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arXiv:2210.05514 · Physical Review Applied

Scanning Cavity Microscopy of a Single-Crystal Diamond Membrane

Jonathan Körber, Maximilian Pallmann, Julia Heupel, Rainer Stöhr, Evgenij Vasilenko, Thomas Hümmer, Larissa Kohler, Cyril Popov, David Hunger

Published Jun 20, 2023• 9 citations
arXivHTMLPDF

Abstract

Efficient optical interfacing of spin-bearing quantum emitters is a crucial ingredient for quantum networks. A promising route therefore is to incorporate host materials as minimally processed membranes into open-access microcavities: it enables significant emission enhancement and efficient photon collection, minimizes deteriorating influence on the quantum emitter, and allows for full spatial and spectral tunability. Here, we study the properties of a high-finesse fiber Fabry-P\'erot microcavity with integrated single-crystal diamond membranes by scanning cavity microscopy. We observe the spatially resolved effects of the diamond-air interface on the cavity-mode structure: a strong correlation of the cavity finesse and mode structure with the diamond thickness and surface topography, prevalent transverse-mode mixing under diamondlike conditions, and mode-character-dependent polarization-mode splitting. Our results reveal the influence of the diamond surface on the achievable Purcell enhancement, which helps to clarify the route towards optimized spin-photon interfaces.

DOI
10.1103/physrevapplied.19.064057

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Efficient optical interfacing of spin-bearing quantum emitters is a crucial ingredient for quantum networks. A promising route therefore is to incorporate host materials as minimally processed membranes into open-access microcavities: it enables significant emission enhancement and efficient photon collection, minimizes deteriorating influence on the quantum emitter, and allows for full spatial and spectral tunability. Here, we study the properties of a high-finesse fiber Fabry-P'erot microcavity with integrated single-crystal diamond membranes by scanning cavity microscopy. We observe the spatially resolved effects of the diamond-air interface on the cavity-mode structure: a strong correlation of the cavity finesse and mode structure with the diamond thickness and surface topography, prevalent transverse-mode mixing under diamondlike conditions, and mode-character-dependent polarization-mode splitting. Our results reveal the influence of the diamond surface on the achievable Purcell enhancement, which helps to clarify the route towards optimized spin-photon interfaces.
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Building a long distance quantum network is one of the big challenges in the field of quantum communication, which requires the development of a quantum repeater. A crucial component of this is an efficient, coherent spin photon interface, and coupling single NV centers to a microcavity is a promising approach therefor. In our experiment, we integrate a diamond membrane to an open access fiber-based Fabry-Perot microcavity to increase the weak ZPL emission of NV centers. Simulations predict the feasibility of a strong enhancement of the ZPL emission efficiency, reaching values of up to 80%. We present a spatially resolved characterization of a coupled cavity-membrane device and report on the current status of the experiment.
Defect centres in diamond are promising building blocks for quantum networks thanks to a long-lived spin state and bright spin-photon interface. However, their low fraction of emission into a desired optical mode limits the entangling success probability. The key to overcoming this is through Purcell enhancement of the emission. Open Fabry–Perot cavities with an embedded diamond membrane allow for such enhancement while retaining good emitter properties. To guide the focus for design improvements it is essential to understand the influence of different types of losses and geometry choices. In particular, in the design of these cavities a high Purcell factor has to be weighed against cavity stability and efficient outcoupling. To be able to make these trade-offs we develop analytic descriptions of such hybrid diamond-and-air cavities as an extension to previous numeric methods. The insights provided by this analysis yield an effective tool to find the optimal design parameters for a diamond-air cavity.
This thesis investigates the coupling of the fluorescence of nitrogen-vacancy (NV) centers in diamond to tunable optical microresonators at ambient conditions, in particular in the regime of Purcell enhancement. We use fiber-based, open-access Fabry-Pérot cavities optimized for high finesse and ultra-small mode volume. Different regimes of cavity enhancement are studied that are complementary to each other: A first experiment relies on a high-finesse cavity with dielectric mirrors. The scaling laws of Purcell enhancement are explicitly demonstrated by a large-range variation of both the cavity mode volume (V = 16 − 600 µm^3 ) and the quality factor (Q = 6 · 10^3 − 2 · 10^6). We detect an enhancement of the emission spectral density by up to a factor of 300. The full potential of this resonator can be exploited with emitters having a linewidth which is narrower than the resonance linewidth of the cavity. This concept holds promise for the implementation of wavelength-tunable, narrow-band single-photon sources as well as the generation of indistinguishable single-photons at ambient conditions. However, for broad-band emitters like the NV center at room temperature, the emission lifetime is not affected noticeably in this configuration. In order to directly observe lifetime changes and Purcell-enhanced single-photon emission, we manufacture fiber-based cavities with silver-coated mirrors having ultra-small mode volumes, as small as V = 1.0 λ^3 = 0.34 µm^3. We demonstrate cavity-enhanced fluorescence imaging, which allows to locate and analyze several single NV centers with one cavity. The Purcell effect is evidenced by an enhanced fluorescence collection of up to 1.6 · 10^6 photons per second from single-NV centers and a tunable variation of the emission lifetime corresponding to an effective Purcell factor of up to 2. We furthermore investigate a benefcial regime of optical confinement where the Fabry-Pérot cavity mode is combined with additional mode confinement by the diamond nanocrystal itself, enabling sub-λ^3 mode volumes. We perform simulations that predict effective Purcell factors of up to 11 for NV centers and of up to 63 for silicon-vacancy centers, revealing a great potential for bright single-photon sources and effcient spin readout at ambient conditions.
Silicon vacancy centers in 4H-silicon carbide (SiC) host a long-lived electronic spin with spin-resolved optical transitions. This makes them a great candidate for implementing a spin–photon interface, an important building block of quantum networks. However, the fraction of coherently scattered photons is intrinsically low, limiting the achievable communication rates. To address this problem, we integrate V2 centers within a SiC membrane into a fiber-based Fabry–Perot microcavity. We find that SiC is uniquely suited for this approach, offering extremely low losses as we evidence by a high cavity finesse F ≈40000 and naturally available maximal cavity–emitter coupling due to the perfect alignment of polarization modes and the dipole orientation. We observe a 13.3-fold Purcell enhancement of the zero phonon line of a single emitter, constituting an efficient source of coherent photons for this novel material platform, and opening the path toward a deterministic spin–photon interface.