Color centers in diamond, among them the negatively charged germanium vacancy (GeV), are promising candidates for many applications of quantum optics, such as a quantum network. For efficient implementation, the optical transitions need to be coupled to a single optical mode. Here, we demonstrate the transfer of a nanodiamond containing a single ingrown GeV center with excellent optical properties to an open Fabry–Pérot microcavity by nanomanipulation utilizing an atomic force microscope. Coupling of the GeV defect to the cavity mode is achieved, while the optical resonator maintains a high finesse of F = 7700, and a 48-fold spectral density enhancement is observed. This article demonstrates the integration of a GeV defect with a Fabry–Pérot microcavity under ambient conditions with the potential to extend the experiments to cryogenic temperatures toward an efficient spin-photon platform.

With increasing advances in quantum technologies and the importance of secure communication, there is great interest in developing a quantum network.1 In such a network, the communication is based on quantum entanglement, and by using quantum key distribution, it provides guaranteed security.2,3 However, to increase the range of the network on a global scale,4 nodes are required due to lossy photon channels that limit the range to about 100 km.5 

Essential for the realization of such nodes in a scalable quantum network are a quantum memory capable of storing and processing quantum information,6 and high rates of coherent photons to establish entanglement in communication schemes. For the interaction between photons and the quantum memory, an efficient interface is necessary. Consequent challenges of overcoming photon losses, maintaining coherence, attaining robustness, and enabling scalability can be tackled by using a quantum repeater as a node in the quantum network.7 One approach to realizing such a repeater is to address a single emitter to gain access to a quantum memory via its spin. By placing the emitter inside a high quality optical resonator,8,9 quantum state writing and reading can be performed efficiently at high rates due to an enhanced spontaneous emission rate.10 

Promising single emitter candidates for realizing a quantum network are color centers, optically active defect centers in diamond.11,12 In order to couple them to the mode of a microcavity, diamond membranes (DMs)13,14 and nanodiamonds (NDs)15 are mainly used. Currently, negatively charged nitrogen vacancies (NV)16 and silicon vacancies (SiV)17 are the most studied color centers. While NV centers show good spin properties and great strides have been made to entangle them,18 they show poor optical access, e.g., a low emission in the zero phonon line (ZPL).19 In contrast, group IV defects, with their interstitial doping atom in the diamond lattice, are promising alternative emitters for this concept.20 Among them is the SiV, which compared to NV, has superior optical properties with a high Debye–Waller (DW) factor of 0.7.20 A drawback is the technical overhead of milliKelvin temperatures, which is required for receiving good spin properties.21 

The negatively charged germanium vacancy (GeV)22,23 is another group IV defect with a heavier impurity atom than the SiV center. The GeV center exhibits a longer excited state lifetime and a comparable DW factor of 0.3–0.6.20,23–25 At 7 K, the phonon-induced spin dephasing is exponentially suppressed resulting in good spin properties already below 1 K, higher than for the SiV center.9,26 In addition to their implementation in diamond waveguides,26,27 the GeV defect was coupled to photonic resonators28,29 and to one tunable Fabry–Pérot (FP) cavity.30 With a cavity formed by the end-facet of an optical fiber and a flat mirror with a bonded DM, a spectral density enhancement (SDE) of 31 was reported.30 

In this work, we demonstrate the transfer of a 200 nm sized ND containing a single GeV defect with non-blinking and narrow emission to an optical open FP microcavity with a high quality factor by utilizing the pick and place technique with an atomic force microscope (AFM). We realized coupling of the strong ZPL of the GeV center to the cavity mode, while observing a 48-fold SDE and maintaining a high finesse. In our approach, the ND with the GeV center is intrinsically aligned with the cavity field, making lateral scanning of the cavity mirror redundant. Therefore, the system is passively very stable with reduced technical overhead.

First, NDs with single GeV defect centers were grown by high pressure high temperature (HPHT)31 synthesis under 1450 °C and 8 GPa. In this process, detonation NDs, fluoroadamantane (C10H15F), and hepta-fluoronaphtalene (C10F8) were used as carbon sources. Germanium triphenyl-chloride (GeC18H15Cl) was added as doping component for germanium. Figure 1(a) shows a scanning electron microscope (SEM) image of the NDs of varying sizes after the synthesis. Subsequently, the NDs were cleaned by acid boiling and diluted in ultrapure water. After an ultrasonic bath for deagglomeration, the ND solution was drop-cast on a sapphire bulk substrate.

FIG. 1.

Characterizing GeV center in NDs. (a) SEM image of the synthesized NDs. (b) Distribution of the ZPL and FWHM of the acquired GeV defects. (c) PL spectrum with a coarse grating of 150 g/mm revealing the ZPL and PSB of the selected ND GeV-I with inserted atomic structure of a GeV. (d) PL with a fine grating of 1200 g/mm showing the ZPL of ND GeV-I at (599.11 ± 0.03) nm and a FWHM of (1.24 ± 0.03) nm under green excitation with (930 ± 20) μW. (e) Saturation curve of ND GeV-I with the saturation intensity I sat=(0.15 ± 0.07) Mc s and saturation power P sat=(3.0 ± 2.2) mW. (f) Off-resonant autocorrelation measurement without background correction of ND GeV-I showing strong anti-bunching of g ( 2 ) ( 0 ) = 0.11 ± 0.04 and an excited state lifetime of τ LT = (2.53 ± 0.20) ns at (60 ± 2) μW.

FIG. 1.

Characterizing GeV center in NDs. (a) SEM image of the synthesized NDs. (b) Distribution of the ZPL and FWHM of the acquired GeV defects. (c) PL spectrum with a coarse grating of 150 g/mm revealing the ZPL and PSB of the selected ND GeV-I with inserted atomic structure of a GeV. (d) PL with a fine grating of 1200 g/mm showing the ZPL of ND GeV-I at (599.11 ± 0.03) nm and a FWHM of (1.24 ± 0.03) nm under green excitation with (930 ± 20) μW. (e) Saturation curve of ND GeV-I with the saturation intensity I sat=(0.15 ± 0.07) Mc s and saturation power P sat=(3.0 ± 2.2) mW. (f) Off-resonant autocorrelation measurement without background correction of ND GeV-I showing strong anti-bunching of g ( 2 ) ( 0 ) = 0.11 ± 0.04 and an excited state lifetime of τ LT = (2.53 ± 0.20) ns at (60 ± 2) μW.

Close modal
The sample was characterized in a confocal microscope (NA = 0.9) at room temperature with an excitation wavelength of 532 nm. NDs with emission from GeV centers showed a strong ZPL at a median wavelength of (602 ± 2) nm and are presented in Fig. 1(b). The observed spectral distribution of the ZPL suggests strain in the NDs.32 With a mean FWHM of (1.3 ± 0.3) nm and a distribution between 0.8 and 2.2 nm, the defects showed narrow linewidths compared to the reported values in Ref. 23. The ND with the identifier GeV-I had good optical properties and was selected for further investigation. Its photoluminescence (PL) is displayed in Figs. 1(c) and 1(d). This color center demonstrated a high and stable emission rate in the ZPL with a high quality factor of Q GeV = λ / δ λ = 483 ± 12. The spectrum shows a small phonon sideband (PSB) with a DW factor above 0.6, which results from different converging evaluations (see the supplementary material). At 658 and 693 nm, emission originated from an unrelated laser and the sapphire bulk, respectively, which was not taken into account for the DW factor. In addition, its saturation curve is illustrated in Fig. 1(e) and was fitted with the function as follows:
I ( P ) = I sat × P P + P sat .
(1)
The determined values for saturation intensity and saturation power subside the presented data in Ref. 27. To evaluate the properties of the emission signal from the emitter, we performed an autocorrelation measurement under off-resonant excitation. The time resolved correlation without any background correction and with the fitting model,
g ( 2 ) ( τ ) = 1 A × exp ( | τ τ LT | ) ,
(2)
can be seen in Fig. 1(f), (see the supplementary material for other emitters). A strong anti-bunching of g ( 2 ) ( 0 ) = 0.11 ± 0.04 was observed. This shows a high rate of emission arising from only a single GeV center within the ND under green excitation, making this a good candidate for quantum optics experiments and the use as a single photon source. An excited state lifetime of τ LT = ( 2.53 ± 0.20 ) ns at an excitation power of 60 μW was determined, in agreement with Ref. 22.

Next, the GeV center containing ND was integrated in the field of the hemispherical FP cavity by placing it in the center of the curved mirror. The concave mirror was fabricated by a CO2 laser ablation process of a SiO2 substrate. By creating multiple mirror structures on one substrate, a versatile mirror array could be fabricated. Varying the pulse lengths results in different structure sizes, as shown in Fig. 2(a). Interferometric analysis of the structure intended for the ND revealed a depth of 2.7 μm and a diameter of 26.4 μm. By coating the structured substrate a distributed Bragg reflector (DBR) was formed, yielding a low transmission between 595 and 690 nm with a minimum of T < 310 ppm at 601 nm. At this wavelength, the mirror coating forms a field antinode around 150 nm in air and 62 nm in diamond away from the mirror surface.

FIG. 2.

Assembly of an FP-resonator with a GeV center containing ND. (a) Microscopic image of the mirror containing the curved structure (circled white) intended for the ND. (b) Confocal scan and AFM images of NDs showing GeV-I. (c) Schematic illustration of the pick and place technique for placing NDs inside a mirror structure using the AFM cantilever. (d) Tilt subtracted AFM image of the spherical mirror containing the transferred ND GeV-I in its center with respective cross sections in x- and y-directions. (e) Confocal scan image of the spherical mirror containing the same ND in its center after transfer. (f) PL of ND GeV-I in the mirror structure with a ZPL at (599.25 ± 0.03) nm and a FWHM of (1.10 ± 0.04) nm using an excitation power of (20 ± 2) μW.

FIG. 2.

Assembly of an FP-resonator with a GeV center containing ND. (a) Microscopic image of the mirror containing the curved structure (circled white) intended for the ND. (b) Confocal scan and AFM images of NDs showing GeV-I. (c) Schematic illustration of the pick and place technique for placing NDs inside a mirror structure using the AFM cantilever. (d) Tilt subtracted AFM image of the spherical mirror containing the transferred ND GeV-I in its center with respective cross sections in x- and y-directions. (e) Confocal scan image of the spherical mirror containing the same ND in its center after transfer. (f) PL of ND GeV-I in the mirror structure with a ZPL at (599.25 ± 0.03) nm and a FWHM of (1.10 ± 0.04) nm using an excitation power of (20 ± 2) μW.

Close modal

Before the mirror integration, the emitter could be identified by comparing the confocal scan with the AFM image in Fig. 2(b). Using the AFM based pick and place technique,33,34 nanomanipulation of ND GeV-I was achieved by transferring the emitter to the center of the desired structure [see Fig. 2(c)].

The mirror containing the ND was scanned with the AFM in Fig. 2(d), whereby the cross sections reveal the size of the ND of (190 × 180 × 130) nm3. It was also determined that the structure used here is elliptical, with radii of curvature RoC X = ( 21.48 ± 0.01 ) and RoC Y = ( 28.94 ± 0.01 ) μm. With the help of a confocal microscope, it could be confirmed that the optical properties were preserved. Figure 2(e) shows the spherical shape of the mirror due to minor fluorescence of the coating. Pollution and other impurities are visible at the edge of the structure, outside the relevant cavity mode. In the center, a peak of fluorescence originating from ND GeV-I is observable, verified by the PL spectrum in Fig. 2(f). The surrounding area of the ND shows a bleached background fluorescence due to previous laser scans with 532 nm.

The spectral density (SD) of the spectrum
SD = Photoncounts FWHM × P
(3)
could be extracted by using the integrated photon counts of the Lorentzian fit and the excitation power that lay within the linear range of the saturation curve. The spectrum in Fig. 2(f) yields a SD of ( 12.7 ± 1.0 ) c/(s GHz mW), which could be compared to the free space emitter that demonstrates a SD of ( 2.03 ± 0.08 ) c/(s GHz mW) in Fig. 1(d). The measurements were performed on different confocal microscopes, so differences in the setup efficiencies had to be taken into account. With the reflected laser signal from the curved mirror and a different excitation beam width at the emitter, a relative excitation efficiency of η ̃ Con exc = η Con 2 exc / η Con 1 exc = 2.3 ± 0.2 followed. Different optical components led to a relative detection efficiency of η ̃ Con det = 1.3 ± 0.2. Considering these efficiencies and comparing the resulting SD from free space emission, to the ND placed in the curved mirror suggested a spectral density enhancement of SDE = SD Con 2 / ( SD Con 1 × η ̃ Con det × η ̃ Con exc ) = 2.1 ± 0.4 (see the supplementary material for complete calculations). This enhancement is attributed to the reflected fluorescence signal of the curved mirror, indicating a beneficial position of the emitter. The twofold SDE suggesting efficient emitter coupling indicates that the emitter is located near the electric field maximum, which is estimated to be at a distance of 62 nm from the mirror surface.

For characterizing the GeV center in the cavity, the curved mirror containing the transferred ND and a second plane mirror with the same substrate and coating were assembled and formed the resonator, as illustrated in Fig. 3(a). After passing a laser clean up filter (600 SP) and the reflection of a dichroic mirror, the excitation laser light (Sirah Matisse 2 DS Dye laser: linewidth below 100 kHz at 600 nm) of a single mode fiber (IVG Fiber: SM CU600PSC) was coupled into one of the cavity structures with the help of a lens (Asphericon: AHL10–08-P-U-780, f = 8 mm, NA = 0.55) mounted to nanopositioners in x, y, and z (Attocube ANP series). An additional nanopositioner with z piezo enabled tunability of the cavity length by scanning the plane mirror. The cavity signal and the reflected light passed the lens, the dichroic mirror, and a filter (605/15 BP) and were collected by a multimode fiber (Thorlabs: MM FG050LGA) in order to guide it to an avalanche photodiode or spectrometer.

FIG. 3.

Characterizing the assembled cavity with a GeV ND. (a) Schematic illustration of the operated optical microcavity setup. (b) Exemplary finesse measurement showing the reflected laser signal over the z piezo voltage of the flat mirror corresponding to the cavity length. (c) Possible finesse values of the microcavity determined by the transmission of the coating compared to the measurement with the ND inside the cavity. (d) PL of resonance n = 51 of the optical microcavity with an excitation wavelength of 587.78 nm and excitation power of (80 ± 12) μW. The spectrum (spectrometer resolution of 0.03 nm) shows a cavity modulated signal of the GeV with a peak of emission at (599.20 ± 0.03) nm.

FIG. 3.

Characterizing the assembled cavity with a GeV ND. (a) Schematic illustration of the operated optical microcavity setup. (b) Exemplary finesse measurement showing the reflected laser signal over the z piezo voltage of the flat mirror corresponding to the cavity length. (c) Possible finesse values of the microcavity determined by the transmission of the coating compared to the measurement with the ND inside the cavity. (d) PL of resonance n = 51 of the optical microcavity with an excitation wavelength of 587.78 nm and excitation power of (80 ± 12) μW. The spectrum (spectrometer resolution of 0.03 nm) shows a cavity modulated signal of the GeV with a peak of emission at (599.20 ± 0.03) nm.

Close modal

The cavity finesse was extracted from the reflection signal by scanning the piezo of the plane mirror, as seen in Fig. 3(b). The obtained values were compared to the theoretical possible finesse given by the coating limit in Fig. 3(c). The cavity maintained a high finesse close to the coating limit. One could receive a maximum finesse of F > 10 300 at 601 nm, while in the measurements a value of F = 7700 ± 1800 was achieved. Comparing the finesse at 648.6 nm before F = 4100 ± 600 and after F = 4000 ± 800 placing the ND in the cavity showed no significant losses due to reduced scattering of the small sized nanoparticle.

With an excitation wavelength of 587.8 nm the cavity modulated ZPL signal of the single GeV center was detected, as shown in Fig. 3(d). With the signal of the emitter at 599.2 nm and the excitation wavelength at neighboring resonances of the resonator, a cavity length of L = ( 15.72 ± 0.06 ) μm was determined. Considering the RoC of ( 25 ± 5 ) μm, this resulted in a beam waist on the flat side of ω 0 = ( 1.5 ± 0.2 ) μm and on the curved side of ω ( L ) = ( 2.5 ± 0.6 ) μm. From this, a mode volume of V = π 4 L ω 0 2 = ( 140 ± 40 ) λ Las 3 followed and with the finesse of F = 5000 ± 1200 at the emitter wavelength the quality factor of the cavity was evaluated to Q Cav = 260 000 ± 60 000. Finally, these parameters of the cavity enabled calculating the Purcell factor with
P = 3 4 π 2 ( λ n ) 3 Q Cav V ,
(4)
and the refraction index n air = 1, giving the values P flat = 152 ± 56 on the flat side and P curv = 57 ± 29 on the curved side.
However, this enhancement is only achieved for Q Cav < Q GeV, which was not the case for our room temperature experiments due to thermal broadening of the emitter.35 Therefore, the corrected formula for the Purcell effect is given by
P * = P × Q GeV Q Cav ,
(5)
yielding the corrected values P flat * = 0.28  ± 0.03 on the flat side and P curv * = 0.12  ± 0.02 on the curved side.

In order to determine the SDE experimentally, we extracted the SD ( 470 ± 120 ) c/(s GHz mW) of the emitter in the microcavity from Fig. 3(d) and compared it to the free space emitter from Fig. 1(d). We evaluated the altered excitation efficiency η ̃ Cav exc = 11 ± 4 in relation to the confocal microscope setup, which was caused by the finesse of the cavity, a more efficient excitation wavelength25 and a wider excitation beam at the emitter. The detection efficiency was corrected by a factor of η ̃ Cav det = 0.45 ± 0.07 due to different optical components. This resulted in a (48 ± 20)-fold SDE (see the supplementary material for complete calculations). Considering that the Purcell effect is limited at room temperature, the enhancement can be attributed to cavity funneling.

In conclusion, we demonstrated the assembly of a tunable and versatile cavity system containing a single GeV inside one ND. The ingrown GeV defect center showed strong, stable, and narrow emission in the ZPL with a small PSB yielding a DW of above 0.6. With a g ( 2 ) ( 0 ) value of 0.11 ± 0.04, it proved to be a high purity single photon source. The small size of the ND enabled integration into the optical resonator mode without significant scattering losses, maintaining a high quality factor Q Cav above 250 000. The nanomanipulation capabilities allowed highly accurate alignment of the pre-characterized ND with GeV center to the cavity field. With this integration into the open, fully tunable FP microcavity photoluminescence measurements could be performed under off-resonant excitation at room temperature, taking advantage of the single emitter emission. We calculated a 48-fold SDE of the emitter and retained a high finesse of F = 7700, including the nanoparticle.

The passively stable platform, with a mode volume V of ( 140 ± 40 ) λ Las 3, does not require additional lateral scanning of the cavity mirror, profits from a low thermal expansion due to the main setup material choice of titanium, and allows for remote realignment with the nanopositioners. Altogether this offers robustness even at cryogenic temperatures.15 Cooling this platform to low temperatures, as shown in earlier work with SiV-centers,15 offers a decreased thermal broadening of the emitter toward the Fourier-transform limit, which suggests an increased Purcell effect of P > 50. In combination with improved coherence of the emission into the ZPL, this results in a higher spectral enhancement of the signal out of the cavity. In order to receive access to a quantum memory, spin access would be a desirable next goal to establish an efficient spin photon interface. With those advances, this hybrid microcavity system is a robust and promising approach for many quantum optic applications, such as a quantum repeater node.

See the supplementary material for additional information on the DW factor, additional autocorrelation measurements for other NDs, nanomanipulation of the ND, and comprehensive calculations of the spectral density enhancement.

The authors thank V. A. Davydov for synthesis and processing of the ND material. The authors thank Jan Schimmel for the fabrication of the CO2 structures and Lukas Antoniuk for experimental support. Funding by the German Federal Ministry of Education and Research (BMBF) within the project QR.X (16KISQ006) is gratefully acknowledged. S.S. acknowledges support of the Marie Curie ITN project LasIonDef (GA n.956387). N.L. acknowledges support of IQst. Most measurements were performed on the basis of the Qudi software suite.36 

The authors have no conflicts to disclose.

Florian Feuchtmayr and Robert Berghaus contributed equally to this work.

Florian Feuchtmayr: Data curation (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Robert Berghaus: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Selene Sachero: Investigation (equal); Methodology (equal). Gregor Bayer: Conceptualization (supporting); Methodology (equal). Niklas Lettner: Investigation (equal). Richard Waltrich: Methodology (supporting). Patrick Maier: Methodology (supporting). Viatcheslav Agafonov: Methodology (equal). Alexander Kubanek: Conceptualization (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal).

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

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Supplementary Material