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Geometry-Dependent Optical Interactions in NV Nanopillar Arrays for Scalable Quantum Sensing

Sondos M. Quqandi

Published Jul 29, 2026• 0 citations
Publisher

Abstract

Nitrogen-vacancy (NV) centers in diamond are versatile room-temperature quantum sensors whose sensitivity is often limited by photon collection. Shaping the diamond into nanopillar antennas increases photon collection, and packing these pillars into a dense array promises to combine this collection gain with the parallel throughput of the widefield quantum diamond microscope. However, optical interactions among the pillars in the array can limit its performance. The study combines finite-difference time-domain (FDTD) simulation with two experimental modalities, a home-built confocal microscope and a widefield microscope, on a (111) diamond parabolic nanopillar array of four geometries spanning two tip diameters (0.4 and 0.8 \textmu m) and edge-to-edge gaps of 0.5-1.6 \textmu m. The pillars enhance NV collection broadband, with band-integrated enhancements of $10.4\times$ and $6.0\times$ for the two tips, consistent with simulation. The geometry-mediated transport is quantified through the photons transported from the pillars to the gap and to neighboring pillars by the gap-to-pillar ratio ($G/P$) and the neighbor fraction ($\chi$): $G/P$ falls with increasing gap across all three methods (widefield $50.4\%\rightarrow31.4\%$, confocal $26.2\%\rightarrow12.1\%$, simulated $\chi$ $7.7\%\rightarrow2.6\%$), establishing the coupling as a robust physical feature governed by the base-to-base gap rather than the pitch. These optical interactions can influence the performance of the sensor where $\chi$ biases the accuracy while $G/P$ blurs the spatial resolution, and both grow as the pitch shrinks, capping the density-for-sensitivity gain and identifying the base-to-base gap and the pillar taper as the geometric parameters a purpose-designed sensing array must control. Whether the transported light is spin-active NV$^-$ emission, and therefore biases ODMR readout rather than merely adding background, remains to be confirmed, and is identified as the central next step toward array-based widefield sensing.

DOI
10.25781/kaust-l015u

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Nitrogen-vacancy (NV) centers in diamond are versatile room-temperature quantum sensors whose sensitivity is often limited by photon collection. Shaping the diamond into nanopillar antennas increases photon collection, and packing these pillars into a dense array promises to combine this collection gain with the parallel throughput of the widefield quantum diamond microscope. However, optical interactions among the pillars in the array can limit its performance. The study combines finite-difference time-domain (FDTD) simulation with two experimental modalities, a home-built confocal microscope and a widefield microscope, on a (111) diamond parabolic nanopillar array of four geometries spanning two tip diameters (0.4 and 0.8 \textmu m) and edge-to-edge gaps of 0.5-1.6 \textmu m. The pillars enhance NV collection broadband, with band-integrated enhancements of 10.4×10.4\times10.4× and 6.0×6.0\times6.0× for the two tips, consistent with simulation. The geometry-mediated transport is quantified through the photons transported from the pillars to the gap and to neighboring pillars by the gap-to-pillar ratio (G/PG/PG/P) and the neighbor fraction (χ\chiχ): G/PG/PG/P falls with increasing gap across all three methods (widefield 50.4%→31.4%50.4\%\rightarrow31.4\%50.4%→31.4%, confocal 26.2%→12.1%26.2\%\rightarrow12.1\%26.2%→12.1%, simulated χ\chiχ 7.7%→2.6%7.7\%\rightarrow2.6\%7.7%→2.6%), establishing the coupling as a robust physical feature governed by the base-to-base gap rather than the pitch. These optical interactions can influence the performance of the sensor where χ\chiχ biases the accuracy while G/PG/PG/P blurs the spatial resolution, and both grow as the pitch shrinks, capping the density-for-sensitivity gain and identifying the base-to-base gap and the pillar taper as the geometric parameters a purpose-designed sensing array must control. Whether the transported light is spin-active NV−^-− emission, and therefore biases ODMR readout rather than merely adding background, remains to be confirmed, and is identified as the central next step toward array-based widefield sensing.
The long-lived electronic spin of the nitrogen-vacancy (NV) center in diamonds is a promising quantum sensor for detecting nanoscopic magnetic and electric fields in various environments. However, the poor signal-to-noise ratio (SNR) of prevalent optical spin-readout techniques presents a critical challenge in improving measurement sensitivity. Here, we address this limitation by coupling individual NVs to optimized diamond nanopillars, thereby enhancing the collection efficiency of fluorescence. Guided by near-field optical simulations, we predict improved performance for tall (≥5 μm) pillars with tapered sidewalls. This is subsequently verified by fabricating and characterizing a representative set of structures using a newly developed nanofabrication process. We observe increased SNR for optimized devices, owing to improved emission collimation and directionality. Promisingly, these devices are compatible with low-numerical-aperture collection optics and a reduced tip radius, reducing experimental overhead and facilitating improved spatial resolution for scanning applications.
Negatively charged nitrogen vacancy centers (NV−) in diamond serve as highly sensitive, optically readable sensors for magnetic fields. Improved sensing approaches rely on NV− centers embedded in diamond nanopillar waveguides, which enable scanning probe imaging and use multi-color laser schemes for efficient spin readout. In this work, we investigate the free-beam coupling of the most relevant laser wavelengths to diamond nanopillars with different geometries. We focus on cylindrical pillars, conical pillars, and conical pillars with an added parabolic dome. We study the effects of the pillar geometry, NV− position, laser wavelength, position of laser focus, and excitation geometry (excitation from the top facet or from the substrate side). We find a pronounced impact of the laser wavelength that should be considered in multi-color excitation of NV−. Within the pillars, exciting laser fields can be enhanced up to a factor of 11.12 compared to bulk. When focusing the laser to the interface between the substrate and the nanopillar, even up to 29.78-fold enhancement is possible. Our results are in accordance with the experimental findings for green laser excitation of NV− in different pillar geometries.
Nitrogen-vacancy (NV) color centers in diamond are excellent quantum sensors possessing high sensitivity and nano-scale spatial resolution. Their integration in photonic structures is often desired, since it leads to an increased photon emission and also allows the realization of solid-state quantum technology architectures. Here, we report the fabrication of diamond nano-pillars with diameters up to 1000 nm by electron beam lithography and inductively coupled plasma reactive ion etching in nitrogen-rich diamonds (type Ib) with [100] and [111] crystal orientations. The NV centers were created by keV-He ion bombardment and subsequent annealing, and we estimate an average number of NVs per pillar to be 4300 ± 300 and 520 ± 120 for the [100] and [111] samples, respectively. Lifetime measurements of the NVs’ excited state showed two time constants with average values of τ1 ≈ 2 ns and τ2 ≈ 8 ns, which are shorter as compared to a single color center in a bulk crystal (τ ≈ 10 ns). This is probably due to a coupling between the NVs as well as due to interaction with bombardment-induced defects and substitutional nitrogen (P1 centers). Optically detected magnetic resonance measurements revealed a contrast of about 5% and average coherence and relaxation times of T2 [100] = 420 ± 40 ns, T2 [111] = 560 ± 50 ns, and T1 [100] = 162 ± 11 μs, T1 [111] = 174 ± 24 μs. These pillars could find an application for scanning probe magnetic field imaging.
Nitrogen-vacancy (NV) centers in diamond are a versatile quantum sensing platform for high sensitivity measurements of magnetic fields, temperature and strain with nanoscale spatial resolution. A common bottleneck is the analysis of optically detected magnetic resonance (ODMR) spectra, where target quantities are encoded in resonance features. Conventional nonlinear fitting is often computationally expensive, sensitive to initialization, and prone to failure at low signal-to-noise ratio (SNR). Here we introduce a robust, efficient machine learning (ML) framework for real-time ODMR analysis based on a one-dimensional convolutional neural network (1D-CNN). The model performs direct parameter inference without initial guesses or iterative optimization, and is naturally parallelizable on graphics processing units (GPU) for high-throughput processing. We validate the approach on both synthetic and experimental datasets, showing improved throughput, accuracy and robustness than standard nonlinear fitting, with the largest gains in the low-SNR regime. We further validate our methods in two representative sensing applications: diagnosing intracellular temperature changes using nanodiamond probes and widefield magnetic imaging of superconducting vortices in a high-temperature superconductor. This deep-learning inference framework enables fast and reliable extraction of physical parameters from complex ODMR data and provides a scalable route to real-time quantum sensing and imaging.
Nitrogen-vacancy (NV) centers in diamond are valuable for quantum computing, sensing, and secure communication due to their stable room-temperature operation. However, efficient photon collection is limited by diamond’s high refractive index, leading to significant internal reflection. This work investigates how varying the sidewall angle of diamond nanopillars can affect photon collection efficiency, using finite element method (FEM) simulations. Results indicate that nanopillars with sidewall angles near 90 degrees optimize collection for x-oriented dipoles, while y-oriented dipoles perform best around 86 degrees. NV center depth within the nanopillar also affects efficiency, with certain depths favoring specific angles. Our findings show that angled sidewalls can enhance photon collection while also provide greater structural stability compared to vertical designs, offering a practical pathway to improve NV center-based quantum devices.
Silicon carbide (SiC) is a promising platform for scalable quantum technologies owing to its well-established, wafer-scale industrial processing. SiC also hosts a variety of optically active color centres including the nitrogen vacancy defect that has a spin-triplet ground state. However, strong phonon coupling in the infrared range limits photon extraction from these defects. Here, we use nanophotonic structures, specifically micro-pillar and micro-disk resonators, to enhance optical collection and spin-readout. The micro-pillar geometry yields a 4-fold increase in photon collection, accompanied by a 2.4-fold reduction in spectral noise in optically detected magnetic resonance measurements. Consequently, the magnetic field sensitivity is improved by 24%. The large mode volume of the micro-disk supports resonances spanning 1150-1250 nm, enabling broadband coupling to nitrogen vacancy emission lines. Our results demonstrate that fabrication of scalable photonic structures efficiently improves performance of silicon carbide color centers for integrated quantum light generation and sensing.