We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully tunable Fabry-Perot cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding confocal measurement. The strong signal amplification results from the Purcell effect. We show that the cavity-enhanced Raman scattering can be harnessed as a narrow-band high-intensity internal light source. The Raman process can be triggered in a simple way by using an optical excitation frequency outside the cavity stop band and is independent of the lateral positioning of the cavity mode with respect to the diamond membrane. The strong Raman signal emerging from the cavity output facilitates in situ mode matching of the cavity mode to single-mode collection optics; it also represents a simple way of measuring the dispersion and spatial intensity profile of the cavity modes. The optimization of the cavity performance via the strong Raman process is extremely helpful in achieving efficient cavity outcoupling of the relatively weak emission of single color centers such as nitrogen-vacancy centers in diamond or rare-earth ions in crystalline hosts with a low emitter density.
We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully tunable Fabry-Perot cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding confocal measurement. The strong signal amplification results from the Purcell effect. We show that the cavity-enhanced Raman scattering can be harnessed as a narrow-band high-intensity internal light source. The Raman process can be triggered in a simple way by using an optical excitation frequency outside the cavity stop band and is independent of the lateral positioning of the cavity mode with respect to the diamond membrane. The strong Raman signal emerging from the cavity output facilitates in situ mode matching of the cavity mode to single-mode collection optics; it also represents a simple way of measuring the dispersion and spatial intensity profile of the cavity modes. The optimization of the cavity performance via the strong Raman process is extremely helpful in achieving efficient cavity outcoupling of the relatively weak emission of single color centers such as nitrogen-vacancy centers in diamond or rare-earth ions in crystalline hosts with a low emitter density.
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.
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.
We employ a fiber-based optical microcavity with high finesse to study the enhancement of phonon sideband fluorescence of nitrogen-vacancy centers in nanodiamonds. Harnessing the full tunability and open access of the resonator, we explicitly demonstrate the scaling laws of the Purcell enhancement by varying both the mode volume and the quality factor over a large range. While changes in the emission lifetime remain small in the regime of a broadband emitter, we observe an increase of the emission spectral density by up to a factor of 300. This gives a direct measure of the Purcell factor that could be achieved with this resonator and an emitter whose linewidth is narrower than the cavity linewidth. Our results show a method for the realization of wavelength-tunable narrow-band single-photon sources and demonstrate a system that has the potential to reach the strong-coupling regime.
The tin-vacancy (SnV) center in diamond is a promising spin-photon interface for quantum networks, combining favorable optical properties with spin coherence above 1K. Unfolding the full potential requires cavity enhancement to increase photon-emitter coupling efficiency. Here, we demonstrate cavity-enhanced light-matter coupling of SnV centers in a fully tunable Fabry-Pérot microcavity operating at temperatures down to 1K with in-situ magnetic field control. We access the diamond-like regime of hybrid cavity modes through integration of low-roughness diamond membranes, where the field is concentrated inside the diamond and Purcell enhancement is maximized. Diamond-like modes deliver a more than two-fold increase in the effective Purcell factor over air-like modes, reaching C0=4.1(1) compared to C0=1.85(5) in the air-like case, while simultaneously relaxing mechanical stability requirements. Resonant probing reveals coherent cavity-emitter coupling with 96% extinction contrast and a coherent cooperativity of C=4.0(14). By applying a magnetic field, we further achieve spin-resolved cavity extinction, observing spin-selective optical transitions with a contrast of Cspin=0.91. These results establish SnV centers in diamond coupled to open Fabry-Pérot microcavities as a promising platform for efficient spin-photon interfaces.
A key requirement for many applications in solid-state quantum sciences is a high fluence of indistinguishable photons. The spontaneous emission of these photons is governed by the coupling of an excited quantum system to electromagnetic vacuum fluctuations. In our experiment, we enhance this coupling by engineering a tunable Fabry-Perot microcavity. The quantum system we study is the nitrogen-vacancy (NV) center in diamond, a workhorse for quantum science and engineering, due to its optical transitions and the coherent electron spin system it hosts. Our device consists of a high-quality, nano-fabricated, single-crystalline diamond membrane bonded to a planar mirror; the cavity is completed by a second, concave mirror. Using piezo positioners, we achieve full spectral and spatial tunability and freedom in selecting NVs with favorable emission properties in our low-temperature (4 Kelvin) experiments. Upon tuning of the cavity into resonance, we find significant enhancement of the 637 nm zero phonon line for several individual NVs which is accompanied by a strong reduction of the overall photoluminescence (PL) lifetime. We infer a 30-fold enhancement of the zero-phonon transition rate at best. The fraction of the PL emission associated to this resonant transition is thereby increased from 3% to 46%. Our results constitute a significant leap on the route towards the implementation of fast long-distance quantum networks, which are currently limited by the photon emission rate in their nodes. Furthermore, our versatile design is readily applicable to other solid-state quantum emitters like color centers in silicon carbide.
An efficient atom–photon interface is a key requirement for the integration of solid-state emitters, such as color centers in diamond, into quantum technology applications. As other solid-state emitters, however, their emission into free space is severely limited due to the high refractive index of the bulk host crystal. In this work, we present a planar optical antenna based on two silver mirrors coated on a thin single crystal diamond membrane, forming a planar Fabry–Pérot cavity that improves the photon extraction from single tin vacancy (SnV) centers and their coupling to an excitation laser. Upon numerical optimization of the structure, we find theoretical enhancements in the collectible photon rate by a factor of 60 as compared to the bulk case. As a proof-of-principle demonstration, we fabricate single crystal diamond membranes with sub-μm thickness and create SnV centers by ion implantation. Employing off-resonant excitation, we show a sixfold enhancement of the collectible photon rate, yielding up to half a million photons per second from a single SnV center. At the same time, we observe a significant reduction of the required excitation power in accordance with theory, demonstrating the functionality of the cavity as an optical antenna. Due to its planar design, the antenna simultaneously provides similar enhancements for a large number of emitters inside the membrane. Furthermore, the monolithic structure provides high mechanical stability and straightforwardly enables operation under cryogenic conditions as required in most spin–photon interface implementations.