver films coated on thediamond surfaces.
Conventional confocal scanning system employed in a a r X i v : . [ phy s i c s . a pp - ph ] F e b Laser 532nmFiber collimatorDM FilterMultimode fiber SensorPulseBlaster cardDAQ (a)
MW Amp.TTLTrigger inInput PIDInputControl AOM(Analog)AOM(TTL) Signal sourceOut MW Switcher (b)
NV spinOtical fiber (c)
Copper wire g e B |0 |-1|+1 Magnetic wire C u r r e n t um B magnetPhotodetectorPhotodetector BS FIG. 2. (a) The schematic of fiber-optical magnetometer setup. DM, long-pass dichroic mirror with the edge wavelength of 658 . | (cid:105) . The fluorescence starting from state | (cid:105) is stronger than that of states | ± (cid:105) , allowing the NV spin to be read out optically. (c) A bulk diamond is attached on the tip of a multi-mode optical fiber. The inset showsthe bulk diamond with the size of 200 × × µ m under the microscope without silver film coated. sensing scheme poses complications in terms of collectivecontrol, signal readout and even the stability, leading to thelab-based demonstrations. Here, these issues can be addressedby using a homebuilt fiber system to excite and detect the NVcenters, as shown in Fig. 2(a). A diamond attached on the tipof a multi-mode optical fiber with a core diameter of 200 µ mhas been mechanically polished and cut into a membrane withdimensions of 200 × × µ m , as shown in Fig. 2(c).The NV center ensemble in the diamond consisted of [N] ≈
55 ppm and [NV − ] ≈ − exhibits an efficient and photostablered PL, which enables optical detection. The ground state isa spin triplet including a singlet state | (cid:105) and a doublet state | ± (cid:105) separated by a temperature-dependent zero-field split-ting (ZFS) D = .
87 GHz in the absence of magnetic field, asshown in the inset of Fig. 2(b). Applying a static magneticfield along the NV axis leads to a splitting of | − (cid:105) and | + (cid:105) states.In the experiment, 532 nm green laser was sent through anacousto-optic modulator (AOM, AA optoelectronic MT250-A0.5-VIS, Analog modulation) and finally coupled into themulti-mode optical fiber. The power was adjusted with abeam-splitter (BS) cube before the diamond, where part ofthe laser light was split-off and measured with a photode-tector (PD, Thorlabs PDA36A). The signal was then input into a proportional-integral-derivative controller (PID, SRSSIM960) to stabilize the laser power. For the sensing proto-col described here, laser pulses on the microsecond timescalefor initializing and reading out the NVs are realized by an-other AOM (AA optoelectronic MT250-A0.5-VIS, TTL mod-ulation). Collected by the same fiber, the photolumines-cence (PL) passed through a 647 nm long pass filter and wasfinally sent to another photodetector (Thorlabs APD430A).Moreover, the output MW was sent through a switch (M-C ZASWA-2-50DR+) to a high-power amplifier (M-C ZHL-16W-43), and finally delivered by a five-turn copper loop withan outer diameter of 0 . . (a)(c) (b)(d) LaserMW Sweep duration tInit. Readout diamond
FIG. 3. (a) Red fluorescence collection with and without the re-flective coating, respectively. The inset shows the diamond coatedwith silver on the tip of fiber, and the fluorescence collection effi-ciency can be enhanced by 2 . µ W. (c) The ZFS changes ∆ Das a function of the pump green laser power. (d) Rabi oscillationswith the pump laser power of 320 mW. The inset show the pulsesequence. The silver film on the diamond surface has affected themicrowave radiation. of reflective film, leading to a low fluorescence excitation effi-ciency, as shown in Fig. 3(b). After coating treatment, almostno green light was transmitted, which indicates the perfect re-flection effect of silver film. In general, diamonds containinghigh concentrations of nitrogen are dark black, resulting in thediamond absorbing part of the green pump laser and tempera-ture rising. The red dots in Fig. 3(c) shows the linear changeof ZFS with laser power, which is consistent with the results ofprevious studies . Note that dD / dT ≈ − / K at roomtemperature . Although coating reflective films on the di-amond could double the green light absorbed by the diamond,the green light heated the silver films and thus led to temper-ature change slope more than twice that of the uncoated dia-mond [blue dots in Fig. 3(c)]. In order to determine the impactof silver films on the MW field, a 392 G magnetic field wasapplied along one of the NV axes. And Rabi oscillations werethen performed under the same MW input power, as shownin Fig. 3(d). The inset shows the Rabi pulse sequence withthe initialization time of 500 µ s, AOM polarization time of1 . µ s and DAQ unit readout pulses length of 300 ns. In theRabi oscillation experiment, the MW frequency was tuned tomatch the NV spin resonance (from m s = m s = − . Themeasurement pulse sequence is depicted schematically inFig. 4(a). The NV − spin state is first optically initialized to m s =
0, and the initialization efficiency is laser power andpulse length dependent . For common noise cancellation, thepulse sequence is applied twice except the MW off in the sec-ond pulse . In this case, the division of two signals denotesthe result of the detection sequence executed once. Settingthe pump laser power to 320 mW and sweeping the initializedduration, the initialization efficiency or the measurement con-trast increases as 1 − e − t / τ , as illustrated in Fig. 4(b), where τ , uncoated and τ , coated are 71 ( ) µ s and 45 ( . ) µ s, respec-tively. Compared with the uncoated diamond, the coated re-duced the time for polarizing NV − spin state. Alternatively,the coated diamond provides a higher contrast with the samepump laser power, and further improves the sensitivity.In order to measure the magnetic field sensitivity of thefiber-based quantum magnetometer, pulsed ODMR was em-ployed. In contrast to cw ODMR, this technique avoids opti-cal and MW power broadening of spin resonances, enablingnearly T ∗ -limited measurements without requiring high Rabifrequency . And pulsed ODMR is linearly sensitive to mag-netic field variations, making the method attractive when highMW field strengths are not available . The pulsed ODMRprotocol is depicted schematically in Fig. 4(c), and the resultwith a Lorentzian resonance line shape is plotted in Fig. 4(d).The sensitivity η B of the measurement is given by the follow-ing relation : η B = √ hg e µ B ∆ ν √ t m C √ R m , (1)where ∆ ν is the full-width at half-maximum (FWHM) of theresonance spectrum, C is the contrast of pulsed ODMR, t m and R m correspond to the measurement time and fluorescence pho-ton emission per sequence operation. Due to the interrogationtime and readout pulse are much smaller than the initializa-tion time , i.e., t π + t read (cid:28) t init , the measurement duration t m ≈ t init . Additionally, the ODMR contrast is strongly de-pendent on the initialization time, yielding η B ≈ √ hg e µ B ∆ ν C ( − e − t init / τ ) √ t init √ R m . (2)Hereafter, we assume R m is independent of t init and thechoice of t π ≈ T ∗ ( ≈ . µ s here) allows nearly T ∗ -limitedlinewidth while preserving PL contrast . Eq. (2) also illus-trates the benefits attained by optimized t init ≈ . τ . Byfixing the MW frequency at point of maximum slop ( f inFig. 4(d)), the output of each pulse sequence operation wasrecorded for 100 s. The calculated power spectral densityfor coated and uncoated diamond are depicted in Fig. 4(e).For frequencies in the range of 1Hz − . ( ) pT / √ Hz with the dia-mond uncoated. After coating reflective films, the sensitiv-ity was enhanced to 35 ( ) pT / √ Hz accompanying the band-width broaden to 1 Hz − . τ = τ , uncoated .The Allan deviation from the pulsed ODMR sequence withthe reflective coating is plotted in Fig. 4(f), and the trace ex-hibits a constant τ − / scaling, which signifies the dominanceof stochastic white noise, like thermally induced electronic LaserMW t Sweep duration π (c)(a)(e) DAQ t Ref. LaserMW Sweep FrequencyRead. π DAQ Ref. (b)
Init.
Read.
Init. (f) S (d) FIG. 4. (a) Pulse sequences of initialization time sweeping. (b)Pulse sequences of pulsed ODMR. (c) The initialization efficiencyas a function of laser duration time t . Both of the two curves arefitted by exponential decay function. (d) The pulsed ODMR withFWHM of 2 .
58 MHz, where f is at the point of maximum slope.(e) Magnetic field sensitivities of coated and uncoated fiber-opticalmagnetometer. The sensitivity is optimized from 71 ( ) pT / √ Hz to35 ( ) pT / √ Hz, with the bandwidth is broadened from 2 . . t ini andwas fitted by Eq. (2), where t ini ≈ . τ gives the best sensitivity.(f) Scaling of Allan deviation from the pulsed ODMR sequence withthe reflective coating. (a) -32 -22Experiment Simulation (b) FIG. 5. (a) Mapping the magnetic field induced by the current-carrying copper-wire mesh with coated fiber-optical magnetometerrespectively. Arrows indicate the directions of the 1 mA current. (b)Simulation of the magnetic field in (a) via Biot-Savart-Laplace equa-tion. noise generated in the detector. A minimum floor for this de-tection is reached for τ = −
30 s, with further averaging(for long detection time τ ) giving no advantage mainly due totemperature-based magnetization fluctuations.Finally, the coated magnetometer probe is incorporated intoa three-axis motorized translation stage with 1 µ m accuracyon each axis. In order to verify the feasibility of scanningimaging in the micron size, this magnetometer was applied tomap the magnetic field induced by a current-carrying copper-wire mesh with a grid pitch and wire diameter of approxi-mately 2 mm and 30 µ m, respectively, as shown in Fig. 2(b).Before imaging, the probe was placed at a 50 µ m distancefrom the mesh, and then, MW frequency was fixed at f inFig. 4(b), so that the probe had the maximum response tomagnetic field variations. Fig. 5(a) shows the 2D magneticfield projected onto NV axis under an injected current 1 mA,where the arrows indicate current directions. Moreover, themagnetic field profile predicted by the Biot-Savart-Laplaceequation agrees very well with the experimental measurement,as shown in Fig. 5(b).In conclusion, we have experimentally demonstrated afiber-based quantum magnetometer coupled with NV centers,which enables practical applications at the micron scale withpicotesla sensitivity. Compared with the previous cw ODMRmeasurement, our magnetometer avoided optical and MWpower broadening of the spin resonances in the pulsed ODMRimplementation, thus allowing a further improvement in sen-sitivity. Additionally, coating the diamond surfaces exposedto the air with reflective films benefits sensitivity in two ways:first, the reflective films enhances the fluorescence collectionefficiency and thus improve the sensitivity by √ η eff30 , where η eff is the efficiency enhancement. And second, the enhancedexcitation efficiency of NV center shortens the initializationtime, which decreases the fraction of time devoted to spinprecession. 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