High-Q cavity interface for color centers in thin film diamond.pdf
Article https://doi.org/10.1038/s41467-024-50667-5 High-Q cavity interface for color centers in thin ﬁlm diamond Sophie W. Ding 1 , Michael Haas1, Xinghan Guo 2, Kazuhiro Kuruma1,3, Chang Jin 1, Zixi Li 2, David D. Awschalom 2,4, Nazar Delegan 2,4, F. Joseph Heremans 2,4, Alexander A. High 2,4 & Marko Loncar 1 Quantum information technology offers the potential to realize unprece- dented computational resources via secure channels distributing entangle- ment between quantum computers. Diamond, as a host to optically-accessible spin qubits, is a leading platform to realize quantum memory nodes needed to extend such quantum links. Photonic crystal (PhC) cavities enhance light- matter interaction and are essential for an efﬁcient interface between spins and photons that are used to store and communicate quantum information respectively. Here, we demonstrate one- and two-dimensional PhC cavities fabricated in thin-ﬁlm diamonds, featuring quality factors (Q) of 1.8 × 105 and 1.6 × 105, respectively, the highest Qs for visible PhC cavities realized in any material. Importantly, our fabrication process is simple and high-yield, based on conventional planar fabrication techniques, in contrast to the previous with complex undercut processes. We also demonstrate ﬁber-coupled 1D PhC cavities with high photon extraction efﬁciency, and optical coupling between a single SiV center and such a cavity at 4 K achieving a Purcell factor of 18. The demonstrated photonic platform may fundamentally improve the perfor- mance and scalability of quantum nodes and expedite the development of related technologies. Diamond, as a host to atom-like defects with optically accessible long-lived spin qubits, has emerged as a compelling platform for applications in quantum sensing1–4 and communication5–8. Among diamond color centers, nitrogen vacancies (NV)1–5, silicon vacancies (SiV)6,7,9–11, and tin vacancies (SnV)8,12–14 are particularly promising and have enabled many state-of-the-art demonstrations of quantum communication. Fundamentally, practical quantum com- munication requires fast and low-loss transfer of quantum informa- tion between spins (stationary qubits/quantum memory) and photons (ﬂying qubits), which translates to ﬁgures of merit like bandwidth and ﬁdelity. Therefore, achieving efﬁcient spin-photon interfaces is crucial for a wide range of applications. In diamonds, these interfaces have been realized by embedding the emitters within optical structures, including microcavities15, micro rings/ microdisks16, waveguides8,11,17, and nanophotonic cavities6,9,12,13,18,19, which have been enabled by steady progress in diamond fabrication techniques. Among all structures, photonic crystal (PhC) cavities are one of the most efﬁcient spin-photon interfaces because they sup- port optical modes with high-quality factors (Q) and small mode volumes (V), which greatly enhances light-matter interactions and allow for efﬁcient control and readout of the emitter spin state. As a result, PhC cavities have been utilized in a diverse range of qubit platforms, including quantum dots20, defects in Si or SiC21–23, and rare-earth ions in host materials24–26. Received: 6 February 2024 Accepted: 17 July 2024 Check for updates 1John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA. 2Pritzker School of Molecular Engi- neering, University of Chicago, Chicago, IL, USA. 3Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo, Japan. 4Center for Molecular Engineering and Materials Science Division, Argonne National Laboratory, Lemont, IL, USA. e-mail: wding@g.harvard.edu; ahigh@uchicago.edu; loncar@g.harvard.edu Nature Communications|        (2024) 15:6358  1 1234567890():,; 1234567890():,; Several methods have been developed to fabricate diamond nanophotonic structures from bulk diamond substrates, including the focused-ion-beam (FIB) milling19, Faraday-cage angled etching27, and approaches based on reactive-ion-beam (RIE) angled etching6,9 (Fig. 1a) or quasi-isotropic etching12,13,18 (Fig. 1b). Both angled and quasi-isotropic etching are the state-of-the-art methods and most popular as a result. However, despite great efforts, these methods typically result in visible PhC cavities with Q factors up to the low ~ 104 range, much lower than the simulated values > 106, likely limited by fabrication imperfections such as roughness of etched surfaces induced by complex under- cutting processes28. In parallel, diamond thin ﬁlms generated by sub- strate thinning and ion-slicing followed by regrowth are explored as the tried-and-true method, due to their potential to simplify and improve fabrication by eliminating undercutting. However, the PhC cavity Qs fabricated using these approaches have been limited to ~103, due to thickness variation and imperfect diamond crystal quality29–31. They are usually incompatible with quantum networking demonstrations for practical reasons, so the bulk machining methods are still the work- horse in the quantum diamond photonics community today. There- fore, a new thin-ﬁlm platform and fabrication pathway that improves fabrication capability and maintains high diamond quality are needed to enhance the performance of photonic components in diamond. In this paper, we demonstrate a thin ﬁlm diamond photonic platform and realize optical cavities in the visible wavelength range featuring record-high Q factors coupled to stable SiV centers. In our approach, we use a high-quality and homogenous thin ﬁlm diamond bonded to a silicon oxide/silicon handle wafer. The ﬁlm is created through ion implantation in bulk diamond, regrowth, electro- chemical etching, and transfer printing32. We design and fabricate 1D and 2D PhC diamond cavities operating at 737 nm wavelength range and measure a Q-factor up to 1.8 × 105, a record for visible PhC cav- ities fabricated in any material (Table 1). We also fabricate 1D PhCs critically coupled to a feeding waveguide and measure loaded Q of 8.4 × 104 (intrinsic Q ~ 1.8 × 105) and a waveguide-cavity coupling efﬁciency of ~65%. We show that this fabrication method exhibits high yield and uniformity: 93% (53 out of 57) of the cavities feature high-Q modes with resonances matched closely to the designed resonances. Finally, we demonstrate the coupling of implanted SiVs to fabricated diamond cavities and observe the three-fold reduction of their radiative lifetime, achieving a Purcell factor of 13. We expect the exceptional cavity performance, high-yield fabrication process, and excellent SiV properties of this new platform to further advance the ﬁeld of quantum photonics as efﬁcient spin-photon interfaces for color centers in diamond and beyond. Fig. 1 | State-of-art high-Q suspended PhC cavities fabrication methods. a Angled etch of a 1D PhC cavity. The structure has a characteristic triangular cross-section due to the nature of the fabrication process. Due to the shallow angle of the ion beam, which would graze the top edge of nearby structures, dense patterns are prohibited. b Quasi-isotropic etch of a 1D PhC cavity. The suspension is achieved by an isotropic O2 reactive-ion etch process. The disadvantage of this method is the long undercutting time that scales with beam width. In both (a) and (b), the structure is carved out of a bulk diamond and usually has visible artifacts/ roughness on the bottom of the nanobeam due to the undercutting. c This work: thin ﬁlm fabrication of 1D and 2D PhC cavities. This approach relies on top-down etching and undercutting of the handle substrate, so it simpliﬁes the fabrication and avoids the roughness of the bottom surface. As a result, this approach leads to exceedingly higher Qs and also allows for more versatile photonic circuits to be realized. Table 1 | Visible and telecom wavelength suspended diamond PhCs and visible wavelength suspended PhCs in other low-loss visible photonic materials Cavity type, material Wavelength (nm) Q V (λ/n)3 Method Reference 1D, diamond 737 8.3 × 104/1.8 × 105 0.5 Thin ﬁlm This work 1D, diamond 637 1.4 × 104 ~ 1 Quasi-isotropic etching Mouradian18 1D, diamond 737 2.0 × 104 0.5 Angle etching Bhaskar6 1D, diamond 660 2.4 × 104 0.5 Photoelectrochemical etching Lee31 1D, diamond 1529 (telecom) 1.8 × 105/2.7 × 105 0.57 Angle etching Burek42 1D, SiN 780 1.1 × 105 0.4 Thin ﬁlm Samutpraphoot45 1D, AlN 403 6.9 × 103 1.6 Thin ﬁlm Sergent46 1D, 4H-SiC 700 7 × 103 0.5 Photoelectrochemical etching Bracher47 1D, GaP 744 3.0 × 104 ~ 1 Monolithic Chakravarthi48 1D, InGaP 841 2.1 × 104 0.64 Monolithic Saber49 2D, diamond 746 1.6 × 105 2.18 Thin ﬁlm This work 2D, diamond 645 8 × 103 0.35 Fib Jung19 2D, diamond 1470 (telecom) 1.8 × 103 2.15 Thin ﬁlm Kuruma29 Article https://doi.org/10.1038/s41467-024-50667-5 Nature Communications|        (2024) 15:6358  2 phonon line (ZPL) FZPL is estimated to be 13 (see “Methods”). This value is much smaller than the theoretical Purcell factor (see “Methods”), likely due to the large displacement of SiV position with respect to the cavity ﬁeld maximum as a result of random bulk implantation. To further improve the Purcell factor, the SiVs can be implanted at the cavity region using the well-established masked implantation technique6,7. We estimate that for SiV mask-implanted at the cavity regions, the cooperativity C, an important ﬁgure-of-merit for evaluat- ing the emitter-cavity coupled system, could reach > 960 (under- coupled, Q ~ 1.8 × 105) or > 440 (critically coupled, Q ~ 8.4 × 104) with experimental values (see “Methods”), which are 9 or 4 times higher than the highest value reported in a previous work6. The realization of such a large C as a result of the high-Q cavity is highly advantageous for high ﬁdelity of control of the spin/nuclear states and networking applications. Discussion We have demonstrated high-Q 1D and 2D PhC cavities using the thin ﬁlm diamond approach. We achieved Q factors up to 1.8 × 105, which is a record high in visible-wavelength PhC cavities in any materials. We also fabricated high-Q 2D PhC cavities thatwere previously challenging using conventional diamond bulk machining approaches and also achieved record-high Q values up to 1.6 × 105. Finally, of interest for practical applications and demonstrations of efﬁcient spin-photon interface, we demonstrated waveguide-coupled 1D PhD cavities fea- turing intrinsic (loaded) Q up to 1.8 × 105 (8.3 × 104) and coupling efﬁ- ciency of 65%. The signiﬁcant improvement of the experimental Q factors in this work can be attributed to the use of a high-optical- quality thin ﬁlm diamond membrane with a smooth surface roughness < 0.3 nm and small thickness variation ~ 1 nm32. Still, Qs are one order of magnitude lower than theoretical predictions, indicating that there is room for improvement. The difference could be due to surface absorption38 and/or optical scatterings due to fabrication imperfec- tions such as lithography error of the air holes in position or radius, surface roughness, sidewall roughness/tilt of air holes39. Lastly, we demonstrated optical coupling between the realized high Q cavity and a single SiV, with a measured Purcell factor of 13, and therefore the immediate compatibility of this platform with color- center cavity QED. Using masked implantation, better overlap between SiV and optical mode can be achieved, resulting in cooperativities > 440 at ~ 100 mK (see “Method”). The lower loss and design ﬂexibility in this platform can fundamentally enhance color-center-based tech- nologies, by allowing for higher single photon rates, higher gate ﬁde- lities, more integrated functionalities, etc. In combination with the robust, high uniformity, and high-yield fabrication process, our thin- ﬁlm diamond platform will hopefully unlock new opportunities for color-center applications for quantum information. Our platform and fabrication approach based on a thin-ﬁlm dia- mond can be applied to a variety of other micro/nanostructures that are important in diamond integrated photonic applications, such as nonlinear photonics40,41 and diamond phononics42,43. In addition, the ﬂexibility of direct bonding makes this approach easily applicable to other substrates besides Si/SiO2 32, which enables the heterogeneous integration of the diamond platform containing color centers onto existing and emerging integrated photonic circuits for quantum net- works, including thin-ﬁlm lithium niobate11, aluminum nitride10, and CMOS-compatible platforms44. Methods Confocal photoluminescence measurement setup In Fig. 3, both spectra for 1D and 2D devices are measured using a commercial spectrometer system with free-space off-resonance Fig. 5 | SiV characterization and its optical coupling to the PhC cavity at 4 K. a An illustration of SiV placed close to the center of PhC cavity (SEM image). SiV is an interstitial defect consisting of Si atoms positioned between two sites with missing carbon atoms in a diamond lattice. b A simpliﬁed level diagram of SiV, featuring A ~ D, 4 optical transitions around 737 nm. c Second-order autocorrela- tion measurement of the C line under on-resonant excitation. The g2(0) < 0.5 con- ﬁrms the single-photon nature of the emitter. d Top panel: the stability of the C line of a SiV over two hours. The plot shows normalized intensity over time. The drift is much less than a linewidth; bottom panel: a scan of the C line at 120 min. e The photoluminescence (PL) spectra of the SiV as the cavity is tuned closer to the C and D lines. Each spectrum is normalized to the highest peak. The D line is getting much brighter as the cavity is tuned into resonance. f The measured lifetime of the SiV on and off-resonance (bulk) with the cavity, which shows a factor of 3 reductions. The two lifetimes correspond to the initial and ﬁnal PL spectra in (e) of the same colors. Article https://doi.org/10.1038/s41467-024-50667-5 Nature Communications|        (2024) 15:6358  6 excitation and collection. A green diode laser (523 nm) is used to scatter off of the cavities via an objective (x100), and the scatter light is sent through a 1800 gr/mm grating and collected by a Si CCD camera cooled to 4 K. The spectrometer used is SpectraPro HRS-750, the supercontinuum laser is SuperK EXTREME. Cross-polarized measurement setup In Fig. 4, the visible scanning laser is M Squared, which can stabilize in 710–790 nm, and the APD is SPCM-AGRH-14-FC from Perkin Elmer. For these measurements, the input polarizer is aligned to the cavity polarization (TE), the half-wave plate is set to 23.5° so that the laser polarization is 45° relative to the cavity polarization, and the output polarization is set to be perpendicular to the cavity polarization. The working principle behind the measurement is that the cavity acts as a polarization ﬁlter when the incident light is resonant with the cavity. As such, off-resonant light is fully blocked by the cross-polarization between P1 and P2, but the light resonant with the cavity has a non-zero polarization component in the direction of P2, resulting in a Lorentzian peak in reﬂection corre- sponding to the cavity resonance. Fiber-coupling measurement setup The waveguide-coupled 1D PhC cavities are measured via ﬁber- coupled reﬂection measurements (shown in Fig. 4e. These measure- ments are performed by inputting light from a tunable laser (M Squared) into the diamond waveguide via a tapered optical ﬁber (S630 HP)33 and measuring the reﬂected light from the optical cavity. Data availability The data supporting this study’s ﬁndings are available from the cor- responding author (S.W.D.) upon request. References 1. Miller, B. S. et al. 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The membrane bonding work is supported by NSF award AM-2240399 and made use of the Pritzker Nanofabrication Facility (Soft and Hybrid Nanotechnology Experimental Resource, NSF ECCS-2025633) and the Materials Research Science and Engineering Center (NSF DMR-2011854) at the University of Chicago. Diamond growth-related efforts were supported by the U.S. Department of Energy, Ofﬁce of Basic Energy Sciences, Materials Science and Engineering Division (N.D.) Author contributions S.W.D., K.K., and M.L. conceived the experiment; S.W.D. fabricated the devices; M.H., S.W.D., and C.J. performed cavity and SiV measurements; S.W.D and M.H. performed data analysis. X.G. and Z.L. prepared the thin- ﬁlm sample; K.K. performed the FDTD simulation; N.D. performed the diamond synthesis; S.W.D wrote the manuscript with input from all co- authors. M.L., A.H., D.D.A., and F.J.H. participated in and supervised the project. Competing interests The authors declare no competing interests. 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Title: High-Q cavity interface for color centers in thin film diamond
Authors: Sophie W. Ding, Michael Haas, Xinghan Guo, Kazuhiro Kuruma, Chang Jin, Zixi Li, David D. Awschalom, Nazar Delegan, F. Joseph Heremans, Alexander A. High, Marko Loncar ​
Publisher: Nature Communications
Date: 2024-07-28 00:00:00
Abstract: Quantum information technology offers the potential to realize unprecedented computational resources via secure channels distributing entanglement between quantum computers. ​ Diamond, as a host to optically-accessible spin qubits, is a leading platform to realize quantum memory nodes needed to extend such quantum links. ​ Photonic crystal (PhC) cavities enhance light-matter interaction and are essential for an efficient interface between spins and photons that are used to store and communicate quantum information respectively. ​ Here, we demonstrate one-and two-dimensional PhC cavities fabricated in thin-film diamonds, featuring quality factors (Q) of 1.8 × 10^5 and 1.6 × 10^5, respectively, the highest Qs for visible PhC cavities realized in any material. ​ Importantly, our fabrication process is simple and high-yield, based on conventional planar fabrication techniques, in contrast to the previous with complex undercut processes. ​ We also demonstrate fiber-coupled 1D PhC cavities with high photon extraction efficiency, and optical coupling between a single SiV center and such a cavity at 4 K achieving a Purcell factor of 18. ​ The demonstrated photonic platform may fundamentally improve the performance and scalability of quantum nodes and expedite the development of related technologies. ​  