Open Access
Add to BibSonomy
Abstract
Large-area, highly uniform microwave field radiation and efficient excitation of fluorescence are the key to achieving high sensitivity sensing of the NV (nitrogen-vacancy) magnetometer. In this paper, we report a compact multipass-laser-beam antenna for NV ensemble color centers sensing. The antenna not only provides a tridimensional uniform magnetic field, but also can be used for efficient excitation of the NV fluorescence. The optimal size of the antenna and the angle of laser incidence are determined by the multi-physics field simulation software COMSOL. For an equivalent excitation power, the designed structure increases the path length of the excitation beam by up to three orders of magnitude, up to the level of m, compared to the conventional direct beam mode. Finally, this method increased the sensitivity by a factor of 60 realized a magnetic field sensitivity of 2.8 nT/√Hz in the range of 10–100 Hz. This work provides an experimental method for the design of integrated NV magnetometers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As a solid-state quantum bit, the diamond nitro-vacancy (NV) color center can be used for the measurement of basic physical quantities such as electric fields [1], magnetic fields [2], temperature [3], strain fields [4], and the detection of cell nuclear spins and protein structures and is widely used in aerospace [5,6], deep sea exploration, biomedicine, micro and nano optics, quantum metrology, quantum sensing and geomagnetic survey [7–9]. The NV color center has a long electron spin coherence time at room temperature and can be optically initialized and readout of electron spins by laser and microwave manipulation [10–14]. In magnetic field sensing NV color center possess picotesla-level sensitivity and nanometer-level spatial resolution [15], and have the advantages of high stability, high compatibility, and fast start-up, making them the best candidates for making high-precision quantum magnetometers. However, to achieve high-sensitivity magnetic field sensing in large-volume ensemble diamond, it is necessary to provide a large-area uniform microwave field for quantum manipulation, on the one hand, the fluorescence emitted from NV centers in a bulk diamond is using direct laser irradiation, of which most color centers cannot be excited [16]. Therefore, the NV excitation efficiency is critically important.
In recent years, researchers have tried to meet the demand for high sensitivity of sensors by making high-performance microwave antennas or trying to obtain efficient fluorescence information, respectively. K. Bayat et al. designed a double-split ring-gap microwave resonator with high microwave transmission efficiency and providing a uniform microwave field [17]; Chen et al. reported a double magnetic ring stereo antenna that provides a uniform microwave fiel, while effectively reducing the interference of the antenna's own thermal noise on the sensing performance [18]. In order to improve the photon detection efficiency, Le Sage et al. proposed a “side-by-side collection” technique with four photodetectors arranged on the side of the diamond with a photon detection efficiency of ≈ 39% [19]. The CPC lens designed by Wolf and the total reflection (TIR) lens designed by Xu et al. Wolf's CPC lens and Xu et al.'s total reflection (TIR) lens achieved fluorescence collection efficiencies of up to 65% and 47%, respectively [20,21]. Clevenson et al. incident a laser at an angle to the diamond waveguide geometry, converting more than 5% of the pumped photons to optically detected fluorescence in magnetic resonance [22]. The above designs play a positive role in improving sensor sensitivity, but they are all individually designed high-performance microwave antenna and fluorescence efficient excitation or collection devices. With the reduction of the magnetometer probe, the precision of the sensor is limited to a certain extent, and there are difficult to integrate and complex processing problems, therefore, the development of high-precision, multifunctional and practical sensor components for the development of chip-scale NV magnetometer is of great significance.
To address the above problems, in this paper, we propose a multipass-laser-beam antenna (MLBA) for improving the sensitivity of diamond ensemble NV color centers. The MLBA consists of a copper ring embedded in a PCB board. The center of the copper ring can generate a large area of uniform microwave field for driving the NV color center, and the polished inner wall of the copper ring can be used for efficient excitation of fluorescence through total reflection of the laser. The single-pass excitation method used in the traditional planar single loop antenna (SLA) limits the excitation efficiency of the large volume system of diamond color centers. As shown in Fig. 1(a), the laser is incident on the diamond through the objective lens, and the fluorescence emitted is collected directly through the objective lens. In the SLA system, standard single-pass laser excitation causes few NV color centers to be excited. In contrast, as shown in Fig. 1(b), the laser enters the interior of the MLBA through a side lens and undergoes multiple reflections from the interior walls of the MLBA to excite more fluorescence photons, which are collected by an objective lens on the upper surface. For an equivalent excitation power, the MLBA increases the path length of the excitation beam by up to several orders of magnitude. Therefore, the MLBA was performed without the additional devices, adding laser optical paths to excite more NV color centers for measurement which is more suitable for low-cost integrated manufacturing of high-precision magnetometers.
Fig. 1. Sketch map of (a) Single-pass path excitation based on SLA structure. (b) Multiple-pass path excitation based on MLBA structure.
2. Basic principles
The NV color center is a point defect in diamond lattice, as shown in Fig. 2(a). It is consisted by the substitution of a C atom by an N atom [23,24], which combines with a vacancy at an adjacent position [25], thus forming a nitrogen-vacancy lattice, or NV color center. As shown in Fig. 2(b). The NV color center is in the spin triplet state in both the ground state 3A2 and the excited state 3E [26]. In the experiment, the NV color center reaches the excited state after the 532 nm laser radiation, One is the emission of fluorescence back to the ground state, and the other is the occurrence of inter-system scramble (ISC) through the sub-stable state 1A1 and 1E back to the ground state, a process that does not emit fluorescence [27]. The probability of these two pathways occurring is limited by the spin state of the NV color-centered electron, which is the key reason why the spin state of that electron can be photo polarized and read out. Therefore, we can read out the spin state of the NV electron by measuring the change in fluorescence intensity. The sensitivity of the DC magnetometer based on ensemble NV color centers is [28]:
Fig. 2. (a) The internal structure of diamond NV color center. (b) The energy level structure of diamond NV color center.
To improve detection sensitivity, special structures could be designed to improve these parameters. For processed diamonds, N (the number of the excited NV centers) is mainly limited by the area laser irradiated which determines the number of NVs excited. And the fluorescence intensity is related to the volume of the excited NV centers. ΔV and C will increase with the microwave radiation intensity, but the uniform microwave radiation can effectively suppress the increase of ΔV. Therefore, highly uniform and strong radiation satisfies the basic requirement of spin manipulation. As for the LMRA, it provides a uniform microwave field while increasing the laser reflection optical path. Thus, the antenna is more suitable for the sensitivity enhancement.
3. Design and simulation
Figure 3(a) shows the prototype photograph of MLBA. It was fabricated on the 1.2 mm-thick FR4 board with a relative dielectric constant of 4.4. Figure. 3(b) shows the geometric dimension parameters of the MLBA, which are designed based on the basic principles of loop antennas and coplanar waveguide antennas [29,30]. For the simulation of MLBA, we use the RF module of the multi-physics field simulation software COMSOL for optimization. The RF module uses a frequency domain solver to solve Maxwell's equations by finite integration techniques. The center of the MLBA is designed to provide a quantum tuned tridimensional uniform microwave magnetic field and is fed and used for 50 Ω impedance matching by introducing two microstrip lines, while a waveguide port is defined at the end of the microstrip lines. Our simulations include S11 parameters in the range of (2.2 GHz ∼ 3.6 GHz) and the radiation power of the antenna at 2.87 GHz.
The inner radius of the antenna r1 and the gap width g1 form capacitive and inductive components respectively. To construct a strong radiation field, the radius of the ring r1 should be as small as possible. One end of the opening is for laser incidence. The open hole width g2, copper ring thickness H could be used for the resonant frequency adjustment. The optimized dimensions of the MLBA are shown in Table 1. The copper ring is embedded in the PCB board. The microstrip line is connected to the lumped port and fed through the SMA connector.
Table 1. Parameters of the MLBA
We simulated the microwave magnetic field distribution of the MLBA at 2.87 GHz by COMSOL RF module, taking the center of the copper ring as the zero point. Figure. 4(a) shows the magnetic field distribution in XY plane. It can be seen in the Fig. 4(a) that the magnetic field intensity of the MLBA is uniform distribution inside the ring area. When the input microwave power is 5w, the magnetic radiation strength is about 60∼80 A/m. Figure. 4(b) shows the simulated and measured values of S11. The simulated value of the antenna resonant frequency is 2.88 GHz, the power loss is -30 dB, the bandwidth is 60 MHz, and the measured value is 2.875 GHz, the power loss is -32 dB, the bandwidth is 62 MHz. The simulated and measured values are almost the same, the small errors are attributed to the inaccuracy of the processing dimensions.
Fig. 4. (a) Simulated distribution of magnetic field on XY plane. (b) Measured (red line) and simulated (black line) S11 results of MLBA. (c) Internal light range of diamond at different incident angles of laser.
Optical path of the laser through the diamond sample is proportional to the excitation efficiency of the fluorescence signal, which is the decisive factor for achieving high sensitivity and analytical accuracy of the diamond NV color-centered quantum sensor. The laser is incident through the open hole g2. The inner wall of the copper ring is set for specular reflection, and the center of the MLBA is set for a circular type IIa diamond with a radius of 3 mm and a height of 1.2 mm (using a circular diamond mainly due to its excellent optical reflection). The laser radius is assumed to be 0.1 mm, and is released from the grid and terminated to the diamond geometry boundary at an angle of 0° from the vertical incident diamond.
As shown in Fig. 4(c), the red line represents the change in the optical path of the laser inside the diamond as a function of the incident angle of multipass configuration. And black represents single-pass configuration. As for multipass, the maximum optical path inside the diamond can reach the order of m in the range of incident angle (-21.8°∼ 21.8°). The optical path inside the diamond can reach 1620 mm when incident at the maximum angle of 16.4°, where the optical absorption and scattering losses on the inner wall of the MLBA and the diamond surface are negligible. For comparison, the maximum optical path length achievable in a single-pass configuration through the top of the sample is plotted in black. The single-pass optical path length is essentially the same as the diamond thickness, which is only a few millimeters.
However, it should be noticed that for simulation the inner wall of the copper ring is set for specular reflection. In practice, light loss occurs when the laser is reflected from the surface of the copper ring, although we polish the surface of the copper ring to Sub-Micron accuracy by fine-tuning. Light loss reduces the number of laser reflections and thus reduces the optical path. In response to this situation, we can increase the laser power to reduce the effect of light loss. Another interesting point is that the diffuse reflection in the light loss will excite another part of the color centers.
4. Results and discussion
The ODMR experiments are performed using MLBA as shown in Fig. 5. The diamond was fabricated using a circular diamond with a radius of 3 mm and a height of 1.2 mm and a concentration of 1018cm-3 in the color center, which was irradiated by 10 MeV electrons for 4 hours and annealed at 850°C for 2 hours. The green 532 nm laser (MW-GL-532) was used for fluorescence excitation of the NV color center. Most of the fluorescence generated by repeated laser excitation spills out through the surface of the MLBA, then passes through the reflector, objective, filter (Thorlabs DMLP650T) and finally the photodiode (Thorlabs FDS1010) to convert the fluorescence signal into an electrical signal. The resonance signal is obtained by sweeping through the microwave field of the characteristic NV electron spin resonance at a central frequency of 2.87 GHz. We compared the fluorescence detection intensity of the traditional single loop antenna (SLA) and MLBA by a single photon count. The 532 nm laser was incident at an incidence angle of 16.4°. As shown in Fig. 6(a), the red and black lines represent the number of detected photons of the confocal system using MLBA and SLA without the external magnetic field, respectively. It can be seen that the number of fluorescence photons detected by the MLBA is 50 times higher than that of the SLA, and the enhancement of the fluorescence signal is mainly attributed to the increasement of the laser optical path inside the diamond, resulting in more NV color centers to be excited.
Fig. 6. (a) Numbers of fluorescence photons detected with the two collection systems by single-photon counter. (b) ODMR experiment results obtained using the SLA and MLBA. (c) The noise spectral densities of the SLA and MLBA.
The ODMR spectra were obtained with the two antennas. Microwave and laser input powers are 19 dBm and 20 mW, respectively. The microwave power is increased to 5W by a microwave amplifier, as shown in Fig. 5. As shown in Fig. 6(b), the red and black lines represent the ODMR spectra of the MLBA and the SLA in the absence of external magnetic field, respectively. The ODMR spectral contrast measured by the SLA is 3%, while for the DMRSA it is as high as 18%, a factor of 6 times for the SLA. This is attributed to the high intensity of microwaves irradiated onto the diamond of the MLBA. As for the linewidth, the MLBA ODMR is 2.6 times narrower than the SLA due to its dependence on the uniformity of the microwave field. The increase in C and the decrease in ΔV demonstrate the availability of the MLBA in improving NV sensor sensitivity. As indicated in Fig. 6(c), the magnetic noise spectral density measured by the lock-in amplifier is observed at low frequencies. The black line shows the result measuring with the SLA, where the noise floor of the magnetic sensitivity is approximately 165.3 nT/√Hz, while the corresponding red line for the MLBA method has a noise floor of approximately 2.8 nT/√Hz. It indicates that the sensitivity of the sensor based on the MLBA is about 60 times that of the SLA, which shows that the MLBA has better application prospects in improving the sensitivity and integrated manufacturing of NV magnetometer.
5. Conclusion
In this paper, we proposed a compact multipass-laser-beam antenna (MLBA) which provides a uniform microwave field while improving the efficiency of the NV color-center excitation. Ultimately, an increase in sensor sensitivity is realized. Compared with the traditional single-loop antenna (SLA), the ODMR signal of the MLBA has higher contrast and narrower linewidth. The sensitivity of magnetic field noise at room temperature is as high as 2.8 nT/√Hz. In addition, the antenna structure can be used as a reflector of laser without adding additional device, which is suitable for low-cost fabrication. The antenna combined with the CPC lens can further improve the collection efficiency of fluorescence signals. In addition, it provides an experimental approach and theoretical basis for the future multi-functional development and applications of microwave antennas and the miniaturized integrated fabrication of solid-state quantum sensors.
Funding
National Natural Science Foundation of China (62004119, 62201333); The Ministry of Education's "Chunhui Plan" collaborative research project (202200056); Basic Research Program of Shanxi Province (20210302124647, 202203021222220).
Acknowledgment
The authors appreciate the support from National Natural Science Foundation of China, The Ministry of Education's “Chunhui Plan” collaborative research project, and the Basic Research Program of Shanxi Province.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
References
1. T. Iwasaki, W. Naruki, K. Tahara, et al., “Direct nanoscale sensing of the internal electric field in operating semiconductor devices using single electron spins,” ACS Nano 11(2), 1238–1245 (2017). [CrossRef]
2. G. Balasubramanian, I. Chan, R. Kolesov, et al., “Nanoscale imaging magnetometry with diamond spins under ambient conditions,” Nature 455(7213), 648–651 (2008). [CrossRef]
3. G.-Q. Liu, X. Feng, N. Wang, et al., “Coherent quantum control of nitrogen-vacancy center spins near 1000 kelvin,” Nat. Commun. 10(1), 1344 (2019). [CrossRef]
4. E. MacQuarrie, M. Otten, S. Gray, et al., “Cooling a mechanical resonator with nitrogen-vacancy centres using a room temperature excited state spin–strain interaction,” Nat. Commun. 8(1), 14358 (2017). [CrossRef]
5. F. Shi, F. Kong, P. Zhao, et al., “Single-DNA electron spin resonance spectroscopy in aqueous solutions,” Nat. Methods 15(9), 697–699 (2018). [CrossRef]
6. F. Casola, T. Van Der Sar, and A. Yacoby, “Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond,” Nat. Rev. Mater. 3(1), 17088 (2018). [CrossRef]
7. M. Hirose and P. Cappellaro, “Coherent feedback control of a single qubit in diamond,” Nature 532(7597), 77–80 (2016). [CrossRef]
8. Y. Y. Hui, O. J. Chen, H.-H. Lin, et al., “Magnetically modulated fluorescence of nitrogen-vacancy centers in nanodiamonds for ultrasensitive biomedical analysis,” Anal. Chem. 93(18), 7140–7147 (2021). [CrossRef]
9. L. H. G. Tizei and M. Kociak, “Spatially resolved quantum nano-optics of single photons using an electron microscope,” Phys. Rev. Lett. 110(15), 153604 (2013). [CrossRef]
10. G. Balasubramanian, P. Neumann, D. Twitchen, et al., “Ultralong spin coherence time in isotopically engineered diamond,” Nat. Mater. 8(5), 383–387 (2009). [CrossRef]
11. P. Wang, Z. Yuan, P. Huang, et al., “High-resolution vector microwave magnetometry based on solid-state spins in diamond,” Nat. Commun. 6(1), 6631 (2015). [CrossRef]
12. M. L. Goldman, A. Sipahigil, M. Doherty, et al., “Phonon-induced population dynamics and intersystem crossing in nitrogen-vacancy centers,” Phys. Rev. Lett. 114(14), 145502 (2015). [CrossRef]
13. M. L. Goldman, M. Doherty, A. Sipahigil, et al., “State-selective intersystem crossing in nitrogen-vacancy centers,” Phys. Rev. B 91(16), 165201 (2015). [CrossRef]
14. M. W. Doherty, N. B. Manson, P. Delaney, et al., “The nitrogen-vacancy colour centre in diamond,” Phys. Rep. 528(1), 1–45 (2013). [CrossRef]
15. Z. Wang, F. Kong, P. Zhao, et al., “Picotesla magnetometry of microwave fields with diamond sensors,” Sci. Adv. 8(32), eabq8158 (2022). [CrossRef]
16. J. Hadden, J. Harrison, A. C. Stanley-Clarke, et al., “Strongly enhanced photon collection from diamond defect centers under microfabricated integrated solid immersion lenses,” Appl. Phys. Lett. 97(24), 241901 (2010). [CrossRef]
17. K. Bayat, J. Choy, M. Farrokh Baroughi, et al., “Efficient, uniform, and large area microwave magnetic coupling to NV centers in diamond using double split-ring resonators,” Nano Lett. 14(3), 1208–1213 (2014). [CrossRef]
18. Y. Chen, H. Guo, W. Li, et al., “Large-area, tridimensional uniform microwave antenna for quantum sensing based on nitrogen-vacancy centers in diamond,” Appl. Phys. Express 11(12), 123001 (2018). [CrossRef]
19. D. Le Sage, L. M. Pham, N. Bar-Gill, et al., “Efficient photon detection from color centers in a diamond optical waveguide,” Phys. Rev. B 85(12), 121202 (2012). [CrossRef]
20. T. Wolf, P. Neumann, K. Nakamura, et al., “Subpicotesla diamond magnetometry,” Phys. Rev. X 5(4), 041001 (2015). [CrossRef]
21. L. Xu, H. Yuan, N. Zhang, et al., “High-efficiency fluorescence collection for NV-center ensembles in diamond,” Opt. Express 27(8), 10787–10797 (2019). [CrossRef]
22. H. Clevenson, M. E. Trusheim, C. Teale, et al., “Broadband magnetometry and temperature sensing with a light-trapping diamond waveguide,” Nat. Phys. 11(5), 393–397 (2015). [CrossRef]
23. J. H. N. Loubser and J. A. van Wyk, “Electron spin resonance in the study of diamond,” Rep. Prog. Phys. 41(8), 1201–1248 (1978). [CrossRef]
24. G. Davies and M. F. Hamer, “Optical studies of the 1.945 eV vibronic band in diamond,” Proc. R. Soc. Lond. A 348(1653), 285–298 (1976). [CrossRef]
25. S. Kitazawa, Y. Matsuzaki, S. Saijo, et al., “Vector-magnetic-field sensing via multifrequency control of nitrogen-vacancy centers in diamond,” Phys. Rev. A 96(4), 042115 (2017). [CrossRef]
26. F. Jelezko and J. Wrachtrup, “Single defect centres in diamond: A review,” Phys. Status Solidi A 203(13), 3207–3225 (2006). [CrossRef]
27. V. R. Horowitz, B. J. Alemán, D. J. Christle, et al., “Electron spin resonance of nitrogen-vacancy centers in optically trapped nanodiamonds,” Proc. Natl. Acad. Sci. USA 109(34), 13493–13497 (2012). [CrossRef]
28. L. Rondin, J.-P. Tetienne, T. Hingant, et al., “Magnetometry with nitrogen-vacancy defects in diamond,” Rep. Prog. Phys. 77(5), 056503 (2014). [CrossRef]
29. F. M. Greene, “The near-zone magnetic field of a small circular-loop antenna,” J. Res. Natl. Bur. Stand., Sect. C 71C(4), 319–326 (1967). [CrossRef]
30. R. Simons and R. Lee, Coplanar waveguide feeds for phased array antennas, Conference on Advanced SEI Technologies pp. 34221991.
