Diamond is a leading quantum photonics platform due to its ability to host qubits based on crystal defects such as nitrogen-vacancy centres. Fabricating nanophotonic devices from defect-rich diamond, which underpins many quantum sensing technologies, promises enhanced performance and integrability of diamond quantum sensors. Here we demonstrate microdisk cavities fabricated from defect-rich diamond that support optical modes with high quality factor (${Q}\sim7\times10^4$ at $1042\,$nm) and show that they exhibit saturable absorption. Power-dependent spectroscopy measurements spanning 979$\,$nm to 1604$\,$nm are used to observe intensity-dependent cavity loss and extract wavelength-dependent absorption coefficients and saturation intensities. At 1047$\,$nm, we observe saturation and measure a saturation intensity of $2.1\,(8)\,$MW/cm$^2$ and an absorption coefficient of $0.53\,(2)\,$cm$^{-1}$. These results provide insight into defect-mediated optical loss in diamond nanophotonics and suggest strategies to harness defect-induced nonlinearities in future diamond photonic devices.
Diamond is a leading quantum photonics platform due to its ability to host qubits based on crystal defects such as nitrogen-vacancy centres. Fabricating nanophotonic devices from defect-rich diamond, which underpins many quantum sensing technologies, promises enhanced performance and integrability of diamond quantum sensors. Here we demonstrate microdisk cavities fabricated from defect-rich diamond that support optical modes with high quality factor (Q∼7×104 at 1042nm) and show that they exhibit saturable absorption. Power-dependent spectroscopy measurements spanning 979nm to 1604nm are used to observe intensity-dependent cavity loss and extract wavelength-dependent absorption coefficients and saturation intensities. At 1047nm, we observe saturation and measure a saturation intensity of 2.1(8)MW/cm2 and an absorption coefficient of 0.53(2)cm−1. These results provide insight into defect-mediated optical loss in diamond nanophotonics and suggest strategies to harness defect-induced nonlinearities in future diamond photonic devices.
Nonlinear optical absorption in commercially available diamond crystals containing nitrogen-vacancy (NV) centers is investigated using femtosecond open-aperture Z-scan measurements at a wavelength of 1032 nm. Experiments were performed on a high-purity electronic-grade diamond and on samples with moderate and high NV concentrations. While the undoped crystal exhibits third-order nonlinear absorption, the NV-doped samples show a pronounced saturable absorption response. To elucidate the microscopic origin of this effect, the Z-scan results are analysed in conjunction with linear transmission spectra, which reveal the presence of additional nitrogen-related defect complexes, including H2 ( NVN− ) centers. Although the excitation wavelength is detuned from the principal zero-phonon lines of both NV and H2 defects, it overlaps with the broader near-infrared absorption band associated with vibronic transitions. A quantitative analysis based on an effective two-level model indicates that the observed saturable absorption is most consistently explained by a dominant contribution from H2 defects, while a weaker off-resonant contribution from NV centers cannot be excluded. The extracted parameters, including the linear absorption coefficient and saturation intensity, are supported by a reconstruction of the defect-related absorbance spectrum and by consistency with previously reported nonlinear refractive index data. These results highlight the importance of complex defect environments in determining the nonlinear optical response of NV-doped diamond and provide insight into defect-specific contributions relevant for diamond-based nonlinear and quantum photonic applications.
Luminescent lattice defects (so called “color centers”) in diamond provide a robust, solid-state architecture for exploring quantum optics, with many facilitating direct access to highly coherent electron and nuclear spins. The past two decades have seen tremendous advances in our ability to prepare and control the quantum states of these atom-like systems. For instance, demonstrations that leverage the diamond nitrogen vacancy (NV–) center as a spin-photon interface have included the generation of indistinguishable photons, a quantum bit memory exceeding 1 second, entanglement of two distance solid-state qubits, and deterministic long distance quantum teleportation. While diamond color centers are an attractive platform for scalable quantum information processing, the diamond solid-state matrix, by its very nature, inhibits efficient photon collection into either free-space or fiber-optical systems. Due to its large refractive index, emitted photons are inevitably trapped within the diamond crystal by total internal reflection, limiting count rates and overall scalability of information processing via quantum optical protocols. To this end, there has been significant progress towards integrating individual diamond color centers within well-defined radiation channels: namely, those provided by monolithic diamond nanophotonic networks. Currently, on-chip low loss (~ few db/cm) diamond waveguide networks are available, as well as key passive photonic elements like micron scale ring and disk resonators and photonic crystal cavities, with high optical quality (Q) factors. Specifically, we recently demonstrated a scalable ‘angled-etching’ nanofabrication method for realizing nanophotonic networks starting from bulk single-crystal diamond substrates (MJ Burek et al., Nano Letters 2012, MJ Burek et al., Nat Comm. 2014). Angled-etching employs anisotropic oxygen-based plasma etching at an oblique angle to the substrate surface, resulting in suspended optical structures with triangular cross-sections. Using this approach, we demonstrated high Q-factor (> 10^5) optical nanocavities fabricated in bulk single-crystal diamond, operating over a wide wavelength range (visible to telecom). Beyond isolated photonic devices, we have further developed free-standing angled-etched waveguides which efficiently route photons between diamond optical nanocavities, while maintaining physical support through attachment to the bulk substrate. Although on-chip diamond nanophotonics realized by angled-etching continue to mature, efficient off-chip optical coupling schemes which provide near seamless transition of photons on-chip into commercial optical fibers remain an outstanding challenge. This is especially pertinent in applications involving single photons, such as quantum optics with diamond color centers. Herein, we demonstrate a high efficiency fiber-optical interface with aforementioned on-chip diamond nanophotonic networks, achieving > 90% power coupling at both telecom and visible wavelengths. The coupling scheme utilizes a single mode optical fiber, with one end fabricated into a conical taper, to adiabatically transition guided light between on-chip tapered diamond waveguides and off-chip optical fiber networks. Additionally, we develop techniques to robustly pigtail single-mode fibers with diamond waveguide couplers, yielding a packaged technology that may potentially enable deployment in harsh environments, including vacuum, sub-Kelvin cryogenics, or even liquid/biological environments. With these newly developed optical integration schemes, single-crystal diamond is now a viable integrated nanophotonics platform, servicing a range of applications, from non-linear optics and chemical sensing, to quantum science and cavity optomechanics.
Defect based qubit systems like the nitrogen vacancy center in diamond have recently emerged as promising candidates for quantum technologies due to their combination of long coherence times, room temperature operation, and robust optical interface. In order to realize many of their proposed applications, defect qubits must be incorporated into scalable devices architectures consisting of photonic, mechanical, or electrical degrees of freedom. Despite much recent progress, many challenges remain for diamond growth and device fabrication. As an alternate approach, we engaged in a search for nitrogen vacancy center analogues in alternative materials with the hope of obtaining a greater degree of control over defect and material properties. Ultimately, we discovered that divacancy-related point defects in all three of the most common forms of silicon carbide- termed 4H, 6H, and 3C- act as analogues to the nitrogen vacancy center in diamond. We chose to focus our research primarily on defects in 3C silicon carbide (termed 'Ky5' defects) because of its availability as a single crystal heteroepitaxial thin film grown on silicon, an advantage that greatly facilitates the fabrication of functional devices. We characterized the spin and optical properties of Ky5 defects in thin film geometries and observed many similarities to the nitrogen vacancy center. We performed the first measurements of spin dynamics in 3C silicon carbide and demonstrate coherent control of defect spins up to room temperature and observe coherence times of up to 22 microseconds. To demonstrate their use in real devices, we designed, fabricated, and characterized photonic crystal cavities in 3C silicon carbide thin films with mode volumes of less than (lambda/n)^3 and Q's as high as 1,500 with integrated Ky5 defects. Additionally, we performed simulations and analysis of the fabricated structures using observed structural imperfections to determine that the Q's are likely limited primarily by the non-vertical structure sidewall angle. Despite the modest Q's of these structures, they can be utilized to generate large local field intensities to enhance optical interactions with Ky5 qubit states within the cavities. We accomplish this by performing cavity enhanced photoluminescence excitation spectroscopy on cavity modes tuned to the zero phonon line of the defects and observe large (as high as 30 times) increases in the luminescence and optically detected magnetic resonance signals originating from the defect states and approximately 2x faster rates of ground state spin initialization. In addition, we use these techniques to probe the photoluminescence dynamics of the Ky5 defects' optical pumping cycle, perform excitation wavelength dependent studies of spin and spectral inhomogeneity, and use the small mode volume and narrowband photoluminescence enhancements of the cavities to observe spectrally distinct subensembles of defects with linewidths as narrow as 25 GHz within the inhomogeneously broadened zero phonon line. Although much is still unknown regarding the properties of these defects, they show great promise as a candidate system for defect qubit based quantum devices and technologies.
The use of diamond colour centres such as the nitrogen-vacancy (NV) centre is increasingly enabling quantum sensing and computing applications. Novel concepts like cavity coupling and readout, laser-threshold magnetometry and multi-pass geometries allow significantly improved sensitivity and performance via increased signals and strong light fields. Enabling material properties for these techniques and their further improvements are low optical material losses via optical absorption of signal light and low birefringence. Here, we study systematically the behaviour of absorption around 700 nm and birefringence with increasing nitrogen- and NV-doping, as well as their behaviour during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen doping yet substitutional nitrogen does not seem to be the direct absorber. Birefringence reduces with increasing nitrogen doping. We identify multiple crystal defect concentrations via absorption spectroscopy and their changes during the material processing steps and thus identify potential causes of absorption and birefringence as well as strategies to fabricate chemical vapour deposition diamonds with high NV density yet low absorption and low birefringence. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.
Diamond color centers such as the nitrogen-vacancy (NV) center have been extensively studied as a popular platform that enables quantum sensing and computing applications. For highly sensitive sensing NV ensembles have significantly stronger signals than individual NV centers and can create strong light fields, which are also essentially required by novel concepts that need a large sensing volume, such as cavity coupling and readout, laser threshold magnetometry and multi-pass geometries. On top of strong signals, low optical material losses are the other key consideration for these techniques, by means of low optical absorption of signal light and low birefringence of the diamond crystal. In this work, we study systematically the behavior of absorption in the NV emission range around 700nm and birefringence with increasing nitrogen- and NV-doping, as well as their behavior during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen-doping yet single substitutional nitrogen atoms (Ns0 or P1 centers) do not seem to be the direct absorber. The high nitrogen doping level in the growth leads to high absorption in the whole UV-Vis range, which is the main contributors to the high absorption at 700 nm. When treating the diamond for an enhanced NV concentration, ‘over irradiating’ the diamond with too high fluence is harmful in the sense of creating neutral vacancies that increase absorption at the wavelength range of interest. Opposite to the absorption, birefringence reduces with increasing nitrogen doping and shows better homogeneity. After-growth treatments show a minor and if any a positive (i.e. reducing) influence on diamond birefringence. Multiple crystal defect concentrations and their changes are identified via absorption spectroscopy during the material processing steps and thus we identify potential causes of absorption and birefringence. From this we suggest strategies to fabricate CVD diamonds with high NV density yet low absorption and low birefringence
A key architectural element of future quantum photonic networks is an efficient light-matter interface to connect electronic and photonic qubit systems. Nanophotonic resonators can be fabricated on-chip to provide such interfaces for atomic-like defect centers in diamond, which are leading qubit candidates. Fabrication advancements have recently lead to the construction of high quality diamond microdisk resonators, which show potential to reach enhancements with Purcell factor CNV ∼ 50. Here, a room-temperature experimental apparatus integrating free space and visible wavelength fiber-taper measurement capabilities is built to characterize diamond microdisk resonators. Using this setup, microdisk wisphering gallery modes with quality factors at visible wavelengths resonant with defect centers as high as Q ∼ 1 × 105, are observed for the first time. Spectral filtering effects of the taper on the microdisk are analysed to reveal that coupling to these disks may be limited by phase matching requirements. By thinning these disks it should be possible to improve coupling while lowering mode volumes, as desired to optimize Purcell factors.