Negatively charged nitrogen-vacancy (NV) centers in diamond have emerged as promising candidates for a wide range of quantum applications, especially quantum sensing of magnetic field. Implementation of nanostructure into diamond is powerful for efficient photon collection of NV centers and chip-scale miniaturization of the device, which is crucial for sensitive and practical diamond magnetometers. However, fabrication of the diamond nanostructure involves technical limitations and can degrade the spin coherence of the NV centers. In this study, we demonstrate the hybrid integration of a silicon nitride grating structure on a single-crystal diamond by utilizing transfer printing. This approach allows the implementation of the nanostructure in diamond using a simple pick-and-place assembly, facilitating diamond-based quantum applications without any complicated diamond nanofabrication. We observed the intensity enhancement in the collected NV emissions both theoretically and experimentally using the integrated grating structure. By applying the increased photon intensity, we demonstrate the improved magnetic sensitivity of the fabricated device. The proposed hybrid integration approach will offer a promising route toward a compact and sensitive diamond NV-based magnetometer.

Negatively charged nitrogen-vacancy (NV) centers in diamond have been considered as promising candidates for quantum technologies, including quantum information and sensing.1–3 As the electron spins in the NV centers can be optically addressed at room temperature to initialize and read out the quantum state with long coherence time, NV centers allow for the sensitive detection of various environmental parameters, e.g., magnetic field, electric field, pressure, and temperature.4–8 In particular, NV centers ensembled in a single-crystal diamond have intensively been investigated as sensitive magnetic sensors.6,9–12 Recently, a DC magnetic sensitivity of 0.9 pT/Hz has successfully been demonstrated using ensembles of NV centers.13 In addition, NV centers allow the vector imaging of the magnetic field owing to their four possible orientations of the axis.14,15

Despite this tremendous progress, the demonstrated magnetic sensitivity of diamond NV-based magnetometers is still several orders of magnitude lower than that of conventional magnetometers.16 One of the critical limitations is the poor collection efficiency of the photons emitted from NV centers because the sensitivity of diamond magnetometers is inversely proportional to the square root of the detected photon counts.2 Owing to the relatively high refractive index of diamond (2.4), the extraction of photons from NV centers is hindered by the total internal reflection inside the diamond. To this end, several studies aimed to improve the photon extraction efficiency of diamond magnetometers using a parabolic concentrator,13 side collection appraoch,17 prisms,18 and surface coating.19 However, these diamond magnetometers are based on a bulky and complicated structure. For the practical application of NV-based magnetometers, it is important to miniaturize the device.10,20

An effective approach to resolve this issue is the implementation of nanostructures in diamonds, which allows for the efficient photon collection and device miniaturization.21 Thus far, nanofabrication of various types of nanophotonic components, a grating structures,22,23 disk resonators,24,25 and photonic crystals,26–31 has successfully been demonstrated in diamond owing to the recent development of diamond nanofabrication technology.3,32,33 However, due to the technological difficulties in nanofabrication of diamonds, particularly single-crystal diamonds, there are a few studies on the nanofabrication of single-crystal diamonds, and there are no reports on the demonstration of nanostructures for the efficient photon extraction from NV ensembles in a single-crystal diamond. Moreover, the rough surface after diamond etching may affect the spin coherence time of the NV centers, resulting in the degradation of the magnetic field sensitivity.34 

In this work, we demonstrate the hybrid integration of a silicon nitride (SiN) circular grating structure on a single-crystal diamond NV substrate to improve the photon extraction and magnetic sensitivity of the NV centers. We utilize transfer printing35–41 to integrate the grating on a single-crystal diamond, as shown in Fig. 1. This simple pick-and-place approach allows the implementation of a prefabricated nanostructure on a single-crystal diamond without any complicated diamond etching processes, facilitating the application of diamond-based quantum photonics. Theoretically, the output efficiency of NV emission is enhanced by 2.1 times through the proposed structure. Experimentally, we achieved an increased intensity of NV emissions by a factor of 1.6 simply by attaching the grating structure to a diamond NV substrate. We further demonstrated that the magnetic sensitivity of the fabricated device can be improved. The proposed approach can potentially enable the development of compact and sensitive diamond magnetometers.

FIG. 1.

Schematic of the proposed hybrid integration process.

FIG. 1.

Schematic of the proposed hybrid integration process.

Close modal

The grating structure was designed to enhance the collection efficiency of NV photons. As shown in the upper panel of Fig. 2(a), the high refractive index of diamond hinders the collection of NV photons by total internal reflection. This reflection can be mitigated by circular gratings, which can guide NV emission to detectors regardless of the NV dipole orientations by modifying the refractive index of the diamond surface. The angle of the traveling photons can be controlled by the grating pitch, enabling the improvement of photon collection efficiency. The lower panel of Fig. 2(a) shows a schematic cross section of the investigated grating structure on diamond. We employed SiN as the grating material, which is transparent to the wavelength of NV emissions and can be processed by using mature nanofabrication technology.42 Simulations were performed based on a two-dimensional finite-difference time-domain (FDTD) method (RSOFT, FullWAVE) to optimize the grating structure for the efficient collection of NV photons from the diamond substrate into an objective lens (numerical aperture of 0.13). In the simulations, an NV center was positioned near the diamond surface and was assumed to be a point dipole source oscillating along the y-direction at 700 nm, where the emission intensity of NV centers is maximal. The thickness and width of the SiN grating were set to be 200 nm and A/2 (A: grating pitch), respectively. Figure 2(b) summarizes the simulated results of the output intensity as a function of A. The intensity was normalized to that obtained without the grating structure. FDTD calculations revealed that the output intensity was the highest for A = 700 and 2100 nm. In this study, we employed A = 2100 nm, thereby enhancing the output intensity by a factor of 2.1. In case if the measurement noise is limited by shot noise of photons emitted by NV centers, the magnetic sensitivity of the NV center is inversely proportional to η (η: the enhancement rate of the photon extraction efficiency from NV centers). The right axis in Fig. 2(b) shows the enhancement factor of the magnetic sensitivity defined by η as a function of the grating pitch. The magnetic field sensitivity can be enhanced approximately 1.5 times by integrating the designed grating structure on diamond in our proposed method. Since the NV centers in electron-irradiated diamond are randomly distributed, we further confirmed the validity of the grating structure by comparing the simulated output intensity with and without the grating as a function of the NV z-position. We defined the enhancement factor as the ratio of the output intensity with and without the grating for a fixed depth. Figure 2(c) shows the enhancement factor as a function of the dipole depth. Even when the NV center was positioned at z = 6 μm, we obtained an enhancement factor of 1.5. It is also confirmed that enhancement factors greater than 1.1 are sustained for z > 6 μm. These results suggest that the proposed grating structure allows for the efficient collection of photons emitted from the ensemble NV centers in diamond. Notably, transfer printing allows hybrid integration with precision <50 nm;36 the grating structure with the proposed approach is also applicable for efficient extraction of single photons from a single NV center. Moreover, further improvement of transfer printing technology [e.g., reducing total contact area with the grating by using polydimethylsiloxane (PDMS) films with nanostructure] will allow the use of the grating with A = 700 nm and narrower pitches.

FIG. 2.

(a) Upper panel: schematics of total internal reflection inside diamond. Lower panel: schematic cross section of the investigated grating structure on diamond. (b) Simulated output intensity and enhancement factor of magnetic sensitivity as a function of A. (c) Simulated results of the enhanced factor as a function of the dipole depth.

FIG. 2.

(a) Upper panel: schematics of total internal reflection inside diamond. Lower panel: schematic cross section of the investigated grating structure on diamond. (b) Simulated output intensity and enhancement factor of magnetic sensitivity as a function of A. (c) Simulated results of the enhanced factor as a function of the dipole depth.

Close modal

To fabricate the designed structure, we prepared air-suspended gratings in a 200 nm-thick SiN layer deposited on a silicon substrate. The grating structure was patterned with polymer resist (ZEP520A, ZEON Corp.) as a mask via electron beam (EB) lithography and etched using dry etching processes. After removing the EB resist with oxygen plasma ashing, the structure was air-suspended through wet etching of tetramethylammonium hydroxide solutions with a critical point dryer. Figure 3(a) shows a scanning electron microscope (SEM) image of the fabricated SiN grating structure. The grating was suspended by two tethers without any distortion. In parallel, a (100) single-crystal diamond (5 × 5 × 0.3 cm3) was irradiated with 2-MeV electrons with a total fluence of 1.0 × 1018 cm2, followed by annealing for 2 h at 1000 °C. Subsequently, we transfer-printed the fabricated grating structure onto a diamond NV substrate. A polydimethylsiloxane (PDMS) adhesive stamp (Gel-Film x4, Gelpak) was attached to the suspended SiN grating, which was then picked up with a quick retraction, as shown in Fig. 1. We then printed the lifted grating structure on the diamond substrate with a slow retraction.37Figure 3(b) presents an optical microscope image of the integrated SiN grating structure. The grating is bonded to the diamond surface through van der Waals force. The SiN grating structure can also be formed by directly depositing a SiN layer and subsequently etching it on top of diamond. However, owing to its low electrical conductivity, it is difficult to perform precise patterning on diamond using electron beam lithography due to a charge-up phenomenon. In addition, diamond chips are typically a few millimeters in size, which makes it difficult to prepare a uniform resist film on diamond by spin coating. We emphasize that the simple pick-and-place operation of transfer printing largely facilitates the fusion of the sophisticated micro/nanostructures and materials whose fabrication technology is undeveloped. We believe that combining parallel integration39 and robotic automation43 with transfer printing would further facilitate device fabrication.

FIG. 3.

(a) SEM image of an air-suspended SiN grating. (b) Optical microscope image of an integrated SiN grating on diamond.

FIG. 3.

(a) SEM image of an air-suspended SiN grating. (b) Optical microscope image of an integrated SiN grating on diamond.

Close modal

The fabricated device was characterized using micro-photoluminescence (μ-PL) spectroscopy. The NV centers were excited with a 532 nm-CW laser (Gem 532, Laser Quantum) which was guided through a single-mode fiber. We used a ×5 objective lens to monitor the sample, irradiate the sample with excitation laser beam, and collect the PL signals. The collected PL was coupled to a multi-mode fiber and analyzed with a fiber optic spectrometer. The red curve in Fig. 4(a) shows a PL spectrum measured for the sample with the SiN grating at an input laser power of 12 mW. For comparison, we measured the PL spectrum for the bare region of the diamond NV substrate near the SiN grating [blue curve in Fig. 4(a)]. The collected photon counts are increased by a factor of up to 1.6 with the SiN grating structure, which is consistent with the simulation results. Figure 4(b) shows the dependence of the PL intensity on the excitation laser power normalized by the intensity of the bare region at 3 mW. We confirmed that the PL intensity measured with the grating is always higher than that measured for the bare diamond NV substrate, regardless of the laser power. The PL intensity of the bare region is the same as that measured for the other four bare regions around the grating. These results suggest that the fabricated grating structure enables the efficient extraction of NV photons.

FIG. 4.

(a) Measured PL spectra (red: grating, blue: bare region). (b) Dependence of the PL intensity on the excitation laser power.

FIG. 4.

(a) Measured PL spectra (red: grating, blue: bare region). (b) Dependence of the PL intensity on the excitation laser power.

Close modal

Finally, we evaluated the magnetic sensitivity of the fabricated device. Figure 5(a) illustrates the principle of lock-in detection employed to evaluate the magnetic sensitivity of the device. When applying an AC magnetic field B with the frequency of fac = 1.0 kHz under a fixed microwave frequency (fMW = 2.8724 MHz), the optically detected magnetic resonance (ODMR) dip fluctuates while following the oscillation of the magnetic fields. This fluctuation in the PL intensity signal was detected using a lock-in amplifier and oscilloscope.6 In this experiment, the PL signals from the NV center were detected using a fiber-connected photodiode with a notch filter (533 nm) and a long-pass filter (>600 nm). To obtain the ODMR spectrum, the NV center was microwave-excited by using a thin copper film located at the bottom of the diamond substrate. Figure 5(b) shows a typical ODMR spectrum for the investigated sample with no external magnetic field, where the dips of the four NV-axes are degenerated. Subsequently, the AC magnetic field was applied to the NV centers using a circular coil (10-mm radius and six turns) attached to the glass plate for the sample holder, as shown in the inset of Fig. 5(a). The sample was optically excited with a strong excitation power of 90 mW by switching from a single-mode to a multi-mode fiber to achieve a better signal-to-noise ratio. We note that after switching the optical fibers, the collected photon counts with the grating increased by 1.3 times when compared to those without grating.

FIG. 5.

(a) Principle of the lock-in detection employed for the evaluation of the magnetic sensitivity of the device. (b) Typical ODMR spectrum for the investigated sample with no external magnetic field. (c) Lock-in voltage detected by the lock-in detection system as a function of B. Error bar represents a standard deviation of the obtained data. (d) Magnified plots inside the dotted green circle of (c).

FIG. 5.

(a) Principle of the lock-in detection employed for the evaluation of the magnetic sensitivity of the device. (b) Typical ODMR spectrum for the investigated sample with no external magnetic field. (c) Lock-in voltage detected by the lock-in detection system as a function of B. Error bar represents a standard deviation of the obtained data. (d) Magnified plots inside the dotted green circle of (c).

Close modal

Figure 5(c) shows the lock-in voltage detected by the lock-in detection system as a function of the magnetic field strength B. Magnetic fields at the investigated NV centers produced by the coil were calculated based on the Biot–Savart law by considering the distance of 1.7 mm from the top surface of the diamond to the coil. In Fig. 5(c), the detected voltages with the grating are higher than those in the bare region, which is consistent with the results shown in Fig. 4(b). To evaluate the minimum detectable magnetic field, we applied a linear fit to the obtained data, as shown by the solid red and blue curves in Fig. 5(c). Figure 5(d) shows the magnified plots of the data inside the dotted green circle in Fig. 5(c). The fittings are not consistent for B < 0.2 μT, and thus, we assumed that the noise floor in this study was approximately 0.2 μV [dotted gray line in Fig. 5(d)]. Using the fitted results and noise floor level, the minimum detectable magnetic field with (without) the grating structure was estimated to be 130 (150) nT, thereby demonstrating the amplification of the magnetic field sensitivity by the integrated grating structure. We consider that the experimental sensitivity in our system may have been limited by the electric noise of several hundred nanovolts. Further improvement of the magnetic sensitivity is possible by extracting residual photons inside the diamond by coating reflective films on the bottom of diamond.19 

In summary, we demonstrated the hybrid integration of a SiN grating structure on a single-crystal diamond NV substrate. FDTD simulations revealed that the proposed structure enhances the output efficiency by a factor of up to 2.1. We observed that the intensity of the NV emission was increased 1.6 times for the fabricated device. Furthermore, the magnetic sensitivity was amplified owing to the increased photon intensity. The proposed hybrid integration approach can also be applied to a single NV center for efficient single-photon extraction.22,44–46 The proposed transfer-printing-based approach potentially allows for the realization of sensitive and compact diamond magnetometers.

This work was supported by the MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) (Grant No. JPMXS0118067395), KAKENHI (Nos. 20H02197, 20H05091, 20K21118, 21K20428, 22H01525, and 22K14289), and a research grant (Basic Research) from The TEPCO Memorial Foundation.

The authors have no conflicts to disclose.

Ryota Katsumi: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Takeshi Ohshima: Resources (equal); Writing – review & editing (equal). Masaki Sekino: Funding acquisition (equal); Resources (equal); Supervision (supporting); Writing – original draft (supporting); Writing – review & editing (equal). Takashi Yatsui: Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Resources (equal); Supervision (lead); Writing – original draft (supporting); Writing – review & editing (equal). Takeshi Hizawa: Methodology (equal); Resources (equal); Writing – review & editing (equal). Akihiro Kuwahata: Data curation (equal); Writing – review & editing (equal). Shun Naruse: Data curation (equal); Writing – review & editing (equal). Yuji Hatano: Resources (equal); Writing – review & editing (equal). Takayuki Iwasaki: Resources (equal); Writing – review & editing (equal). Mutsuko Hatano: Funding acquisition (equal); Resources (equal); Writing – review & editing (equal). Fedor Jelezko: Supervision (supporting); Writing – review & editing (equal). Shinobu Onoda: Resources (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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